NEC Questions and Answers based on 2017 NEC - October 2018

Figure 01

Here's the follow-up to yesterday's newsletter.
This includes the answers to the questions sent, so you can see how you did.

Note: The answers to these questions are based on the 2017 NEC.

Q1. According to the NEC, conductors must be protected at what amperage?
A1. Except as permitted by 240.4(A) through (G), conductors must be protected against overcurrent in accordance with their ampacity after ampacity correction and adjustment as specified in 310.15.

Table 310.15(B)(16) contains the most commonly used conductor ampacities based on up to three current carrying conductors in a raceway or directly buried in an ambient temperature of 86F. If any other conditions apply, such as more than three current carrying conductors, or a different ambient temperature, the conductor ampacities found on this table must be corrected and/or adjusted accordingly. Author's Comment:

Section 240.4 used the term Overcurrent. According to Article 100 definition, Overcurrent is any current in excess of the rated current of equipment or conductor ampacity. Overcurrent can be from an overload condition, a short-circuit, or a ground fault. See the Article 100 definition of Overcurrent and its Informational Note.

One example of where this definition is important is in 240.4(A). Where a power loss hazard exist, 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 [240.4(A)].

Keep in mind that different applications (such as conductors for motors) have their own set of requirements [240.4(G)]. Many different Code sections must be understood to properly size conductor overcurrent protection.

Q2. Does the NEC allow overcurrent protection for conductors to be rounded up to the next larger size?
A2. Absolutely, the next higher standard rating of overcurrent protection device listed in 240.6 (above the ampacity of the ungrounded conductors being protected) is permitted, provided all of the following conditions are met [240.4(B)]:

(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 [310.15(B)(2)(a)], conductor bundling adjustment [310.15(B)(3)(a)], or both, doesn't correspond with the standard rating of a fuse or circuit breaker in 240.6(A).
(3) The overcurrent protection device rating doesn't exceed 800A.Figure 01

Q3. Does the NEC rule for rounding up of overcurrent protection for conductors change above any specific amperage?
A3. Yes, is does. If the circuit's overcurrent protection device exceeds 800A, the conductor ampacity, after the application of ambient temperature correction [310.15(B)(2)(a)], conductor bundling adjustment [310.15(B)(3)(a)], or both, must have a rating of not less than the rating of the overcurrent protection device defined in 240.6 [240.4(C)] .

Author's Comment:
Conductors to overcurrent devices rated over 800A are typically installed in parallel. In these cases, the ampacity rating applies to the set of parallel conductors instead of the individual conductors.

Example: A 1,200A overcurrent device feeds conductors parallel in three raceways? What is the minimum ampacity needed for the conductors?
Answer: 1,200A. The parallel conductors must have an ampacity of 1,200A. 1,200A/ 3 raceways = 400A minimum ampacity needed in each parallel set. 600 kcmil rated 420A at 75°C is the minimum size ungrounded conductors.

Q4. What does the Code require for overcurrent protection of flexible cords, flexible cables, and fixture wires?
A4. Flexible cord and flexible cable must be protected by an overcurrent protection device in accordance with its ampacity as specified in Table 400.5(A)(1) or Table 400.5(A)(2). Fixture wires must be protected against overcurrent in accordance with their ampacity as specified in Table 402.5. Supplementary overcurrent protection, as discussed in 240.10, can provide this protection [240.5(A)].

Branch-circuit overcurrent protection must be provided for cords for listed appliances or luminaires. If flexible cord is used with a specific listed appliance or luminaire, the conductors are considered protected against overcurrent when used within the appliance or luminaire listing requirements 240.5(B)(1)].

Author's Comment:
• The NEC only applies to premises wiring, not to the supply cords of listed appliances and luminaires.

The branch circuit overcurrent protection device can be used to protect fixture wires when they are tapped to the following circuits [240.5(B)(2)]:
(1) 20A“18 AWG, up to 50 ft of run length
(2) 20A“16 AWG, up to 100 ft of run length
(3) 20A“14 AWG and larger
(4) 30A-14 AWG and larger
(5) 40A-12 AWG and larger
(6) 50A-12 AWG and larger

Flexible cord used in listed extension cord sets is considered protected against overcurrent when used within the extension cord's listing requirements 240.5(B)(3).

Q5. Where in the Code are standard ampere ratings for overcurrent protection devices?
A5 The standard ratings in amperes for fuses and inverse time breakers are shown in Table 240.6(A). The use of fuses and inverse time circuit breakers with nonstandard ampere ratings are permitted [240.6(A)].

Additional standard ampere ratings for fuses include 1, 3, 6, 10, and 601.

Author's Comment:
• Fuses rated less than 15A are sometimes required for the overcurrent protection of fractional horsepower motor circuits [430.52], motor control circuits [430.72], small transformers [450.3(B)], and remote control circuit conductors [725.43].

The ampere rating of an adjustable circuit breaker is equal to its maximum long time pickup current setting [240.6(B)].

The ampere rating of adjustable trip circuit breakers that have restricted access to the adjusting means is equal to their adjusted long time pickup current settings [240.6(C)].