Ecmweb 8747 Nec Code Quandaries 2017 Pr
Ecmweb 8747 Nec Code Quandaries 2017 Pr
Ecmweb 8747 Nec Code Quandaries 2017 Pr
Ecmweb 8747 Nec Code Quandaries 2017 Pr
Ecmweb 8747 Nec Code Quandaries 2017 Pr

Stumped by the Code? Rules for sizing transformer secondary conductors?

Jan. 19, 2017
Your most pressing National Electrical Code (NEC) questions answered

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.

The primary overcurrent device can protect the secondary conductors of a 2-wire system if the primary device doesn’t exceed the value determined by multiplying the secondary conductor ampacity by the secondary-to-primary 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

About the Author

Mike Holt

Mike Holt is the owner of Mike Holt Enterprises (www.MikeHolt.com), one of the largest electrical publishers in the United States. He earned a master's degree in the Business Administration Program (MBA) from the University of Miami. He earned his reputation as a National Electrical Code (NEC) expert by working his way up through the electrical trade. Formally a construction editor for two different trade publications, Mike started his career as an apprentice electrician and eventually became a master electrician, an electrical inspector, a contractor, and an educator. Mike has taught more than 1,000 classes on 30 different electrical-related subjects — ranging from alarm installations to exam preparation and voltage drop calculations. He continues to produce seminars, videos, books, and online training for the trade as well as contribute monthly Code content to EC&M magazine.

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