When service protection can be higher than ampacity

Jul 1, 1995 12:00 PM, Stallcup, James G.

Protecting service-entrance conductors when there are motors and other loads.

The basic requirement in Sec. 230-90(a) is that the fuse size or circuit breaker setting must not be greater than the current-carrying capacity of the service-entrance conductors. Either fuses or a circuit breaker may be used as the "over-current device" to protect service-entrance conductors. When fuses are used, there must be a fuse in each "hot" wire, but a fuse is normally never permitted in a grounded circuit conductor per Sec. 240-22. (See Exceptions to Sec. 240-22.)

For example: If the service is No. 4 type THWN copper conductors, the maximum fuse size or circuit breaker setting with 75 [degrees] C terminals per Sec. 110-14(c), Ex. 2 would be 80A, since 85A is the carrying capacity of the conductor. There are five exceptions to the basic requirement for sizing the "overcurrent device" of a service, where the overcurrent protection device (OCPD) can be larger than the allowable ampacity of the service-entrance conductors.

Sizing OCPD for motor loads (Ex. 1)

When motors comprise all or most of the load, protection of the service conductors can be according to the rules for motors. For motors, the Code allows the conductors to be overfused to permit the motor to start and run.

Calculation Note: Do motor calculations separately in conformity with Sec. 430-24 for the conductors, and Sec. 430-52 and Sec. 430-62 or Sec. 430-63 as applicable for overcurrent protection. After you make those computations add your results to the other loads calculated per Sec. 220-10(a) and (b) of the NEC.

For example: If the service supplies one or more motors with other loads, the service-entrance conductors and overcurrent protection device due to motor loads must be calculated as follows, given the loads as indicated. Don't forget to add 25% for the largest motor (Sec. 430-24) and for continuous loads [Sec. 220-10(b)].

Problem: What is the size OCPD and THWN copper conductors required for the service? Although this isn't necessarily true in all cases, we'll assume the lighting and the appliance loads are continuous for the purposes of this example.

Sizing OCPD based on loads

Step 1: Calculating loads (120/240V 1-phase system)

230-42(a); 220-10(b); Table 430-148; 220-14

* Lighting load

40A x 125% 5 50.0A

* Receptacle load

37.5A x 100% 5 37.5A

* Appliance load

30A x 125% 5 37.5A

* Heat or A/C load

10.08kVA x 100% 4240V 542.0A

Other load subtotal: 167.0A

* Motor load

5 hp (240v) 528A x 100% 528.0A 1 hp (120V) 516A x 100% 516.0A(*) 2 hp (120V) 524A x 100% 524.0A

* Ignore for figuring largest line

* 25% of largest motor 7.0A

Motor subtotal = 59.0A Total load 5226.0A

Step 2: Selecting OCPD based on load per Sec. 230-90(a) and Sec. 220-10(b), and allowing the next higher standard size per Sec. 240-3(b): 226A allows 250A.

Solution: The size of the OCPD based upon calculated load is 250A.

Sizing OCPD based on motor load

Step 1: Calculating load

430-52(a); 430-62(a); 240-3(f); Table 430-152

Motor loads (assume inverse-time circuit breakers)

* 5 hp 528A x 250% 570.0A

* 1 hp 516A x 100% 516.0A(*)

* 2 hp 524A x 100% 524.0A

* Ignore for figuring largest line

Total motor component: 94.0A

Step 2: Other loads

* Lighting load 5 50.0A

* Receptacle load 5 37.5A

* Appliance load 5 37.5A

* Heating load 5 42.0A

Subtotal other load: 167.0A Total load 5261.0A

Step 3: Selecting OCPD [Sec.430-62 (a)] 250A is the next size below 261A

Solution: The size OCPD based upon motor loads is 250A. Note: In most cases the calculated load procedure results in a similar OCPD. However, in cases where there is a very large motor that is disproportionate to the other load, the Sec. 430-63 result will be significantly higher, and should be followed in order to prevent nuisance tripping.

Sizing conductors

Step 1: Calculating load based on 100%

215-2; 220-10(a)

* Lighting load

40A x 100% 5 40.0A

* Receptacle load

37.5 x 100% 5 37.5A

* Appliance load

30A x 100% 5 30.0A

* Heat load

42A x 100% 5 42.0A

* Motor loads

* 5 hp 5 28A x 100% 5 28.0A

* 1 hp 5 16A x 100% 5 16.0A(*)

* 2 hp 5 24A x 100% 5 24.0A

* Ignore for figuring largest line

* Largest motor load

28A x 25% 5 7.0A

Total load 5 208.5A

Step 2: Selecting conductors

310-10; Table 310-16

* Calculated load at 100% 5 208.5A

* Conductors are No. 4/0 (230A)

Check per Sec. 110-14(c)(2):

208.5A load on 4/0 conductor won't cause terminations to exceed 75 [degrees] C, OK.

Solution: Although the 4/0 THWN conductors calculated under the provisions of Sec. 430-24 are lower in ampacity than the rating of the overcurrent device calculated under the provisions of Sec. 430-63, this arrangement is still allowable. Note that the running overload protection for the motors contributes to the overall protection for these circuits. In this case the fact that the OCPD (250A) is the next higher standard size above the conductor ampacity (230A) and thereby qualifies under Sec. 240-3(b) is a coincidence and the spread could exceed the next higher standard size and still be within the requirements of the Code.

Sizing the neutral

Step 1: Calculating load (original loads assumed to be evenly distributed, so line-to-neutral loads have the same ampere value as on previous calculations)

220-22; 220-10(a)

* Lighting load

40A x 100% 5 40.0A

* Receptacle load

37.5A x 100% 5 37.5A

* Appliance load

30A x 100% 5 30.0A

Subtotal nonmotor load: 107.5A

* Motor loads

1 hp 5 16A x 100% 5 16.0A(*)

2 hp 5 24A x 100% 5 24.0A

* Ignore in favor of the 2 hp motor

* Largest motor load

2 hp 5 24A x 25% 5 6.0A

Subtotal of motor contribution: 30.0A

Total load 51 37.5A

Step 2: Selecting conductors

310-10; Table 310-16137.5A requires No. 1/0 at 75 [degrees] C

Solution: The size of the neutral conductor is No. 1/0 THWN copper.

Summary: As proven by the calculation based upon the 1993 NEC, the over-current protection device can be of greater rating than the ampacity of the service-entrance conductors due to staring currents of motors.

James G. Stallcup is the owner of Grayboy & Associates in Fort Worth, Tex., a consulting firm specializing in the National Electrical Code.