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
Copyright © 2003 Mike Holt Enterprises,Inc.
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