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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 following: Figure 2402
(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.
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)].
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
Step 3. Minimum secondary conductor ampacity.
Secondary Ampacity = Primary Protection Rating x (PriV/SecV
Secondary Ampacity = 70A x (480/208 x 0.333)
Secondary Ampacity = 70A x (0.77)
Secondary Ampacity = 54A
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