Electrical Thumb Rules-(Part-15)


Selection of MCB

MCB curve Type of Load Residential Commercial
B curve Resistive Loads Incandescent lights Incandescent lights
Geyser  Boilers
 Heater  Heaters
Fan blower heaters Oil radiator heaters
Slight Inductive Loads Florescent Lights Florescent lights
Small motors (FHP) High pressure mercury vapor lamps
C curve Slight Inductive Loads Fans & small pumps Sodium vapor lamps
Window / Split ACs
Lights with ballasts
 General household equipment
D curve Inductive Loads Water lifting pumps Florescent lights
UPS ID & FD fans
  Small control transformers
  Medium size motors
  Refrigerators for commercial use


Type of MCB

MCB Curve Type of Load Response Tripping Application Uses
B curve Resistive loads MCBs react quickly to overloads 3 To 5 times F.L current (0.04 To 13 Sec) Domestic & Commercial applications Suitable for incandescent lighting, socket-outlet, bulbs, heaters etc. Protection of DG sets (since DG sets have low short-circuit capacity)
C curve Slightly inductive loads MCBs react more slowly, 5 To 10 times F.L current  (0.04 To 5 Sec) Commercial and Industrial applications


Highly Inductive loads such as motors, air conditioners, fluorescent lighting lights, fans & household electrical appliances.
D curve Inductive loads MCBs are slower 10 To 20 times F.L current  (0.04 To 3 Sec) Commercial and Industrial applications


Very high inrush Inductive currents, Small transformers, welding machines. UPS, small motor & pumps, x-ray machines etc. Note, however, that MCBs with Type K characteristics may provide better protection in some applications of this type.
K curve Inductive loads MCBs are slower 8 To 10 times F.L current  (0.04 To 3 Sec)   Placing them between the traditional Type C and Type D breakers. In most cases, they allow improved cable protection to be provided in circuits that include motors, capacitors and transformers, where it would previously have been necessary to use Type D devices. This enhanced protection is achieved without increasing the risk of nuisance tripping.


Selection of RCCB

Type of RCCB Sensitive Application
Type AC Sensitive to AC Currents Only Suitable for most domestic and commercial applications.
Type A Sensitive to AC Currents + Pulsating DC Currents (Produced by Rectifier, Thyristors) Used where there are a lot of “electronic” loads, such as computer equipment or lighting systems with electronic ballasts.
Type B Sensitive to AC Currents + Pulsating DC Currents+ Pure DC Currents Use in photovoltaic (PV) solar energy installations because the PV panels produce a DC Output and some types of fault can result in the leakage of DC Currents to Earth.
Type B+ Similar to Type B, but respond to ac leakage currents over a wider frequency range Type B and Type B+ devices can be used wherever a Type AC or Type A device is specified, as they provide the same functionality as these types and more.



TYPE AC Current 50Hz AC Current 50Hz To 1KHZ Pulsating Current with DC Component Multi Frequency Current Generated By 1Phase Inverter Multi Frequency Current Generated By 3Phase Inverter


Sensitivity of RCCB

RCCBs Application
30 mA personal protection domestic installation / direct contact
100 mA limited personal protection / indirect contact
300 mA building / fire protection


MCB Enclosure Size

MCB Rating (A) Min. Enclosure Size
Height Width Depth
100A 370mm 216mm 72mm
125A 310mm 180mm 83mm
225A 370mm 217mm 72mm
250A 380mm 195mm 83.5mm
300A to 400A 506mm 381mm 153mm
600A to 800A 520mm 420mm 200mm
1000A to 1200A 704mm 554mm 173mm
1600A to 3000A 1016mm 608mm 615mm


Switch Gear Protection

Switch Gear Protection Isolation Control
Over Load Short Circuit
Circuit Breaker YES YES YES YES
Contactor NO NO NO YES
Disconnector NO NO YES NO


Type of Faults

Types of Fault Reason Consequences  Protective Device to be used
Overload When Equipment tries to run beyond its rated capacity, or there is a fault in the equipment E.g. When you keep a heater on without any water in it. It can lead to reduction in life of equipment, Failure of insulation and hence damaging the equipment. MCB / RCBO
Short Circuit Insulation Failure, Shorting of the Phase to Phase or Phase and Neutral Wires. High Inrush Current, causing permanent damage to equipment and may lead to a Fire. MCB
Earth Fault Short circuit between Phase and Earth Conductor. Can result in Fire due to sparking. RCBO / RCCB
Earth Leakage Human Body Touching Live Wires. Insulation failure   RCBO / RCCB
Over Voltage Opening of Neutral Connection increase in Phase To Phase Voltage of 440V, Surge through Lighting or transients, Over voltage from Utility. Damage to sensitive Electronic Equipment. OV protection Device
Under Voltage Drop in supply voltage, starting of heavy loads Damage of Equipment, Flickering of Lights. UV relays


 MCB Type (BS EN 60898-2)

Trip Type instantaneous
Trip (< 0.1 s)
Load Type Typical Load
B 3 to 5 In (AC) Resistive Heaters, showers, cookers, socket outlets.
4 to 7 In (DC)
C 5 to 10 In (AC) Inductive Motors, general lighting circuits, power supplies.
7 to 15 In (DC)
D 10 to 20 In High Inductive Transformers, motors, discharge lighting circuits, computers


Relays for Transformer 

Capacity of Transformer Relays on HV Side Relays on LV Side Common Relays
Generator Transformer 3 Nos Non-Directional O/L Relay – – Differential Relay or
1 no Non-Directional E/L Relay Overall differential Relay
and/or standby E/F + REF Relay Over flux Relay
  Buchholz Relay
  OLTC Buchholz Relay
  PRV Relay
  OT Trip Relay
  WT Trip Relay
220 /6.6KV Station Transformer 3 Nos Non-Directional O/L Relay 3 Nos Non-Directional O/L Relay Differential Relay
1 no Non-Directional E/L Relay Over flux Relay
and/or standby E/F + REF Relay Buchholz Relay
  OLTC Buchholz Relay
  PRV Relay
  OT Trip Relay
  WT Trip Relay
132/33/11KV up to 8 MVA 3 Nos O/L Relay 2 Nos O/L Relays Buchholz Relay
1 no E/L Relay 1 no E/L Relay OLTC Buchholz Relay
    PRV Relay
    OT Trip Relay
    WT Trip Relay
132/33/11KV up to 8 MVA to 31.5 MVA 3 Nos O/L Relay 3 Nos O/L Relay Differential Relay
1 no Directional E/L Relay 1 no E/L Relay Buchholz Relay
    OLTC Buchholz Relay
    PRV Relay
    OT Trip Relay
    WT Trip Relay
132/33KV, 31.5 MVA & above 3 Nos O/L Relay 3 Nos O/L Relay Differential Relay
1 no Directional E/L Relay 1 no E/L Relay Over flux Relay
    Buchholz Relay
    OLTC Buchholz Relay
    PRV Relay
    OT Trip Relay
    WT Trip Relay
220/33 KV, 31.5MVA & 50MVA , 220/132KV, 100 MVA 3 No O/L Relay 3 Nos O/L Relay Differential Relay
1 no Directional E/L Relay 1 no Directional E/L Relay Over flux Relay
    Buchholz Relay
    OLTC Buchholz Relay
    PRV Relay
    OT Trip Relay
    WT Trip Relay
400/220KV 315MVA 3 Nos Directional O/L Relay 3 Nos Directional O/L Relay Differential Relay
1 no Directional E/L relay. 1 no Directional E/L relay. Over flux Relay
Restricted E/F relay Restricted E/F relay Buchholz Relay
3 Nos Directional O/L Relay for action   OLTC Buchholz Relay
  PRV Relay
    OT Trip Relay
    WT Trip Relay
    Over Load (Alarm) Relay


Relays for Transmission & Distribution Lines Protection

Lines to be protected Relays to be used
400 KV Transmission Line Main-I: Non switched or Numerical Distance Scheme
Main-II: Non switched or Numerical Distance Scheme
220 KV Transmission Line Main-I : Non switched distance scheme (Fed from Bus PTs)
Main-II: Switched distance scheme (Fed from line CVTs)
With a changeover facility from bus PT to line CVT and vice-versa.
132 KV Transmission Line Main Protection : Switched distance scheme (fed from bus PT).
Backup Protection: 3 Nos. directional IDMT O/L Relays and
1 No. Directional IDMT E/L relay.
33 KV lines Transmission Line Non-directional IDMT 3 O/L and 1 E/L relays.
11 KV lines Transmission Line Non-directional IDMT 2 O/L and 1 E/L relays.


Selection Chart for 3Ph Induction Motor

Motor Rating,415V,3Ph Full Load Current (A) CONTACTOR (A) OVER LOAD RELAY (A) BACK UP FUSE (A) Cable Size
DOL Starter STAR-DELTA Starter
0.75 0.52 1.6 16   1.0 To 1.6   4 1.5 1.5    
1 0.75 2 16   1.6 To 2.5   6 1.5 1.5    
2 1.5 3.5 16   3.0 To 4.5   10 1.5 1.5    
3 2.2 5 16   4.5 To 7.0   10 1.5 1.5    
5 3.7 7.5 16   6.5 To 10   16 1.5 1.5    
7.5 5.5 11 16 16 10 To 15 4.5 To 7.0 16 2.5 1.5 2.5 1.5
10 7.5 14 16 16 13 To 20 6.5 To 10 20 2.5 2.5 2.5 2.5
12.5 9.3 18 25 16 13 To 20 10 To 15 25 4 2.5 4 2.5
15 11 21 25 16 15 To 22 13 To 20 25 6 4 6 4
20 15 28 32 18 24 To 30 13 To 20 32 10 6 10 6
25 18.5 35 40 25 25 To 30 15 To 22 50 16 10 16 10
30 22.5 40 50 25 32 To 50 24 To 30 50 16 16 16 16
35 26 47 70 32 32 To 50 25 To 30 63 25 16 25 16
50 37 66 70 40 57 To 70 32 To 50 80 35 25 35 25
60 45 80 95 50 70 To 105 32 To 50 100 50 35 50 35
75 55 100 125 70 100 To 150 40 To 57 100 70 50 70 50
90 67.5 120 140 70 100 To 150 57 To 70 160 95 70 95 70
100 75 135 140 95 100 To 150 70 To 105 160 95 70 95 70
125 90 165   125   70 To 105 160     150 95
150 110 200   125   100 To 150 200     185 150


MCB Selection Chart For Motor Protection

Kw HP 1Phase 230V DOL
3Phase 400V DOL
3 Phase 400V Star Delta
Full Load Current MCB Selection Full Load Current MCB Selection Full Load Current MCB Selection MCB Selection
0.18 0.24 2.8 10 0.9 2  —  —  —
0.25 0.34 3.2 10 1.2 2  —  —  —
0.37 0.5 3.5 10 1.2 2  —  —  —
0.55 0.74 4.8 16 1.8 3  —  —  —
0.75 1.01 6.2 20 2 3  —  —  —
1.1 1.47 8.7 25 2.6 6  —  —  —
1.5 2.01 11.8 32 3.5 10  —  —  —
2.2 2.95 17.5 50 4.4 10  —  —  —
3 4.02 20 63 6.3 16 6.3 16 10
3.75 5.03 24 80 8.2 20 8.2 20 10
5.5 7.37 26 80 11.2 25 11.2 32 16
7.5 10.05 47 125 14.4 40 14.4 40 25
10 13.4  —  — 21 50 21 50 32
15 20.11  — 27 100 27 63 40
18.5 24.8  —  — 32 125 32  — 50
22 29.49  —  — 38 125 38 63
30 40.21  —  — 51 125 51  — 63



Code Type of Relay
1 Master Element
2 Time-delay Starting or Closing Relay
3 Checking or Interlocking Relay
4 Master Contactor
5 Stopping Device
6 Starting Circuit Breaker
7 Rate of Change Relay
8 Control Power Disconnecting Device
9 Reversing Device
10 Unit Sequence Switch
11 Multifunction Device
12 Over speed protection
13 Synchronous-Speed Device
14 Under speed Device
15 Speed or Frequency Matching Device
16 Data Communications Device
17 Shunting or Discharge Switch
18 Accelerating or Decelerating Device
19 Starting-to-Running Transition Contactor
20 Electrically-Operated Valve
21 Distance protection Relay
21G Ground Distance
21P Phase Distance
22 Equalizer circuit breaker
23 Temperature control device
24 Volts per hertz relay
25 Synchronizing or synchronism-check device
26 Apparatus thermal device
27 Under voltage relay
27P Phase Under voltage
27S DC under voltage relay
27TN Third Harmonic Neutral Under voltage
27TN/59N 100% Stator Earth Fault
27X Auxiliary Under voltage
27 AUX Under voltage Auxiliary Input
27/27X Bus/Line Under voltage
27/50 Accidental Generator Energization
28 Flame Detector
29 Isolating Contactor
30 Annunciator Relay
31 Separate Excitation Device
32 Directional Power Relay
32L Low Forward Power
32N Watt metric Zero-Sequence Directional
32P Directional Power
32R Reverse Power
33 Position Switch
34 Master Sequence Device
35 Brush-Operating or Slip-ring Short Circuiting Device
36 Polarity or Polarizing Voltage Device
37 Undercurrent or Under power Relay
37P Under power
38 Bearing Protective Device / Bearing Rtd
39 Mechanical Condition Monitor
40 Field Relay / Loss of Excitation
41 Field Circuit Breaker
42 Running Circuit Breaker
43 Manual Transfer or Selector Device
44 Unit Sequence Starting Relay
45 Atmospheric Condition Monitor
46 Reverse-Phase or Phase Balance Current Relay or Stator Current Unbalance
47 Phase-Sequence or Phase Balance Voltage Relay
48 Incomplete Sequence Relay / Blocked Rotor
49 Machine or Transformer Thermal Relay / Thermal Overload
49RTD RTD Biased Thermal Overload
50 Instantaneous Overcurrent Relay
50BF Breaker Failure
50DD Current Disturbance Detector
50EF End Fault Protection
50G Ground Instantaneous Overcurrent
50IG Isolated Ground Instantaneous Overcurrent
50LR Acceleration Time
50N Neutral Instantaneous Overcurrent
50NBF Neutral Instantaneous Breaker Failure
50P Phase Instantaneous Overcurrent
50SG Sensitive Ground Instantaneous Overcurrent
50SP Split Phase Instantaneous Current
50Q Negative Sequence Instantaneous Overcurrent
50/27 Accidental Energization
50/51 Instantaneous / Time-delay Overcurrent relay
50Ns/51Ns Sensitive earth-fault protection
50/74 Ct Trouble
50/87 Instantaneous Differential
51 Phase Inverse Time Overcurrent IDMT (Time delay phase overcurrent )
51G Ground Inverse Time Overcurrent
51LR AC inverse time overcurrent (locked rotor) protection relay
51N Neutral Inverse Time Overcurrent
51P Phase Time Overcurrent
51R Locked / Stalled Rotor
51V Voltage Restrained Time Overcurrent
51Q Negative Sequence Time Overcurrent
52 AC circuit breaker
52a AC circuit breaker position (contact open when circuit breaker open)
52b AC circuit breaker position (contact closed when circuit breaker open)
53 Exciter or Dc Generator Relay
54 Turning Gear Engaging Device
55 Power Factor Relay
56 Field Application Relay
57 Short-Circuiting or Grounding Device
58 Rectification Failure Relay
59 Overvoltage Relay
59B Bank Phase Overvoltage
59P Phase Overvoltage
59N Neutral Overvoltage
59NU Neutral Voltage Unbalance
59P Phase Overvoltage
59X Auxiliary Overvoltage
59Q Negative Sequence Overvoltage
60 Voltage or current balance relay
60 Voltage or Current Balance Relay
60N Neutral Current Unbalance
60P Phase Current Unbalance
61 Density Switch or Sensor
62 Time-Delay Stopping or Opening Relay
63 Pressure Switch Detector
64 Ground Protective Relay
64F Field Ground Protection
64R Rotor earth fault
64REF Restricted earth fault differential
64S Stator earth fault
64S Sub-harmonic Stator Ground Protection
64TN 100% Stator Ground
65 Governor
66 Notching or Jogging Device/Maximum Starting Rate/Starts Per Hour/Time Between Starts
67 AC Directional Overcurrent Relay
67G Ground Directional Overcurrent
67N Neutral Directional Overcurrent
67Ns Earth fault directional
67P Phase Directional Overcurrent
67SG Sensitive Ground Directional Overcurrent
67Q Negative Sequence Directional Overcurrent
68 Blocking Relay / Power Swing Blocking
69 Permissive Control Device
70 Rheostat
71 Liquid Switch
72 DC Circuit Breaker
73 Load-Resistor Contactor
74 Alarm Relay
75 Position Changing Mechanism
76 DC Overcurrent Relay
77 Telemetering Device
78 Phase Angle Measuring or Out-of-Step Protective Relay
78V Loss of Mains
79 AC Reclosing Relay / Auto Reclose
80 Liquid or Gas Flow Relay
81 Frequency Relay
81O Over Frequency
81R Rate-of-Change Frequency
81U Under Frequency
82 DC Reclosing Relay
83 Automatic Selective Control or Transfer Relay
84 Operating Mechanism
85 Pilot Communications, Carrier or Pilot-Wire Relay
86 Lock-Out Relay, Master Trip Relay
87 Differential Protective Relay
87B Bus Differential
87G Generator Differential
87GT Generator/Transformer Differential
87L Segregated Line Current Differential
87LG Ground Line Current Differential
87M Motor Differential
87O Overall Differential
87PC Phase Comparison
87RGF Restricted Ground Fault
87S Stator Differential
87S Percent Differential
87T Transformer Differential
87V Voltage Differential
88 Auxiliary Motor or Motor Generator
89 Line Switch
90 Regulating Device
91 Voltage Directional Relay
92 Voltage And Power Directional Relay
93 Field-Changing Contactor
94 Tripping or Trip-Free Relay
Abbreviation Code
AFD Arc Flash Detector
CLK Clock or Timing Source
CLP Cold Load Pickup
DDR Dynamic Disturbance Recorder
DFR Digital Fault Recorder
DME Disturbance Monitor Equipment
ENV Environmental data
HIZ High Impedance Fault Detector
HMI Human Machine Interface
HST Historian
LGC Scheme Logic
MET Substation Metering
PDC Phasor Data Concentrator
PMU Phasor Measurement Unit
PQM Power Quality Monitor
RIO Remote Input/output Device
RTD Resistance Temperature Detector
RTU Remote Terminal Unit/Data Concentrator
SER Sequence of Events Recorder
TCM Trip Circuit Monitor
LRSS  Local/Remote selector switch
VTFF  Vt Fuse Fail

Suffixes Description

_1 Positive-Sequence
_2 Negative-Sequence
A Alarm, Auxiliary Power
AC Alternating Current
AN Anode
B Bus, Battery, or Blower
BF Breaker Failure
BK Brake
BL Block (Valve)
BP Bypass
BT Bus Tie
BU Backup
C Capacitor, Condenser, Compensator, Carrier Current, Case or Compressor
CA Cathode
CH Check (Valve)
D Discharge (Valve)
DC Direct Current
DCB Directional Comparison Blocking
DCUB Directional Comparison Unblocking
DD Disturbance Detector
DUTT Direct Under reaching Transfer Trip
E Exciter
F Feeder, Field, Filament, Filter, or Fan
G Ground or Generator
GC Ground Check
H Heater or Housing
L Line or Logic
M Motor or Metering
MOC Mechanism Operated Contact
N Neutral or Network
O Over
P Phase or Pump
PC Phase Comparison
POTT  Pott: Permissive Overreaching Transfer Trip
PUTT Putt: Permissive Under reaching Transfer Trip
R Reactor, Rectifier, or Room
S Synchronizing, Secondary, Strainer, Sump, or Suction (Valve)
SOTF Switch On To Fault
T Transformer or Thyratron
TD Time Delay
TDC Time-Delay Closing Contact
TDDO Time Delayed Relay Coil Drop-Out
TDO Time-Delay Opening Contact
TDPU Time Delayed Relay Coil Pickup
THD Total Harmonic Distortion
TH Transformer (High-Voltage Side)
TL Transformer (Low-Voltage Side)
TM Telemeter
TT Transformer (Tertiary-Voltage Side)
U Under or Unit
X Auxiliary
Z Impedance


Harmonic Effects

Harmonic R Phase Y Phase B Phase Phase Rotation Sequence Harmonic Effect
Rotation Rotation Rotation
Fundamental 120° 240° R-Y-B  
3th 3×0°= 3×120°=360°=0° 3×240°=720°=0° No Rotation          (In Phase) Adds Voltages or Currents in Neutral Wire causing Heating
9th 9×0°= 9×120°=1080°=0° 9×240°=2160°=0°
15th 15×0°= 15×120°=1800°=0° 15×240°=3600°=0°
21th 21×0°= 21×120°=2520°=0° 21×240°=5040°=0°
5th 5×0°= 5×120°=600°=(600-720)=(-120°) 5×240°=1200°=(1200-2400)=(-240°) Rotate Against Fundamental (-) (B-Y-R) Motor Torque Problems
11th 11×0°= 11×120°=1320°=(1320-1400)=(-120°) 11×240°=2640°=(2880-2640)=(-240°)
17th 17×0°= 17×120°=2040°=(2040-2160)=(-120°) 17×240°=4080°=(4320-4080)=(-240°)
23th 23×0°= 23×120°=2760°=(2760-2880)=(-120°) 23×240°=5520°=(5760-5520)=(-240°)
7th 7×0°= 7×120°=840°=(840-720)=(+120°) 7×240°=1680°=(1680-1440)=(+240°) Rotate with Fundamental (+) (R-Y-B) Excessive Heating Effect
13th 13×0°= 13×120°=1560°=(1560-1440)=(+120°) 13×240°=3120°=(3120-2880)=(+240°)
19th 19×0°= 19×120°=2280°=(2280-2160)=(+120°) 19×240°=4560°=(4560-4320)=(+240°)
25th 25×0°= 25×120°=3000°=(3000-2880)=(+120°) 25×240°=6000°=(6000-5760)=(+240°)

Difference Between High Bay-Low Bay and Flood Light Fixture


  • In the lighting industry, the term “bay” means to illuminate any large area.
  • High Bay fixtures and Low Bay fixtures are used to for illumination in Buildings with higher ceilings like warehouse lighting, industrial lighting, Commercial lighting, retail lighting, and gym lighting.
  • High Bay Lighting and Low Bay Lighting are mounted at high level via a pendant, chain, or directly to a ceiling or ceiling girder.

Type of Lighting Fixture for Larger Area Illumination:

  • There are three type of lighting fixture to illuminate large open Area
  • Low Bay Lighting Fixtures
  • High Bay Lighting Fixtures
  • Flood Lights

 (1) Low Bay Light Fixture.

  • As the name says, these bay lights are often used with lower ceilings in open areas.
  • Low bay lights are designed to illuminate open areas with ceilings Between 12 foot to 20 foot.
  • Anything use over this height treat as high bays, and anything lower is very uncommon in large open area facilities, and would require a different type of light fixture.
  • The reflectors or lens for low bays also spread the light far out to maintain a desired lighting level.



  • Ware House.
  • Petrol Station.
  • Retail Store.

(2) High Bay Light Fixture.

  • As their name implies, high-bay lights are used to illuminate spaces with high ceilings. That usually means ceilings ranging from 20 feet to 45 feet.
  • These light is effective at high Ceiling Level to provide well distributed and uniform light for open areas.
  • They need specifically reflectors (for HPS / MH bulbs) or lens angles to ensure light reaches the floor evenly and reduces wasted light. Different kinds of reflectors can accomplish different kinds of illumination tasks for high-bay lights. Aluminum reflectors make light from the fixtures flow directly downward to the floor, while prismatic reflectors create a more diffused lighting useful for illuminating shelves and other elevated objects in a space.
  • High-ceiling location has more space to fill, hence a high-bay by definition is a powerful light source that can brighten up a large area.
  • High-bay lighting is provides clear, uniform lighting of what’s below it with little glare.
  • Numerous types of fixtures can be used as a high-bay lights like LED lights, induction lights, metal halide lights, and fluorescent lights.
  • For instance, LED lights offer extremely long life and energy efficiency but require a bigger initial investment, while traditional incandescent lights are less expensive to purchase initially but don’t last as long and use more energy.
  • There are several types of fixtures available for high-bay lights. Round high-bay lights, linear high-bays, architectural high-bays and grid-mount high-bays.



  • Whenever a large indoor space needs to be illuminated, high bay lighting is usually appropriate. These area is typically vast and cover a lot of vertical as well as horizontal space. This need powerful lighting to provide the appropriate Lux levels to adequately illuminate.
  • High bay lighting fixtures typically hang from the ceiling via hooks, chains or pendants, or they may be fixed to the ceiling directly (similar to troffer lights).  
  • Various industries and facilities require high bay lighting. Some of the most common are
  • Industrial facilities.
  • Manufacturing facilities.
  • School and university gymnasiums.
  • Municipal facilities like community centers or recreation centers.
  • Commercial applications like department stores.
  • Airport hangar or any large open area industrial and commercial space with relatively high ceilings

Choosing the Correct High Bay & Low Bay Fixture

  • Choosing the right High Bay fixture can make the difference between a successful lighting project or
  • A light designed for a warehouse is a totally different than a light designed for a gymnasium or a factory floor. In gymnasium or a factory floor, a light can distribute in the area evenly while in a warehouse, a light can light up the face of the shelves and on the path way between two shelves.

(A) Lumen Output of Lamps:

  • We cannot be assumed that 100% of the lamp output will be emitted from the fitting or that the light output will be constant over its operational lifetime.
  • The actual total illumination levels that can be provided by an installed commercial light fitting will depend on the Light Output Ratio:
  • As an example, an industrial or warehouse high bay light fitting with a LOR of 70%, this indicates that 30% of the lamp’s light output is lost due to the design of the fitting.
  • The light output ratio is need to be consider in commercial lighting installation because when a lamp is positioned in a light fitting (such as an industrial 400W metal halide high bay) losses of light occur within the fitting itself.

(B) Beam Angle:

  • For maximum light coverage, we need to select a beam configuration that matches the height of the high bay light.


  • The common beam angles used for high-bay lighting are 60°, 90° and 120°.
  • The narrow beam angle creates a more focused beam enabling a high lux level on the floor or the platform.
  • The wider beam angle ensures large open areas with lower roof heights receive an excellent spread of light.


Beam Angle

Beam Angle

Ceiling Height


Up to 4 meter


4 to 6 meter


6 to 8 meter


8 to12 meter


Beam Angle & Applications

Beam Angle



Spot Lights Stadium Lights


Spot Lights Stadium Lights


Residential and Architectural Lighting


Commercial and Industrial Lighting


Commercial and Industrial Lighting


Low Ceiling Gas Stations and Public Spaces


Industrial Lighting Parking Garages


Beam Angle & Fitting Type

Beam Angle

Type of Fitting

4° To 9°

Spot Light

20° To 35°

Flood Light

36° To 49°

Wide Flood Light

More than 60°

Very Wide Flood Light

(C) Glare:

  • When there is an excessive contrast between the dark areas and bright areas in the direction of viewing, then glare can occur. When there is too much light, it will cause glare.
  • Glare can happen during daytime and nighttime. Examples of where glare can occur includes moving from a shaded location into bright sunlight, and the reflection of light from a surface which is shiny.


(D) Fixtures Shape

  • Circular fixtures creates circular beams; rectangular fixtures creates rectangular beams.
  • Round LED high bays certainly have their universal application, but if we are going to illuminate a long workbenches or a production line, we may get more efficient results from a rectangular linear high bay

(3) Flood Light Fixture.

  • A floodlight is called Flood Light because it illuminate evenly a large area with high intensity of Light.
  • Flood lights are a general method for illuminating areas where a conventional mounting arrangement of Fixtures may or may not be an available and we can also change direction Light or tilt Angle.


  • The flood light have an asymmetric throw of light which can be angled into the space to be illuminate.
  • Flood light illuminate uniformly in all directions and its exposure range can be adjusted.
  • Flood Lights utilizing light bulbs of high power to illuminate a big outdoor location.
  • Flood light is able to equably shine in all directions. Besides, the shine angels could change freely and is able to generate shadow. It is most widely used to illuminate the whole area.
  • When we install floodlights, we should need to care about glare because the brightness of the fitting is high and it angled close to horizontal.
  • Flood Light is different from spot light. Its light beam is highly diffuse without direction. Therefore, its shadow is gentle and transparency.


  • Floodlights are broad beamed, high intensity lights often used to illuminate outdoor playing fields while an outdoor sports event.
  • Flood light is a good choice for lighting and decoration of construction sites, squares, parks, arts venues.
  • Flood light also use as a object Lighting.
  • Factory buildings, stadiums, golf courses, shops, hotels, subway stations, gas stations, buildings.
  • Sculptures and other indoor and outdoor applications.

Difference between High Bay and Low Bay Lighting.

  • Normally, there is a confusion between high bay light and low bay light because both looks like same and having same applications except installation height and intensity of illumination and lumen output.
  • High bay and low bay fixtures both are typically suspension mounted using chains or hooks, but they may also have the option of being surface mounted depending on the fixture.
  • Actually both are not same lights. There are some differences between them.
  • The wattage
  • The wattage or applications of both are different. The wattage and application determines whether to call them high bay or low bay.
  • If the wattage used is above 100 Watts then it is called  high bay. Those using below 100 Watts are called low bay fixtures.
  • The Mounting Height
  • The  low bay light fixtures are used in areas where the bottom of the fixture is up to 20 feet or less above the floor.
  • They are usually spread the light evenly. They also contain optical refractors which cover the lamp thereby reducing glare. Their widespread distribution improves the vertical illumination and also permits spacing as much as 2 or more times the mounting height.
  • High bay lighting fixtures, they are mostly used in areas where the bottom of the fixture is 20 feet or more above the floor.
  • They allowing for a more concentrated beam spreading with a prominent downward component. High wattage is needed so as to illuminate the space properly.


Spacing between lights



15 feet

12 feet to 15 feet
20 feet

15 feet to 18 feet

30 feet

20 feet to 25 feet


Height and lumens


10 to 15 feet

 10,000 to 15,000 lumens

15 to 20 feet

16,000 to 20,000 lumens
25 to 30 feet

 33,000 lumens


Low Bay / High Bay Lighting Fixtures


Installation height Distance Fixture To Fixture
50 Watt 3 Meter

3 To 6 Meter

90 Watt

4 Meter 6 Meter

120 To 150 Watt

5 Meter 6 To 8 Meter
200 Watt 7 Meter

9 To 10 Meter

300 Watt 8 Meter

More than 10 Meter

Difference between Spot Light and Flood Light.

  • A spotlight casts a narrow beam of light, usually no wider than 45°. This beam is more concentrated and easier to point and control.
  • A floodlight can have a beam spread of up to 120°. It can illuminate a larger amount of space with the same wattage and lumen output as a spotlight.
  • Flood Lights is generally utilized for highlighting the architectural appearance of an outstanding or historically Building.
  • By utilizing flood lights, we can boost the in-depth framework of a building.

Determining beam width:

  • The width of a light’s beam in degrees is not always helpful. It should be much easier to know the beam width in feet, from a given distance away.
  • There is a simple formula to know Beam width
  • Beam Width =Angle of Beam x 0.018 x Distance from Light Bulb
  • If we have an 80 degree floodlight, and we want to know how wide the beam will be from 10 feet away.
  • Beam Width = 80 degrees x 0.018 x 10 feet = 14.4 feet wide

LED Vs Metal Halide

LED Watt

Metal Halide Watt
20W to 50W


30W to 75W

40W to 125W


50W to 175W


60W to 225W


80W to 250W


100W to 350W


120W to 400W

150W to 500W



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