Abstract of National Electrical Code for Transformer’s Protection

Abstract of National Electrical Code for Transformer’s Protection:

NEC, Code 450.4: (Calculate over current Protection on the Primary)

  • According to NEC 450.4, “each transformer 600 volts, nominal, or less shall be protected by an individual over current device installed in series with each ungrounded input conductor.
  • Such over current device shall be rated or set at not more than 125% of the rated full-load input current of the auto transformer.
  •  Further, according to NEC Table 450.3(B), if the primary current of the transformer is less than 9 amps, an over current device rated or set at not more than 167% of the primary current shall be permitted. Where the primary current is less than 2 amps, an over current device rated or set at not more than 300% shall be permitted.
  • Example: Decide Size of circuit breaker (over current protection device) is required on the primary side to protect a 75kva 440v-230v 3ø transformer.
  • 75kva x 1,000 = 75,000va
  • 75,000va / (440V x √3) = 98.41 amps.
  • The current (amps) is more than 9 amps so use 125% rating.
  • 98.41 amps x 1.25 = 123amps
  • Use 125amp 3-pole circuit breaker (the next highest fuse/fixed-trip circuit breaker size per NEC 240.6).
  • The over current device on the primary side must be sized based on the transformer KVA rating and not sized based on the secondary load to the transformer.

NEC, Code 450.3B:(Calculate over current Protection on the Secondary)

  • According to NEC Table 450.3(B), where the secondary current of a transformer is 9 amps or more and 125% of this current does not correspond to a standard rating of a fuse or circuit breaker, the next higher standard rating shall be required. Where the secondary current is less than 9 amps, an over current device rated or set at not more than 167% of the secondary current shall be permitted.
  • Example: Decide Size of circuit breaker (over current protection device) is required on the secondary side to protect a 75kva 440v-230v 3ø transformer.
  • We have Calculate the secondary over current protection based on the size of the transformer, not the total connected load.
  • 75kva x 1,000 = 75,000va
  • 75,000va / (230V x √3) = 188.27 amps. (Note: 230V 3ø is calculated)
  • The current (amps) is more than 9 amps so use 125% rating.
  • 188.27 amps x 1.25 = 235.34 amps.
  • Therefore: Use 300amp 3-pole circuit breaker (per NEC 240.6).

NEC, Section 450-3(a):(Transformers over 600 volts, Nominal)

  • For primary and secondary protection with a transformer impedance of 6% or less, the primary fuse must not be larger than 300% of primary Full Load Amps (F.L.A.) and the secondary fuse must not be larger than 250% of secondary F.L.A.

NEC, Section 450-3(b):(Transformers over 600 volts, Nominal)

  •  For primary protection only, the primary fuse must not be larger than 125% of primary F.L.A.
  •  For primary and secondary protection the primary feeder fuse must not be larger than 250% of primary F.L.A. if the secondary fuse is sized at 125% of secondary F.L.A.

NEC, Section 450-3(b):(Potential (Voltage) Transformer)

  • These shall be protected with primary fuses when installed indoors or enclosed

NEC, Section 230-95(Ground-Fault Protection of Equipment).

  • This section show that 277/480 volt “wye” only connected services, 1000 amperes and larger, must have ground fault protection in addition to conventional over current protection.
  • The ground fault relay (or sensor) must be set to pick up ground faults which are 1200 amperes or more and actuate the main switch or circuit breaker to disconnect all ungrounded conductors of the faulted circuit.

NEC, Section 110-9 – Interrupting Capacity.

  • Any device used to protect a low voltage system should be capable of opening all fault currents up to the maximum current available at the terminal of the device.
  • Many over current devices, today, are used in circuits that are above their interrupting rating.
  • By using properly sized Current Limiting Fuses ahead of these devices, the current can usually be limited to a value lower than the interrupting capacity of the over current devices.

NEC, Section 110-10 – Circuit Impedance and Other Characteristics.

  • The over current protective devices, along with the total impedance, the component short-circuit withstand ratings, and other characteristics of the circuit to be protected shall be so selected and coordinated so that the circuit protective devices used to clear a fault will do so without the occurrence of extensive damage to the electrical components of the circuit.
  •  In order to do this we must select the over current protective devices so that they will open fast enough to prevent damage to the electrical components on their load side.

About Jignesh.Parmar
Jignesh Parmar has completed M.Tech (Power System Control), B.E(Electrical) from Gujarat University. He has more than 13 years experience in Power Transmission-Power Distribution-Electrical energy theft detection-Electrical Maintenance-Electrical Projects(Planning-Designing-coordination-Execution). He is Presently associate with one of the leading business group as a Deputy Manager at Ahmedabad,India. He is Freelancer Programmer of Advance Excel and design useful Excel Sheets of Electrical Engineering as per IS,NEC,IEC,IEEE codes. He is technical Author for "Electrical Mirror" and "Electrical India" Magazines. He is Technical Blogger and Familiar with English, Hindi, Gujarati, French languages. He wants to Share his experience & knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.

9 Responses to Abstract of National Electrical Code for Transformer’s Protection

  1. dhar1111 says:

    hi,

    very intresting electrical site,

    I have a doubt .Most of the trasformers secondery is busducted.Is it mandetory according to IE rules. please clarify me.

    Sridhar
    Hyderabad

  2. nagendra sarma says:

    hai sir,
    this is very interesting to students like me,as i learned more information about trsnsformers,i want to know latest trends in electrical mechines.please update portal with this

    P.NAGENDRA SARMA
    ANANTAPUR

  3. Willie says:

    I like to address the following and think is values from diferent voltages were used in the explanation provided…

    ■75,000va / (440V x √3) = 98.41 amps.
    ■The current (amps) is more than 9 amps so use 125% rating.
    ______
    ■123 amps x 1.25 = 112.76 amps

    [[[ shouldn’t this be 98.41Amps x 1.25 = 123AMps ?? ]]]
    The 112.76 Primary Protection comes from using (480v x √3) but since you are using (440v x √3) the Amps is 98A while on a 480v x 3√ is 90.25Amps x 1.25 = 112.8Amps

    Just to clariy , good work in posting these articles…… xfmr do tend to comfuse people alot…..

    best regards
    Willie

  4. Dhanapalmariappan says:

    Sir,
    If the load connected to the transformer is less than its capacity, can we set the protective device based on the connected load?

    • Willie says:

      Good day Dhanapalmariappan
      If referring to the secondary side of the transformer than keep in mind anything originating from a transformer will be considered a service point and will need to meet the number of mains like any other utility service (may consist of 1 thru 6 circuit breakers or fuses)
      Now your question yes you could limit the feeder and size on the secondary side of a transformer, (I would like to ask why would this be the case) also if you are limiting the load to an specific panel/disconnect on the secondary side because you are planning to have more than one secondary overcurrent protective devices should be aware that the combined load of the overcurrent devices shall not exceed the total rating of the transformer secondary capacity…..

      For example a 75KVA transformer will have a total of 5 disconnects or fuses each rated at 60A by adding this rating will exceed the secondary side capacity (208A)
      Primary protection
      75kva / 480v/3ph = 90A
      Secondary Max Load
      75kva / 120/208v/3ph = 2008A
      Having 5 disconnects on the secondary side rated at 60A = this could yield a total possible connected load of 300A this will put the transformer in an overworked mode not allow or permitted, therefore you should be limiting the load of each disconnect so that it will not exceed the secondary capacity (Ampacity)…. This is for 600V or less…. not this refers to actual load and not rating of disconnecting means
      For 600V or more there is a section that allows the combined rating of the secondary side to be no more than 300% of the primary side in example if primary is 90A x 300% = 270A the combined O.C.P. connected to the secondary side of the transformer can be more than 270A – note that this refers to the rating of the disconnecting means rather than the actual connected load as previously described

      Thank you hope this helps
      willie

  5. Willie says:

    Dhanapalmariappan in a nut shell or a simple yes or no

    Yes you can protect to the actual connected load and not the full load of the secondary side…. this will leave space for other future connections although very uncommon practice unless specify conditions exist……… it is permissible….. Also don’t forget the O.C.P should also protect the wire.

    Regards
    Willie

  6. vishal says:

    Dear sir,

    If we are going for a 11 kV ht panel feeding say 3 no’s of transformers each 250 kVA, should we go for vcb’s in each outgoing of the panel or a vcb in incomer of panel and load break switches as outgoings. If we use LBS i doubt if sufficient protection will be provided to transformers. Please comment on best possible switchgear selction to be made for this panel.

  7. sakil says:

    I want Installed 500 Kva Transformer Pl. guide

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