Table 450.3(A) Maximum Rating or Setting of Overcurrent Protection for Transformers Over 1000 Volts (as a Percentage of Transformer-Rated Current).

An overcurrent device for the 12,470 volt primary side of this transformer is needed. Standard sizes for overcurrent devices over 1000 volts are not mentioned in the NEC.
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An overcurrent device for the 12,470 volt primary side of this transformer is needed. Standard sizes for overcurrent devices over 1000 volts are not mentioned in the NEC.

Code Change Summary: Notes under Table 450.3(A) were expanded.

NEC Table 450.3(A) provides the maximum rating or setting for an overcurrent device for transformers over 1000 volts. As well, the table also applies when the primary side of the transformer is over 1000 volts and the secondary side is 1000 volts or less.

For example, Table 450.3(A) would be used for a stepdown transformer with a 12,470 volt primary and a 480 volt secondary (see image).

The notes under almost any NEC® table are always of great importance.

In the 2011 NEC®, note 1 under Table 450.3(A) allowed the next higher STANDARD overcurrent protective device (OCPD) rating to be used when the transformer calculation resulted in a non-standard size OCPD.

For example: The 500 kVA, 3-phase transformer in the image has a 12,470 volt primary. To size a fuse for the primary side, use the formula in the image as seen below:

The issue with the above math is that the NEC® only specifies standard sizes of fuses and circuit breakers in section 240.6 which is only for overcurrent devices rated 1000 volts and less. Standard sizes of overcurrent devices above 1000 volts are not listed in the NEC®. The math above is for a 12,470 volt installation. The next standard size OCPD above 69.45 amps at 12,470 volts is anybody’s guess.

In the 2014 NEC®, note 1 was changed to allow the next higher standard rating or setting for fuses and circuit breakers 1000 volts and below, or the next higher COMMERCIALLY AVAILABLE rating or setting for fuses and circuit breakers above 1000 volts.

Below is a preview of the NEC®.  See the actual NEC® text at NFPA.ORG for the complete code section. Once there, click on the “free access” tab and select the applicable year of NFPA 70 (National Electrical code).

2011 Code Language:

Table 450.3(A) Notes:

  1. Where 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 next higher standard rating or setting shall be permitted.

2014 Code Language:

Table 450.3(A) Notes:

  1. Where 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:

a. The next higher standard rating or setting for fuses and circuit breakers 1000 volts and below, or

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

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Table 450.3(A) Maximum Rating or Setting of Overcurrent Protection for Transformers Over 1000 Volts (as a Percentage of Transformer-Rated Current).

Below is a Real Question from our Electrical Continuing Education Courses for Electrical License Renewal:

Which of the following is true when sizing a circuit breaker for the primary side of a 7,200 volt transformer with 7% impedance?

A: The circuit breaker can be sized based on 600% of the transformer rated current.
B: The circuit breaker must be sized based on 300% of the transformer rated current.
C: The next higher standard size for the circuit breaker shall be used.
D: The next higher commercially available circuit breaker rating can be used.
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