Sizing Conductors Based on Temperature Rating
There are many different rules for sizing and protecting conductors and equipment. It's not enough to simply use a No. 12 wire and 20A breaker or fuse to cover yourself in all applications. The general rule is that conductors must be protected in accordance with their ampacity as determined by Sec. 310-15, except as permitted or required in Secs. 240-3(b) through (g).
An overcurrent protection device (OCPD) opens a circuit to prevent excessive heat from damaging conductors and related equipment. Overcurrent protection devices have two current ratings: overcurrent and amperes interrupting current (AIC).
Overcurrent rating. If the current flowing through the protection device exceeds the device setting for a significant period, the protection device will open. The overcurrent rating of an OCPD is listed in amperes, such as 15A, 20A, or 30A. The NEC lists the standard sizes of OCPDs in Sec. 240-6(a).
Interrupting rating (short-circuit) [Sec. 110-9]. OCPDs, such as circuit breakers and fuses must have an interrupting rating sufficient for the maximum possible fault-current available on the line side terminals of the equipment [Sec. 240-1 FPN].
If the OCPD is not rated to interrupt at the available fault-current, it could explode while attempting to clear the fault, and/or the downstream equipment could suffer serious damage causing possible hazards to persons and property.
The NEC states unless marked otherwise, the amperes interruption current (AIC) rating for circuit breakers is 5000A [Sec. 240-83(c)] and 10,000A for fuses [Sec. 240-60(c)]. They are typically marked with an AIC rating of 10,000A.
Continuous load factors. Size OCPDs no less than 125% of the continuous load, and 100% of the noncontinuous load [Sec. 210-20(a), and Sec. 215-3]. Conductors must have an ampacity (before the application of any adjustment or correction factors) no less than 125% of the continuous load, plus 100% of the noncontinuous load [Sec. 210-19(a) and Sec. 215-2(a)] in accordance with the terminal temperature rating limitations of Sec. 110-14(c).
Example: Branch circuit continuous load. Q. What size OCPD and conductor does the NEC require for a 100A continuous load?
(a) 150A (c) 125A (b) 100A (d) any of these
A. The answer is (c), 125A.
Step 1. The branch circuit OCPD must have an ampacity of not less than 125% of the continuous load [Sec. 210-20(a)].
100A x 1.25 = 125A protection.
Step 2. Branch circuit conductors must have an ampacity no less than 125% of continuous load before applying an ampacity adjustment factor [Sec. 210-19(a)].
100A x 1.25 = 125A.
Therefore, use a No. 1 THHN rated 130A at 75øC [Sec. 110-14(c)(2)(b)].
Next higher overcurrent device rating [Sec. 240-3(b)]. Where the ampacity of a conductor does not correspond with the standard ampere rating of a fuse or circuit breaker as listed in Sec. 240-6(a), you can use the next size up device - only permitted if the conductors do not supply multi-outlet receptacle branch circuits for portable cord- and plug-connected loads, and the next size up does not exceed 800A.
Example: Feeder continuous load. Q. What size OCPD/feeder conductor does the NEC require for a 104A continuous load?
(a) 150A - No. 1/0 (b) 150A - No. 1 (c) 125A - No. 2 (d) 125A - No. 1
A. The answer is (b), 150A OCPD with No. 1 THHN.
Step 1. The OCPD must have an ampacity no less than 125% of the continuous load [Sec. 215-3].
104A x 1.25 = 130A = 150A protection [Sec. 240-6(a)].
Step 2. The feeder conductor must have an ampacity of not less than 125% of the continuous load before any ampacity adjustment [Sec. 215-2(a)].
104A x 1.25 = 130A.
No. 1 THHN is rated 130A at 75øC [Sec. 110-14(c)(2)(b)] and the 150A overcurrent protection device can protect it.
Circuits with overcurrent protection over 800A [Sec. 240-3(c)]. If the circuit OCPD exceeds 800A, the circuit conductors must have an ampacity not less than the rating of the OCPD, per Sec. 240-6(a).
Example: Feeder continuous load. Q. What size feeder conductor does the NEC require for a 1200A service/feeder paralleled in three raceways?
(a) 400kcmil (c) 600kcmil (b) 500kcmil (d) any of these
A. The answer is (c), 600kcmil. The feeder OCPD must have an ampacity not less than the rating of the OCPD (1200A).
Step 1. Determine ampacity per parallel conductor, 1200A/3 = 400A.
Step 2. Select a conductor using Table 310-16 that has an ampacity of 400A at 75øC [Sec.110-14(c)(2)(b)]. A 600kcmil conductor is required.
Small conductors [Sec. 240-3(d)]. Unless specifically permitted in Sec. 240-3(e) through Sec. 240-3(g), overcurrent protection shall not exceed 15A for No. 14, 20A for No. 12, 30A for No. 10 copper, and 15A for No. 12, and 25A for No. 10 aluminum and copper-clad aluminum - after taking the ampacity correction factor into account.
Specific equipment requirements [Sec. 240-3(g)]. When sizing conductors and OCPD, the Code has specific requirements. For example, rules for sizing panelboards, service conductors, and feeder conductors are all listed in separate sections.
Motors [Secs. 430-22(a) and 430-52(c)]. According to Sec. 420-22, branch circuit conductors are sized at no less than 125% of the motor full-load current listed in Tables 430-147 to 430-150, not the motor nameplate amperes [Sec. 430-6(a)]. The motor short-circuit ground-fault protection device must be sized at no more than the motor full-load current rating listed in Tables 430-147 to 430-150, multiplied by the percentages shown in Table 430-152. According to Table 430-152, inverse time circuit breakers are sized at 250% of the motor FLC.
Example: Motors. Q. What size branch circuit conductor and short-circuit protection (circuit breaker) is required for a 2-hp motor rated 230V (12A FLC)?
(a) No. 14, 15A breaker (b) No. 12, 20A breaker (c) No. 12, 30A breaker (d) No. 14, 30A breaker
A. (d) No. 14 with a 30A protection device.
Short-circuit protection: The short-circuit protection (circuit breaker) must be sized at no more than 250% of motor full-load current.
12A x 2.5 = 30A [Sec. 240-6(a) and Sec. 430-52(c)(1) Exception No. 1].
Conductors must be sized no less than 125% of the motor full-load current [Sec. 430-6(a) and Sec. 430-22(a)].
12A x 1.25 = 15A, Table 310-16, No. 14.