In case you missed it, you can click here to view Conductor Sizing, Part 1.

By Mike Holt, NEC Consultant

I In Part 1 we learned the correct sizing of conductors is more complicated than it appears. One reason is the way temperature affects the resistance of a conductor. A higher temperature means less current-carrying capacity for a given size conductor. With Part 1 complete, you’re now ready to select the minimum conductor size from the table. There is a catch: you can’t use a column with a temperature that exceeds the temperature rating of any device in a given run in accordance with 110.14(C)(1).

Branch-circuits

The ampacity of branch-circuit conductors must be at least the maximum load to be served [210.19(A)(1)]. So size the conductors to carry the larger of:

1. 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); or

2. The load after the application of correction or adjustment factors.

Question: What’s the minimum size branch circuit conductor (THHN) for a 45A continuous nonlinear load that requires three ungrounded conductors and a neutral (four current-carrying conductors)?

(a) 8 AWG      (b) 6 AWG      (c) 4 AWG      (d) none of these

Answer: (c) 4 AWG.

Here are the key points for solving this problem:

• Step 1a. Since the load is 45A continuous, the conductor must have an ampacity of at least 56A (45A x 1.25) [210.19(A)(1)(a)] before the application of any ampacity adjustment or correction.
• Step 1a. 4 AWG is rated 70A at 60ºC on Table 310.15(B)(16), so it is suitable to meet the 56A requirement of 210.19(A)(1)(a) and the terminal requirements of 110.14(C)(1); before applying conductor ampacity adjustment and/or correction.
• Step 2a. Because the neutral is a current-carrying conductor per 310.15(B)(5)(c), you must apply Table 310.15(B)(3) adjustment factor of 80% [210.19(A)(1)(b)].
• Step 2b. 4 THHN has an ampacity of 95A at 90ºC [Table 310.15(B)(16)].
• Step 2c. Adjustment factor for four current-carrying conductors is 0.80 of the ampacity of the conductor in accordance with the 90ºC insulation rating of the conductor, not the 60ºC equipment terminal rating [310.15(B) and Table 310.15(B)(3)(a)].
• Step 2d. Adjusted Ampacity = 95A x 0.80 = 76A. The conductor after adjustment is rated 76A which is adequate for the 45A load.

Feeders

The ampacity of feeder conductors must be sized to carry at least the maximum load to be served [215.2(A)(1)]. So size the conductors to carry the larger of 215.2(A)(1)(a) or (b):

(c) 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); or

(d) The load after the application of correction or adjustment factors.

Taps

Feeders bring another twist to conductor sizing: feeder tap rules. A tap is a conductor (other than a service conductor) with overcurrent protection rated higher than normally allowed in 240.2.

You can run feeder taps without overcurrent protection (at the tap location), up to 10 ft. But only if the ampacity of the tap conductor is at least the:

1. Calculated load per Article 220, and

2. Rating of the device or OCPD supplied by the tap conductors.

Also, the tap conductors must:

1. Not extend beyond the equipment they supply.

2. Be in a raceway, if they leave the enclosure,

3. Have an ampacity at least one-tenth of the rating of the OCPD that protects the feeder.

Question: Using the 10-foot tap rule, what’s the minimum size conductor you need to supply a 200A OCPD, if the tap is from feeder conductors protected by a 400A OCPD?

(a) 3 AWG     (b) 1/0 AWG      (c) 3/0 AWG      (d) 250 kcmil

Answer: (c) 3/0 AWG

The ampacity of the tap conductors must be at least the rating of:

• The 200A OCPD they terminate into, and

• One-tenth the rating of the OCPD protecting the feeder conductors.

A 3/0 AWG conductor:

• Is rated 200A at 75ºC, and

• Has an ampacity of at least 40A (one-tenth the rating of the 400A feeder OCPD (400/10) [Table 310.15(B)(16)].

What if you need a longer tap? If you don’t need to go more than 25 ft, you’re in luck [240.21(B)(2)]. However, you must satisfy three requirements to get that additional 15 ft.:

1. The ampacity of the tap conductors must be at least one-third the rating of the OCPD that protects the feeder.

2. The tap conductors must terminate in a single circuit breaker (or set of fuses) rated no more than the tap conductor ampacity per 310.15 [Table 310.15(B)(16)].

3. The tap conductors must be protected from physical damage by being enclosed in a manner approved by the authority having jurisdiction (e.g., within a raceway).

Question: Using the 25-foot tap rule, what’s the minimum size conductor to supply a 200A OCPD, if the tap is from feeder conductors protected by a 400A circuit?

(a) 3 AWG      (b) 1/0 AWG      (c) 3/0 AWG      (d) 250 kcmil

Answer: (c) 3/0 AWG

The tap conductors must have an ampacity of at least the 200A rating of the OCPD they terminate into and at least one-third the rating of the OCPD protecting the feeder conductors. A 3/0 AWG conductor is rated 200A at 75ºC and has an ampacity of at least 133A (one-third the rating of the 400A feeder OCPD (400/3) [Table 310.15(B)(16)].

The great outdoors

And just when you thought the feeder conductor fun was over, we have one more twist to talk about. The good news is there’s no limit on how long an outside feeder-tap can be without overcurrent protection at the point it receives its supply [240.21(B)(5)]. But only if:

1. The tap conductors are suitably protected from physical damage in a raceway or manner approved by the authority having jurisdiction.

2. The tap conductors terminate at a single circuit breaker (or single set of fuses) that limits the load to the ampacity of the conductors.

3. The OCPD for the tap conductors is an integral part of the disconnecting means, or is located immediately adjacent to it.

4. The disconnecting means is at a readily accessible location (either outside the building), or nearest the point of entry of the conductors.

Branch vs. Feeder

It can be hard to remember whether you size the OCPD first for branch-circuits or for feeders. The rules differ because each type of circuit serves a different purpose, but that fact alone may not help you keep things straight.

Perhaps another way of looking at it will help. The conductors of a branch-circuit must be big enough to supply the branch-circuit loads. But the conductors of a feeder circuit must be big enough to serve the feeder OCPD.

In case you missed it, you can click here to view Conductor Sizing, Part 1.