National Electrical Code Basics: Sizing and Protecting Branch-circuit Conductors

People involved in electrical affairs should study the procedures and details of calculations per the National Electrical Code® (NEC) rules, observing their wording and spirit. This article aims to determine the size and the protection of the conductors feeding the branch-circuit loads.

 

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Branch-circuit Ampere Rating

NEC® Article 210 postulates the general requirements for branch-circuit conductor sizing and overcurrent protection.

The maximum permissible ampere rating or setting of the overcurrent protective device establishes the branch-circuit rating (210.18). The ampere rating of a circuit depends on the rating of the circuit breaker or fuse protecting the conductor, not on the conductor size in the circuit. When using conductors of higher ampacity, the ampere rating or setting of the overcurrent device still determines the circuit rating.

Other than individual branch circuits (multiple-outlet branch circuits) are rated 15 A, 20 A, 30 A, 40 A, and 50 A (210.23).

The basis of these five ratings is the ampacities of N° 14, 12, 10, 8, and 6 AWG copper conductors (Table 310.16). Circuits larger than 50 A may serve only individual branch circuits (210.22) and shall supply only non-lighting outlet loads (210.23(D)).

Example 1. Typically, the circuit breaker or fuse rating matches the conductor ampacity. A 20 A circuit usually employs an N° 12 AWG conductor. When using an N° 10 AWG conductor to reduce voltage drop or increase mechanical strength (Table 310.16 shows an ampacity of 30 A for a TW copper conductor), it is still a 20 A circuit.

Example 2.   A multioutlet lighting branch circuit has an 8 AWG TW copper conductor (Table 310.16 shows an ampacity of 40 A) protected by a 20 A circuit breaker. What’s the circuit ampere rating? See figure 1.

 

Figure 1. Multioutlet lighting branch circuit.

 

The 20 A circuit breaker rates the circuit as 20 A.

 

Branch-circuit Conductor Ampacity

Section 210.19(A)(1) – not more than 600 V – affirms that the ampacity of the minimum branch-circuit conductor size must not be less than the larger of

   1. The noncontinuous load plus 125% of the continuous load – per Section 310.14 (Ampacities for Conductors Rated 0 Volts - 2000 Volts).

A continuous load operates for 3 hours or more, such as in stores, office buildings, schools, and outdoor lighting (see Article 100).

This rule is equivalent to limiting a continuous load to 80% of the conductor’s ampacity – it restricts the circuit’s load, not the circuit conductor’s ampacity.

Example 3. An individual branch circuit feeding a continuous, single-phase load of 38 A requires a conductor ampacity of at least 1.25 x 38 A = 47.5 A. Table 310.16 shows that a N° 6 AWG TW conductor has an ampacity of 55 A. And according to Section 210.18, the circuit rating is 50 A.

   2. The maximum load served after applying adjustment or correction factors per Section 310.15 (Ampacity Tables).

This rule changes the ampacity of the conductors after applying the adjustment and correction factors. This change in conductor ampacity is unlike limiting the load on a circuit, as stated in the previous rule.

After applying the correction and adjustment factors to the ampacities, the conductors must withstand 100% of the noncontinuous load plus 100% of the continuous load.

Ambient temperature correction factors. Tables 310.16 through 310.21 give the normal, maximum, and continuous ampacities of copper and aluminum conductors. Tables 310.15(B)(1) and 310.15(B)(2) display correction factors for the ampacity tables 310.16 through 310.21 for ambient temperatures other than 30°C and 40°C.

Example 4. What’s the ampacity of three N° 1 AWG THHN conductors installed in a raceway at an ambient temperature of 37.80°C?

Table 310.16 shows an allowable ampacity of 145 A in the 90°C column. Note 1 of 

Table 310.16 references Section 310.15(B) for ampacity correction factors where the ambient temperature is other than 30°C.

Table 310.15(B)(1) shows a correction factor of 0.91 in the 36-40°C row, and 90°C column. Then, 145 A x 0.91 = 131.95 A.

Adjustment factors for more than three current-carrying conductors. Table 310.15(C)(1) shows adjustment factors to reduce (derate) the ampacity when there are more than three current-carrying conductors in a raceway or cable.

Example 5.  Six N° 1 AWG THHN copper conductors installed in one conduit supply power to a couple of 3-phase motors. Terminations temperature rating = 90°C. What’s the true ampacity of these conductors? See figure 2.

 

Figure 2. Six conductors in a conduit.

 

Table 310.16 shows an allowable ampacity of 145 A under the 90°C column. Table 310.15(C)(1) shows an 80% adjustment factor for six conductors. Then, 145 A x 0.80 = 116 A.

High ambient temperature and more than three current-carrying conductors. Both adjustment and correction factors apply under this condition.

Example 6. What’s the true ampacity of six N° 1 AWG THHN copper conductors installed in a raceway at 37.80°C?

From table 310.16, the allowable ampacity under the 90°C column is 145 A.

Table 310.15(C)(1) shows an 80% adjustment factor for six conductors in one raceway.

Table 310.15(B)(1) shows a correction factor of 0.91 in the 36-40°C row, and 90°C column. Then, 145 A x 0.80 x 0.91 = 105.56 A.

   3. The conductor must comply with 110.14(C) “Temperature limitations” for equipment terminations. The temperature rating related to the ampacity of a conductor must not surpass the lowest temperature rating of any connected termination, conductor, or device.

Terminations. For equipment rated 100 A or less or marked for N° 14 AWG through N°1 AWG conductors – unless listed and identified for use with higher temperature rated conductors – the conductor ampacity is based on a 60°C rating even though the conductor rating allows higher temperatures such as 75°C or 90°C. (110.14(C)(1)(a)).

For equipment rated over 100 A or marked for conductors larger than N° 1 AWG, the conductor ampacity is based on a 75°C rating even though the conductor rating allows a higher temperature such as 90°C. (110.14(C)(1)(b)).

Connecting a 90°C conductor to a 60°C terminal will transfer excessive heat to the terminal because the insulation will have a higher temperature for a particular ampacity.

The Exception No. 1 to 210.19(A)(1)(a) allows an ampacity of the branch-circuit conductors of not less than the sum of the continuous load plus the noncontinuous load if the assembly, including the overcurrent device of the branch-circuit, is listed for operation at 100% of its rating.

Example 7. Size a THWN copper conductor to supply a 23.50 A continuous load. Consider the terminal ratings.

Computed load by 210.19(A)(1)(a): 1.25 x 23.50 = 29.37 A.

Terminal ratings by 110.14(C)(1)(a): terminated at 60°C.

Table 310.16 shows the THWN copper conductor in the 75°C column. Yet, see the ampacities in the 60°C column. Then, select a conductor N° 10 AWG THWN copper with an ampacity of 30 A.

 

Overcurrent Protection of Conductors

The general rule to protect conductors against overcurrent is by using fuses, or circuit breakers rated no higher than the conductor´s ampacity. The overcurrent protection device guards the conductor’s insulation against the damage that excessive currents – overloads and short-circuits – cause.

NEC® Section 210.20 establishes the rating or setting of the overcurrent protective devices to protect the branch-circuit conductors and equipment.

Similarly to Section 210.19(A)(1), for conductor ampacity, Section 210.20(A) states that where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the rating of the overcurrent device shall not be less than the noncontinuous load plus 125% of the continuous load.

Demanding 125% of the continuous load is equivalent to limiting the load to 80% of the branch-circuit ampere rating. The NEC® has this requirement because the heat generated by constant current through the protective device may cause excessive temperature in the adjacent conductors.

Example 8. What’s the maximum permitted continuous load on a branch circuit protected by a 20 A circuit breaker?

Because the ampere rating of a branch circuit depends on the rating of the circuit breaker or fuse protecting the conductor, this circuit is rated 20 A.

The maximum permitted load is 0.80 x 20 = 16 A.

The Exception to 210.20(A) allows the ampere rating of the overcurrent device to be not less than the sum of the continuous load plus the noncontinuous load if the assembly, including the overcurrent device protecting the branch circuit, is listed for operation at 100% of its rating.

Section 240.4 establishes that conductors other than flexible cords, flexible cables, and fixture wires shall be protected against overcurrent following their ampacities specified in 310.14.

Example 9. What´s the largest size of a fuse or circuit breaker setting permitted to protect an N° 2 AWG THWN copper conductor?

According to table 310.16, the conductor’s ampacity is 115 A.  100 A or 110 A standard ampere ratings are allowed sizes. However, Section 240.4(B) allows the use of a 125 A circuit breaker under certain conditions. See example 10.

 

Small Conductors

Section 240.4(D) states the maximum overcurrent protection for small conductors – after applying the correction factor for ambient temperature and the adjustment factor for more than three conductors – as shown in table 1.

 

Size AWG	A
18 copper	7
16 copper	10
14 copper	15
12 aluminum	15
12 copper	20
10 aluminum	25
10 copper	30

Table 1. Maximum overcurrent protection of small conductors (Based on NEC® Section 240.4(D)).

 

Overcurrent Devices Rated 800 A or Less

Section 240.4(B) allows the use of the next larger standard ampere rating of fuses or nonadjustable circuit breakers when

• The conductors protected are not part of a branch circuit supplying more than one receptacle for cord-and-plug-connected portable loads. 
• The standard ratings do not conform to the conductor’s ampacity.
• The next larger standard does not exceed 800 A.

Table 240.6(A) shows the standard ampere ratings for fuses and inverse time circuit breakers.

Example 10. Table 310.16 shows an ampacity of 115 A for an N° 2 AWG THWN copper conductor. There is no standard 115 A overcurrent protective device. 

Table 240.6(A) shows the following standard size is 125 A. The 125 A fuse or inverse time circuit breaker is acceptable to protect the conductor.

 

Overcurrent Devices Rated Over 800 A

Section 240.4(C) states that the conductor’s ampacity shall be equal to or greater than the ampacity of the overcurrent protection device when it is rated over 800 A.

This section does not allow the use of the next larger standard ampere rating.

Example 11. Three parallel-connected sets of four 500 kcmil THW copper conductors – 3 phases plus neutral each group – in separate conduits constitute a lighting feeder (assume noncontinuous load and 75°C rated terminals). Compute the nearest standard ampere rating of a fuse or nonadjustable circuit breaker to protect the conductors. See figure 3.

 

Figure 3. Lighting feeder.

 

Table 310.16 shows an ampacity of 380 A in the 75°C column for each conductor. The ampacity per phase leg is 3 x 380 A = 1 140 A. Then, Table 240.6(A) shows 1 000 A as the nearest standard.

 

An Additional Example

Example 12. An individual branch circuit supplies a 38 A, single-phase, continuous load.

   a. What´s the minimum ampere rating? 1.25 x 38 A = 47.5 A. The minimum ampere rating is 50 A. This circuit breaker or fuse protects the conductor (210.18).

   b. What’s the termination temperature rating? 60°C (110.14(C)(1)(a)).

   c. List some acceptable smallest-size typical copper conductors for the application (Table 310.16, 60°C column). N° 6 TW, THW, or THHN.

   d. List some acceptable smallest-size typical aluminum conductors for the application (Table 310.16, 60°C column). N° 4 TW, THW, or THHN.

 

Summary

The rating or setting of the overcurrent protective device establishes the ampere rating of a branch circuit.

The branch circuit’s ampere rating must be equal to or lower than the conductor ampacity.

The ampacity of branch-circuit conductors must be equal to or higher than the supplied load.

Continuous loads require a reduction in the current allowed in a branch-circuit conductor.

Tables 310.16 to 310.21 give the conductors’ normal, maximum, continuous ampacities in cables or raceways. These ampacities may need a correction for ambient temperature and an adjustment for more than three conductors in a cable or raceway.

 

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