|NEC Article 240 Page 3 of 11
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
(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
(2) The secondary conductors terminate in a single circuit
breaker or set of fuses that have a rating not greater than the conductor
(3) The secondary conductors are protected from physical
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
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.
= VA/(E x 1.732)
I = 45,000/(480 x 1.732)
I = 54A
Primary overcurrent protection device rating [450.3(B)].
= 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
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 =
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