220.60 Noncoincident loads

[Ryan Ray]

If AC is omitted as the smaller load, is it still increased 25% if it's the largest motor? Tom Henry's Calculations book states that you use the next highest rated motor and not the AC if it's omitted, but I don't believe this is true. Does anyone know if I'm, correct?

 

[wwhitney]

The language in 220.60 on dealing with this question has changed every cycle the last couple cycles, and so I think there's still some debate about what it actually tells you to do. But here's what it should say:

You have two non-coincident loads, say the A/C and the Heat. Calculate the total load as if there was no Heat, just A/C. And then calculate the total load as if there was A/C and no Heat. Use whichever is bigger.

So yes, if the A/C is omitted as the smaller load, the 25% extra for the largest motor would ignore the A/C and use the next largest motor.

Again, maybe they didn't get the language correct to say this, but it is obviously the correct formulation.

Cheer, Wayne

 

[LarryFine]

Calculate the total load as if there was no Heat, just A/C. And then calculate the total load as if there was A/C and no Heat.

 

[Ryan Ray]

The language in 220.60 on dealing with this question has changed every cycle the last couple cycles, and so I think there's still some debate about what it actually tells you to do. But here's what it should say:

You have two non-coincident loads, say the A/C and the Heat. Calculate the total load as if there was no Heat, just A/C. And then calculate the total load as if there was A/C and no Heat. Use whichever is bigger.

So yes, if the A/C is omitted as the smaller load, the 25% extra for the largest motor would ignore the A/C and use the next largest motor.

Again, maybe they didn't get the language correct to say this, but it is obviously the correct formulation.

Cheer, Wayne

Nothing in the NEC states to use the next largest if omitted, but 220.60 states "Where the motor is part of the noncoincident load and is not the largest of the noncoincident loads, 125% of the motor load shall be used in the calculation if it's the largest motor." It appears you can still use the AC as the largest motor load.