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NEC Article 240 Page 3 of 11 1/11/2004
Overcurrent Protection

240.21 Location in Circuit

A new subsection was added to read:

C) Transformer Secondary Conductors
(6) Secondary Conductors Not Over 25 ft.
Secondary conductors can be run up to 25 ft. if installed in accordance with the following:
(1) The secondary conductors have an ampacity that (when multiplied by the ratio of the secondary-to-primary voltage) is at least 1/3 of the rating of the overcurrent device protecting the primary of the transformer.
(2) The secondary conductors terminate in a single circuit breaker or set of fuses that have a rating not greater than the conductor ampacity.
(3) The secondary conductors are protected from physical damage.

Intent: The new subsection provides the language necessary for secondary conductors longer than 10 ft. but not more than 25 ft. This rule was missing in the previous editions of the NEC. The 1999 Code contained the requirement for secondary conductors up to 25 ft. for industrial installations only [240-21(c)(3)].

Author's Comment: The secondary conductor must be sized no smaller than 1/3 the ampacity of the primary protection device based on the secondary/primary ratio. For all practical purposes this can be ignored, because typically the secondary conductors only supply one load. However, if the secondary conductors are to supply multiple loads, then the conductors must be sized to the 1/3 rule as follows:

Step 1. Primary current rating.
I = VA/(E x 1.732)
I = 45,000/(480 x 1.732)
I = 54A

Step 2. Primary overcurrent protection device rating [450.3(B)].
Primary Protection = 54A x 1.25
Primary Protection = 68A
Next size up permitted 70A [240.3(B) and Table 450.3(B) Note 1].

Step 3. Minimum secondary conductor ampacity.
Secondary Ampacity = Primary Protection Rating x (PriV/SecV x 0.333)
Secondary Ampacity = 70A x (480/208 x 0.333)
Secondary Ampacity = 70A x (0.77)
Secondary Ampacity = 54A

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