Art. 430, Part II provides the requirements for motor circuit conductors for circuits rated 1,000V or less. Why is this Part II necessary, since Art. 310 already provides requirements for circuit conductors and even has ampacity tables? If you look a bit more closely, you’ll see Art. 310 provides the requirements for “general” circuit conductors. The conductors for motor circuits are special case conductors because of inrush current.
Starting a motor can take up to five times more current than running the same motor. So why don’t we size the conductor at five times the motor full-load current rating? Because the inrush period is temporary, so the amount of heat generated into the conductor is capped at a much smaller value. It turns out that sizing the motor circuit conductor at 125% will handily accommodate this extra heat.
When determining conductor size for a single continuous-duty motor, the solution is pretty straightforward. Size the conductor so its ampacity is at least 125% of the motor full-load current rating [430.22]. But there are variations. For example, if it’s a DC motor with a single-phase, half-wave rectifier bridge, then the ampacity must be at least 190% of the motor full-load current rating [430.22(A)(1). And no matter what kind of motor you have, you need to follow 430.6(A)(1) when making this determination.
If the motor application is noncontinuous, what ramification does that have? If your answer is “more heat,” you are correct. More starts in a given time means more residual heat. And that means you go to Table 430.22(E) to get a duty cycle multiplier to use. The multiplier is based partly on whether the motor is a five-minute rated, 15-minute rated, 30- or 60-minute rated, or continuous-rated motor. It’s also based on the classification of service.
What if you have several motors on a circuit, or you have a motor and other loads on a circuit? Then it’s not just a matter of upsizing the conductors 125% (or any of those other things if you have a variation). You have to make four calculations, and then add the results of all four together. This sum is the lowest ampacity that you can allow your circuit conductors to be [430.24]:
- 125% of the full-load current rating of the highest-rated motor (per 430.6(A)).
- Sum of the full-load current ratings of all the other motors in the group (per 430.6(A)).
- 100% of the noncontinuous nonmotor load.
- 125% of the continuous nonmotor load.
This might seem like somebody had too much time on their hands when coming up with this process, but look at it carefully and you will see the logic.
- 125% of the full-load current rating of the highest-rated motor is really an extension of the single motor calculation.
- The total of all the other motors because they are there. Why not 125%? Because that’s already allowed for by the highest-rated motor.
- The noncontinuous nonmotor loads don’t have inrush current and aren’t continually heating the conductors.
- The continuous nonmotor loads don’t have inrush current, but they are continually heating the conductors.
