NEC Questions and Answers based on 2020 NEC^® - June 2021

Figure 01

For EC&M Magazine
By Mike Holt, NEC^® Consultant
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 2020 NEC.

Q1. What are the Code rules for ambient temperature correction to conductor ampacity?
A1. Ampacities for ambient temperatures other than those shown in the ampacity tables must be corrected in accordance with Table 310.15(B)(1).

Author’s Comment:
►The conductor ampacities contained in Table 310.16 are based on the condition where the ambient temperature is between 78ºF and 86ºF, and no more than three current-carrying conductors are bundled together; such ampacity must be corrected in accordance with Table 310.15(B)(1) if the ambient temperature is lower than 78ºC or greater than 86ºC. Figure 01

Ambient Temperature Below 30ºC (86ºF) Example
Question: What is the ampacity of a 12 THHN conductor when installed in an ambient temperature of 50ºF?
(a) 20A (b) 25A (c) 31A (d) 35A
Answer: (d) 35A
Solution: Corrected Conductor Ampacity = Table 310.16 Ampacity × Ambient Temperature Correction Factor from Table 310.15(B)(1)
The conductor ampacity for 12 THHN is 30A at 90ºC [Table 310.16].
The correction factor for a 90ºC conductor installed in an ambient temperature of 50ºF is 1.15 [Table 310.15(B)(1)].
Corrected Ampacity = 30A × 115%
Corrected Ampacity = 34.50A, round to 35A
Note: Ampacity increases when the ambient temperature is less than 30ºC (86ºF).

Ambient Temperature Above 86ºF (30ºC) Example
Question: What is the ampacity of a 6 THWN-2 conductor installed in an ambient temperature of 50ºC?
(a) 35A (b) 53A (c) 62A (d) 75A
Answer: (c) 62A
Solution:
The conductor ampacity for 6 THWN-2 is 75A at 90ºC [Table 310.16].
The correction factor for a 90ºC conductor installed in an ambient temperature of 50ºC is 0.82 [Table 310.15(B)(1)].
Corrected Ampacity = 75A × 82%
Corrected Ampacity = 61.50A; round to 62A

(2) Raceways and Cables Exposed to Sunlight on Rooftops. Where raceways or cables are exposed to direct sunlight and located less than 7⁄8 in. above the roof, a temperature adder of 60°F (33°C) must be added to the outdoor ambient temperature to determine the ambient temperature correction in accordance with Table 310.15(B)(1) [310.15(B)(2)].

Author’s Comment:
►The reason for the temperature adder is because the air inside raceways and cables that are in direct sunlight is significantly hotter than the surrounding air.

Example
Question: What is the ampacity of 6 THWN-2 in a raceway located ½ in. above the roof, where the ambient temperature is 90°F?
(a) 40A (b) 41A (c) 42A (d) 44A
Answer: (d) 44A
Solution:
Corrected Temperature = 90°F + 60°F adder [310.15(B)(2)]
Corrected Temperature = 150°F
The temperature correction factor for 150°F = 0.58 [Table 310.15(B)(1)]
6 THWN-2 is rated 75A at 90°C [Table 310.16]
Corrected Ampacity = 75A × 58%
Corrected Ampacity = 43.50A, round to 44A

Ex: Type XHHW-2 insulated conductors are not subject to the rooftop temperature adder.

Informational Note 1: See the ASHRAE Handbook—Fundamentals (www.ashrae.org) as a source for the ambient temperatures in various locations.

Q2. What are the Code rules for conductor bundling adjustment to conductor ampacity?
A2. (1) Four or More Current-Carrying Conductors. Where four or more current-carrying conductors are contained within a raceway that is longer than 24 in., the ampacities contained in Table 310.16 must be adjusted (reduced) in accordance with Table 310.15(C)(1).

Where multiconductor cables are installed without maintaining spacing for a continuous length longer than 24 in., the ampacities contained in Table 310.16 must be adjusted (reduced) in accordance with Table 310.15(C)(1).

Author’s Comment:
►The neutral conductor might be a current-carrying conductor, but only under the conditions specified in 310.15(E). Equipment grounding conductors are never considered current carrying [310.15(F)].

Author’s Comment:
►Conductor ampacity adjustment (reduction) is required when four or more current-carrying conductors are bundled together because heat generated by current flow is not able to dissipate as quickly as when there are fewer current-carrying conductors.

Ampacity Adjustment Example
Question: What is the adjusted ampacity of four current-carrying 12 THWN-2 conductors in a raceway?
(a) 20A (b) 24A (c) 29A (d) 32A
Answer: (b) 24A
Solution:
Adjusted Conductor Ampacity = Table 310.16 Ampacity × Bundled Ampacity Adjustment Factor from Table 310.15(C)(1)
12 THWN-2 is rated 30A at 90ºC [Table 310.16].
The adjustment factor for four current-carrying conductors is 80 percent [Table 310.310.15(C)(1)].
Adjusted Ampacity = 30A × 80%
Adjusted Ampacity = 24A

Ampacity Adjustment Example 2
Question: What is the adjusted ampacity of eight current-carrying 12 THWN-2 conductors in a raceway?
(a) 16A (b) 21A (c) 35A (d) 43A
Answer: (b) 21A
Solution:
Adjusted Conductor Ampacity = Table 310.16 Ampacity × Bundled Ampacity Adjustment Factor from Table 310.15(C)(1)
12 THWN-2 is rated 30A at 90ºC [Table 310.16].
The adjustment factor for eight current-carrying conductors is 70 percent [Table 310.310.15(C)(1)].
Adjusted Ampacity = 30A × 70%
Adjusted Ampacity = 21A

Q3. What are the Code rules for bundling adjustment to conductor ampacity for short raceways?
A3. Conductor ampacity adjustment from Table 310.15(C)(1) does not apply to conductors in raceways having a length not exceeding 24 in [310.15(C)(1)(b)].

Ampacity Adjustment—Raceway Not Exceeding 24 Inches Example
Question: What is the ampacity of five 3/0 THWN-2 conductors in a raceway that does not exceed 24 in. in length?
(a) 150A (b) 195A (c) 205A (d) 225A
Answer: (d) 225A
3/0 THWN-2 is rated 225A at 90ºC [Table 310.16].