Stumped by the Code? Rules for sizing transformer secondary conductors?
Q. What are the NEC rules for sizing transformer secondary conductors?
A. A set of conductors supplying single or separate loads is permitted to be connected to a transformer secondary without overcurrent protection in accordance with 240.21(C)(1) through (6). Note: The permission of the “next size up” protection rule when the conductor ampacity doesn’t correspond with the standard size overcurrent protection device of 240.4(B) doesn’t apply to transformer secondary conductors.
(1) Overcurrent Protection by Primary Overcurrent Protection Device. The primary overcurrent protection device sized in accordance with 450.3(B) is considered suitable to protect the secondary conductors of a 2-wire (single-voltage) system, provided the primary overcurrent protection device doesn’t exceed the value determined by multiplying the secondary conductor ampacity by the secondary-to-primary transformer voltage ratio.
Example: What’s the minimum size secondary conductor required for a 2-wire, 480V to 120V transformer rated 1.50kVA with 60°C terminals (see Figure)?
Solution:
Primary Current = VA ÷ E
VA = 1,500VA
E = 480V
Primary Current = 1,500VA ÷ 480V = 3.13A
Primary Overcurrent Protection [450.3(B)] = 3.13A × 1.67 = 5.22A, or 5A Fuse
Secondary Current = 1,500VA ÷ 120V = 12.50A
Secondary Conductor = 12 AWG, rated 20A at 60°C [Table 310.15(B)(16)]
The 5A primary overcurrent protection device can be used to protect 12 AWG secondary conductors because it doesn’t exceed the value determined by multiplying the secondary conductor ampacity by the secondary-to-primary transformer voltage ratio.
Overcurrent Protection Device = 20A × (120V ÷ 480V) = 5A fuse
Answer: 12 AWG conductor.
Secondary conductors can be run up to 10 ft without overcurrent protection if installed as follows:
(1) The secondary conductor must have an ampacity not less than:
a. The calculated load in accordance with Art. 220, and
b. The rating of the overcurrent protection device termination or equipment containing overcurrent protection devices supplied by the tap conductors.
Secondary conductors located outside a building or structure can be of unlimited length, without overcurrent protection at the point they receive their supply, if they’re installed as follows:
(1) The conductors are protected from physical damage within a raceway or manner approved by the authority having jurisdiction.
(2) The conductors terminate at a single overcurrent protection device that limits the load to the ampacity of the outside secondary conductors.
(3) The terminating overcurrent protection device for the outside secondary conductors is part of the building feeder disconnect.
(4) The building feeder disconnecting means is readily accessible, either outside the building, or nearest the point of entrance of the outside secondary conductors.
Transformer secondary conductors must be installed in accordance with Sec. 240.21(B)(3).
Secondary conductors can be run up to 25 ft without overcurrent protection if they comply with all of the following:
(1) The secondary conductors must have an ampacity that’s not less than the value of the primary-to-secondary voltage ratio multiplied by one-third of the rating of the overcurrent protection device protecting the primary of the transformer.
(2) Secondary conductors terminate in a single overcurrent protection device that limits the load to the secondary conductor ampacity.
Service conductors must be protected against overload in accordance with Sec. 230.90 and Sec. 230.91.
Overcurrent protection must be installed as close as practicable to the storage battery terminals.
These materials are provided to us by Mike Holt Enterprises in Leesburg, Fla. To view Code training materials offered by this company, visit www.mikeholt.com/code
