NEC Tables · Transformers

Table 450.3(B) — transformer protection (≤1000 V)

Primary-only or primary-plus-secondary — the two protection schemes and the percentages each allows.

Free · NEC 2023 · Updated
600%1
Any location · Circuit Breaker4
maximum OCPD per Table 450.3(B)
Exam trapCurrents of 9 A and 2 A flip the percentages entirely — small transformers get different math. Check the current before quoting 125%.

Protection method decides the row: primary-only protection allows 125% of primary current, while the primary-and-secondary scheme lets the primary device grow to 250% when the secondary is protected at 125%. The note allowing next-size-up applies where marked.

Protection percentages

Location LimitationsTransformer Rated ImpedancePrimary Protection over 1000 VoltsSecondary Protection2
Location LimitationsTransformer Rated ImpedanceCircuit Breaker4Fuse RatingCircuit Breaker4Fuse RatingCircuit Breaker or Fuse Rating
Any locationNot more than 6%600%1300%1300%1250%1125%1
Any locationMore than 6%and not more than 10%400%1300%1250%1225%1125%1
Supervised locations only3Any300%1250%1Not requiredNot requiredNot required
Supervised locations only3Not more than 6%600%300%300%5250%5250%5
Supervised locations only3More than 6%and not more than 10%400%300%250%5225%5250%5
Notes that change the answer — 450.3(B)

1Where the required fuse rating or circuit breaker setting does not correspond to a standard rating or setting, a higher rating or setting that does not exceed the following shall be permitted:

(1) The next higher standard rating or setting for fuses and circuit breakers 1000 volts and below, or

(2) The next higher commercially available rating or setting for fuses and circuit breakers above 1000 volts.

2Where secondary overcurrent protection is required, the secondary overcurrent device shall be permitted to consist of not more than six circuit breakers or six sets of fuses grouped in one location. Where multiple overcurrent devices are utilized, the total of all the device ratings shall not exceed the allowed value of a single overcurrent device. If both circuit breakers and fuses are used as the overcurrent device, the total of the device ratings shall not exceed that allowed for fuses.

3A supervised location is a location where conditions of maintenance and supervision ensure that only qualified persons monitor and service the transformer installation.

4Electronically actuated fuses that may be set to open at a specific current shall be set in accordance with settings for circuit breakers.

5A transformer equipped with a coordinated thermal overload protection by the manufacturer shall be permitted to have separate secondary protection omitted.

Continued

Protection MethodPrimary ProtectionSecondary Protection2
Primary only protection125%1167%300%Not requiredNot required
Primary and secondary protection250%3250%3250%3125%1167%
Notes that change the answer — 450.3(B)

1Where 125 percent of this current does not correspond to a standard rating of a fuse or nonadjustable circuit breaker, a higher rating that does not exceed the next higher standard rating shall be permitted.

2Where secondary overcurrent protection is required, the secondary overcurrent device shall be permitted to consist of not more than six circuit breakers or six sets of fuses grouped in one location. Where multiple overcurrent devices are utilized, the total of all the device ratings shall not exceed the allowed value of a single overcurrent device.

3A transformer equipped with coordinated thermal overload protection by the manufacturer and arranged to interrupt the primary current shall be permitted to have primary overcurrent protection rated or set at a current value that is not more than six times the rated current of the transformer for transformers having not more than 6 percent impedance and not more than four times the rated current of the transformer for transformers having more than 6 percent but not more than 10 percent impedance.

Values derived from the 2023 National Electrical Code for exam study. Verify installations against the adopted code and local amendments.

Race your codebook — 3 lookups on this table

Q1

A single-phase transformer with only a 2-wire (single-voltage) secondary supplies a Class 1 circuit. The designer wants to protect the secondary conductors using overcurrent protection installed only on the primary side of the transformer. Which section allows this and specifies the maximum protection value based on the secondary-to-primary voltage ratio?

  1. A 724.45(B)
  2. B 724.45(E)
  3. C 724.45(D)
  4. D 724.45(C)
Show answer & explanation

Correct: C — 724.45(D)

724.45(D) specifically permits primary-side overcurrent protection for a 2-wire transformer secondary supplying Class 1 conductors, sized by the secondary-to-primary voltage ratio and per 450.3. 724.45(E) covers the analogous case for electronic power sources, not transformers, so it doesn't apply here.

Q2

A contractor wants to field-connect a two-winding transformer as an autotransformer to buck the voltage up on a job. Before doing so, which section requires that the transformer be identified for use at the elevated voltage?

  1. A 450.5(A)(4)
  2. B 450.3(B)
  3. C 450.4(A)
  4. D 450.4(B)
Show answer & explanation

Correct: D — 450.4(B)

450.4(B) is the specific rule requiring that a transformer field-connected as an autotransformer be identified for use at the elevated voltage. 450.4(A) instead covers overcurrent protection sizing for autotransformers 1000V or less, and 450.3(B) covers overcurrent protection for transformers 1000V or less generally — neither addresses the identification requirement for field-connected autotransformers.

Q3

A single-phase autotransformer rated 1000 volts or less has a rated full-load input current of 40 amperes. You need to size the individual overcurrent device installed in series with each ungrounded input conductor. Which section specifies the maximum percentage of rated input current this device may be set at?

  1. A 450.4(A)
  2. B 450.5(A)(2)
  3. C 450.6(B)
  4. D 450.3(B)
Show answer & explanation

Correct: A — 450.4(A)

450.4(A) governs overcurrent protection for autotransformers 1000V or less, setting the 125 percent (or 167 percent under 9A) rule for the input conductor device. 450.3(B) instead covers overcurrent protection for general (two-winding) transformers 1000V or less, not autotransformers, and 450.5(A)(2)/450.6(B) address grounding autotransformers and secondary tie overcurrent protection, not the basic input-conductor device sizing.

Keep going

Transformer Sizing Calculator Free calculator Article 450 practice questions 22 questions
Loomi · NEC exam prep

Every table. Timed lookups. Spaced repetition.

Loomi drills all 252 NEC tables the way the exam tests them — find the value fast and apply the note behind it. Try everything free for 3 days.

Get launch access