Multiple Motors on a Circuit vs Combination Motor Loads Conductor Sizing

Motors are used all over the world; in fact, 90 percent of global power is consumed by motors. There are motors that do all kinds of jobs such as move conveyor belts, lift heavy equipment, and even run large and small equipment. This is a big reason for motors using so much power throughout the world. 

In this article we will be going over sizing the conductors for multiple motors on a circuit and combination motor loads. Article 430 of the National Electrical Code covers motors, and the rules and requirements for all motor-type installations. Section 430.22 covers the requirements for single motors on circuit conductors. It tells us to multiply the Full Load Current, or FLC, of the motor (found in Tables 430.248 for single phase motors and 430.250 for three-phase motors) by 125 percent. Then we would go to Table 310.16 in the NEC to size the conductors. Sizing conductors for multiple motors isn’t that different. Let’s take a look.

Section 430.24 of the NEC covers conductor sizing for multiple motors. We go to the same Tables as before — 430.248 for single phase and 430.250 for three-phase motors — to get the FLC, measured in amps. We take the FLC of the largest motor and multiply it by 125 percent, and then add the FLCs from the rest of the motors on that circuit. 

Let’s say we have four motors. One motor is three-phase, 10 hp and 208 volts. A second motor is three-phase, 7.5 hp and 208 volts. Our third motor is three-phase, 1.5 hp and 208 volts, and our fourth motor is three-phase, 1 hp and 208 volts. The first motor mentioned is 10 hp, has a FLC of 30.8 amps according to Table 430.250, and is the largest. We take that 30.8 and multiply it by 125 percent which gives us 38.5 amps. The FLCs of the remaining motors, the 7.5 hp, 1.5 hp, and 1 hp, are then added together. That gives us a total of 56.6 amps for the three smaller motors. We then add together our 56.6 amps and 38.5 amps, to give us a grand total of 98.1 amps. 

Okay, now that we know the total amps for this multi-motor circuit, let’s go to Table 310.16 in the NEC to size the conductors. We haven’t mentioned temperature ratings for terminals up to this point, but that’s okay because the NEC helps us with that. If we don’t have any knowledge of the temperature rating of the terminals, we can’t just assume one. This is a huge mistake that a lot of electricians make. NEC code section 110.14(C)(1) says that if we are dealing with a load or loads that are 100 amps or less, we use the 60 degree column; if the loads are over 100 amps, we use the 75 degree column. Since the amperage of our example motors comes to less than 100 amps, we would use the 60 degree column. (If you know the temperature rating of the terminals, we would use that to size the conductors).

Looking at Table 310.16, we can see that 98.1 amps isn’t an option so we need to round up to 110 amps. For this multi-motor load, it looks like we need a 1-AWG conductor.

The codes for sizing the conductors for a multimotor and combination-load piece of equipment can be found in NEC section 430.25. It states that equipment ampacity for multimotors and combination-loads shall not be higher than the ampacity marked on the equipment. It then refers us to Section 430.7(D)(10) for equipment that is factory-wired, and Section 430.7(D)(2) for equipment that is not factory-wired. In both cases it states that the equipment needs a visual nameplate. This nameplate needs to be visible and marked with the manufacturer’s name, the rating in volts, frequency, number of phases, minimum supply circuit conductors ampacity, and the maximum ampere rating of the short-circuit and ground-fault protective device. It then refers us back to 430.24 to calculate the conductor sizes — which, if you remember, is the code section we used earlier to size conductors for multiple motors.

I hope this article is helpful to you as you work on motors and size their conductors.

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