By Mike Holt for EC&M Magazine

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

Note: These questions are based on the 2014 NEC.

Q1. What is the Code requirement for sizing branch-circuit conductors?
A1. Branch-circuit conductors must have an ampacity of not less than the maximum load to be served. Conductors must be the larger of 210.19(A)(1)(a) or (b):

(a) Conductors must be sized no less than 125 percent of the continuous loads, plus 100 percent of the noncontinuous loads, based on the terminal temperature rating ampacities as listed in Table 310.15(B)(16).

(b) Conductors must be sized to the maximum load to be served after the application of any adjustment or correction factors.

Ex 1: If the assembly and the overcurrent device are both listed for operation at 100 percent of its rating, the conductors can be sized at 100 percent of the continuous load.

Author’s Comment:
Equipment suitable for 100 percent continuous loading is rarely available in ratings under 400A.
See the definition of “Continuous Load” in Article 100.
See 210.20 for the sizing requirements for the branch-circuit overcurrent device for continuous and noncontinuous loads.

Question: What size branch-circuit conductors are required for a 4-wire circuit of a 45A nonlinear continuous load, if the equipment terminals are rated 75ºC?
(a) 10 AWG (b) 8 AWG (c) 6 AWG (d) 4 AWG
Answer: (c) 6 AWG

Since the load is 45A continuous, the conductors must be sized to have an ampacity of not less than 56A (45A x 1.25). According to the 75ºC column of Table 310.15(B)(16), a 6 AWG conductor is suitable, because it has an ampere rating of 65A at 75ºC. This satisfies the portion of the calculation discussed in 210.19(A)(1)(a).

For 210.19(A)(1)(b), we need to address ambient temperature and conductor bundling.

Because the neutral in this example is considered a current-
carrying conductor [310.15(B)(5)(c)], there are four current-carrying conductors. Table 310.15(B)(3)(a) requires an adjustment factor of 80% for this example.

6 AWG rated 75A at 90ºC x 0.80 = 60A after adjustment factors which is adequate for the 45A load. If we’d selected an 8 AWG rated 55A at 90ºC, based on the 45A load, it would be too small for the load after applying the adjustment factor (55A x 0.80 = 44A).

Informational Note 4: To provide reasonable efficiency of operation of electrical equipment, branch-circuit conductors should be sized to prevent a voltage drop not to exceed 3 percent. In addition, the maximum total voltage drop on both feeders and branch circuits shouldn’t exceed 5 percent.

Author’s Comment:
Many believe the NEC requires conductor voltage drop, as per Informational Note 4 to be applied when sizing conductors. Although this is often a good practice, it’s not a Code requirement because Informational Notes are only advisory statements [90.5(C)].

The NEC doesn’t consider voltage drop to be a safety issue, except for sensitive electronic equipment [647.4(D)] and fire pumps [695.7].

Q2. What does the Code require for branch circuits that supply more than one receptacle?
A2. Branch circuits that supply more than one receptacle must have an ampacity not less than the rating of the circuit overcurrent device [210.3 and 210.19(A)(2)].

Author’s Comment:
See Article 100 definition of receptacle. A branch circuit with one duplex receptacle has two receptacles and must comply with 210.19(A)(2).

Q3. What is the minimum ampacity required for household range and cooking appliance circuits?
A3. Branch-circuit conductors that supply household ranges, wall-mounted ovens or counter-mounted cooking units must have an ampacity not less than the rating of the branch circuit, and not less than the maximum load to be served. For ranges of 8¾ kW or more rating, the minimum branch-circuit ampere rating is 40A [210.19(A)(3)].

Ex 1: Conductors tapped from a 50A branch circuit for electric ranges, wall-mounted electric ovens and counter-mounted electric cooking units must have an ampacity not less than 20A, and must have sufficient ampacity for the load to be served. The taps must not be longer than necessary for servicing the appliances.

Q4. What is the Code minimum size requirement for branch circuit overcurrent protection?
A4. Branch-circuit overcurrent devices must have a rating of not less than 125 percent of the continuous loads, plus 100 percent of the noncontinuous loads [210.20(A)].

Author’s Comment:
See 210.19(A)(1) for branch-circuit conductor sizing requirements.

Ex: If the assembly and the overcurrent devices are both listed for operation at 100 percent of their rating, the branch-circuit overcurrent device can be sized at 100 percent of the continuous load.

Author’s Comment:
Equipment suitable for 100 percent continuous loading is rarely available in ratings under 400A.

(B) Conductor Protection. Branch-circuit conductors must be protected against overcurrent in accordance with 240.4.

(C) Equipment Protection. Branch-circuit equipment must be protected in accordance with 240.3.