Ecmweb 19247 Nec Code Quandaries May 2018 Pr 2
Ecmweb 19247 Nec Code Quandaries May 2018 Pr 2
Ecmweb 19247 Nec Code Quandaries May 2018 Pr 2
Ecmweb 19247 Nec Code Quandaries May 2018 Pr 2
Ecmweb 19247 Nec Code Quandaries May 2018 Pr 2

Stumped by the Code? Code Requirement for Sizing the Overcurrent Protection of Conductors

May 15, 2018
Your most pressing National Electrical Code (NEC) questions answered

All questions and answers are based on the 2017 NEC.

Q. What is the Code requirement for sizing the overcurrent protection of conductors?

A. Except as permitted by Sec. 240.4(A) through (G), conductors must be protected against overcurrent in accordance with their ampacity after ampacity correction and adjustment as specified in Sec. 310.15.

(A) Power loss hazard. Conductor overload protection isn’t required, but short-circuit overcurrent protection is required where the interruption of the circuit will create a hazard; such as in a material-handling electromagnet circuit or fire pump circuit.

(B) Overcurrent protection devices rated 800A or less. The next higher standard rating of overcurrent protection device listed in Sec. 240.6 (above the ampacity of the ungrounded conductors being protected) is permitted, provided all the following conditions are met:

(1) The conductors aren’t part of a branch circuit supplying more than one receptacle for cord-and-plug-connected loads.

(2) The ampacity of a conductor, after the application of ambient temperature correction [Sec. 310.15(B)(2)(a)], conductor bundling adjustment [Sec. 310.15(B)(3)(a)], or both, doesn’t correspond with the standard rating of a fuse or circuit breaker in Sec. 240.6(A).

(3) The overcurrent protection device rating doesn’t exceed 800A.

Note: This “next size up” rule doesn’t apply to feeder tap conductors [Sec. 240.21(B)] or transformer secondary conductors [Sec. 240.21(C)].

(C) Overcurrent protection devices rated over 800A. If the circuit’s overcurrent protection device exceeds 800A, the conductor ampacity, after the application of ambient temperature correction [Sec. 310.15(B)(2)(a)], conductor bundling adjustment [Sec. 310.15(B)(3)(a)], or both, must have a rating of not less than the rating of the overcurrent protection device defined in Sec. 240.6.  

(D) Small conductors. Unless specifically permitted in Sec. 240.4(E) or (G), overcurrent protection must not exceed the following after any ambient temperature and number of conductors have been applied:

(1) 18 AWG copper — 7A

(2) 16 AWG copper — 10A

(3) 14 AWG copper — 15A

(4) 12 AWG aluminum/copper-clad aluminum — 15A

(5) 12 AWG copper — 20A

(6) 10 AWG aluminum/copper-clad aluminum — 25A

(7) 10 AWG copper — 30A

Note: The general rule for common circuit conductor and overcurrent protection sizes, such as 14 AWG with 15A protection, 12 AWG with 20A protection, and 10 AWG with 30A protection [240.4(D)(5), (6), and (7)] are often used but there are many conditions where the overcurrent protection is safely permitted to exceed the ampacity of the conductors as specified in Sec. 240.4(E) or (G).

(E) Tap conductors. Tap conductors must be protected against overcurrent as follows:

(1) Household ranges, cooking appliances and other loads [Sec. 210.19(A)(3) and (4)]

(2) Fixture wire [Sec. 240.5(B)(2)]

(3) Location in circuit [Sec. 240.21]

(4) Reduction in ampacity size of busway [Sec. 368.17(B)]

(5) Feeder or branch circuits (busway taps) [Sec. 368.17(C)]

(6) Single motor taps [Sec. 430.53(D)]

(F) Transformer secondary conductors. The primary overcurrent protection device sized in accordance with Sec. 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.

Here’s an example calculation that shows you how to determine the proper protection.

What’s the minimum secondary conductor size required for a 2-wire, 480V to 120V transformer rated 1.5kVA with 60°C terminals (Fig. 1) ?

Solution: Primary current = VA ÷ E 

VA = 1.5kVA x 1,000 = 1,500VA

E = 480V

Primary current = 1,500VA ÷ 480V = 3.13A

Primary overcurrent protection [Sec. 450.3(B)] = 3.13A × 1.67 = 5.22A, or 5A fuse

Secondary conductor must have an ampacity no less than five times the primary protection ampere rating.

Secondary conductor ampacity = Primary protection ampacity × (primary voltage ÷ secondary voltage)

Secondary conductor ampacity = 5A × 480V ÷ 120V = 5A × 4 times = 20A

Answer: 12 AWG rated 20A at 60°C [per Table 310.15(B)(16)]

(G) Overcurrent protection for specific applications. Overcurrent protection for specific equipment and conductors must comply with the requirements referenced in Table 240.4(G).

For example, air-conditioning and refrigeration equipment, and their circuit conductors, must be protected against overcurrent in accordance with Sec. 440.22. Note: Typically, the branch-circuit ampacity and overcurrent protection size are marked on the equipment nameplate [Sec. 440.4(A)]. Air-conditioning and refrigeration nameplate values are calculated by the manufacturer as follows:

Branch-circuit conductor size [440.32]
18A × 1.25 = 22.50A, 10 AWG rated 30A at 60°C [Sec. 110.14(C)(1)(a), Table 310.15(B)(16)]

Branch-circuit overcurrent protection size [Sec. 440.22(A)]
18A × 2.25 = 40.50A, 40A maximum overcurrent protection size [Sec. 240.6(A)]

If the nameplate calls for fuses, then fuses must be used to comply with the manufacturer’s instructions [Sec. 110.3(B)].

What about overcurrent protection for motors? Motor circuit conductors must be protected against short circuits and ground faults in accordance with Sec. 430.52 and Sec. 430.62 [430.51]. Here’s an example calculation that shows you how to determine the proper protection.

What size branch-circuit conductor and overcurrent protection device (circuit breaker) is required for a 7½-hp, 230V, 3-phase motor with 60°C terminals? (Fig. 2).

Solution: Table 240.4(G) directs us to several parts of Art. 430.

Step 1: Determine the branch-circuit conductor size [Table 310.15(B)(16), 430.22, and Table 430.250]:

FLC = 22A [Table 430.250]

22A × 1.25 = 28A [240.6(A)]

Step 2: Determine the branch-circuit overcurrent protection size [240.6(A), 430.52(C)(1) Exception No. 1, and Table 430.250].

Inverse Time Breaker:

22A × 2.50 = 55A

Answer: 10 AWG rated 30A at 60°C branch-circuit conductor [110.14(C)(1)(a), Table 310.15(B)(16)] with a 60A (next size up) overcurrent protection device [240.6(A)].

Motor control circuit conductors must be sized and protected in accordance with Sec. 430.72.

Remote-control, signaling, and power-limited circuits conductors must be protected against overcurrent in accordance with Sec. 725.43.          

Holt is the owner of Mike Holt Enterprises, Inc. in Leesburg, Fla. He can be reached at www.mikeholt.com
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.

Voice your opinion!

To join the conversation, and become an exclusive member of EC&M, create an account today!

Sponsored Recommendations

Electrical Conduit Comparison Chart

CHAMPION FIBERGLASS electrical conduit is a lightweight, durable option that provides lasting savings when compared to other materials. Compare electrical conduit types including...

Fiberglass Electrical Conduit Chemical Resistance Chart

This information is provided solely as a guide since it is impossible to anticipate all individual site conditions. For specific applications which are not covered in this guide...

Considerations for Direct Burial Conduit

Installation type plays a key role in the type of conduit selected for electrical systems in industrial construction projects. Above ground, below ground, direct buried, encased...

How to Calculate Labor Costs

Most important to accurately estimating labor costs is knowing the approximate hours required for project completion. Learn how to calculate electrical labor cost.