Why my numbers don't match

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Ponchik

Ponchik

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CA
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Electronologist
I am tasked to install a circuit for a welder. The machine name plate says 11.4KVA and 92 amps at 240 V 3 phase.

When I do the math myself it doesn't match with the machine.

11400 / 1.732 x 240 =
47.5 / 1.732 = 27.43 amps
27.43 x 1.25 = 34.28 amps
40A circuit breaker

Where is my mistake?

TIA
 
I agree there is an approximately three-to-one discrepancy, but I don't know why.

Maybe it has something to do with the type of load a welder is, duty cycles, etc.
 
Is it a spot welder? Those usually have a short duration of current, so a smaller circuit can be used. The manufacturer usually will supply the OCP and wire size requirements. You have to be careful of voltage drop though. Also use pvc conduit instead of emt.
 
hillbilly1 said:
Is it a spot welder? Those usually have a short duration of current, so a smaller circuit can be used. The manufacturer usually will supply the OCP and wire size requirements. You have to be careful of voltage drop though. Also use pvc conduit instead of emt.
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Not spot welder. I think it is TIG welder, although I will confirm welder type tomorrow since the name plate doesn't match. The Miller website does not list the KVA or amp rating of their welders, interesting.

I am curious why use PVC? I hate PVC unless it goes underground or I use it for GEC installation.
 
Ponchik said:
Not spot welder. I think it is TIG welder, although I will confirm welder type tomorrow since the name plate doesn't match. The Miller website does not list the KVA or amp rating of their welders, interesting.

I am curious why use PVC? I hate PVC unless it goes underground or I use it for GEC installation.
Click to expand...
If it is a spot welder, because of the high amperage bursts, emt or rigid, unless aluminum acts as a choke.
 
I think the KVA is a long time rating, but it can use more current for short periods of time.

If you look at welders in the code book, I think there are factors that let you install a circuit that is smaller than 92 amps. It might even get you back down to the 30-40 amp range you are calculating.
 
So I think its basically rated for a 30% duty cycle (27 amps/92 amps). From 630.31, you get a .55 multiplier on the ampacity. So 92* 0.55 = 50.6 amps. So it would get 60 amp wire. That's how I see it.

The 125% factor would not apply to a welder.
 
Look at page 2 for required amperage at a given input voltage, it also shows 22KVA when at 60Hz, but only 12KW(?, that's some spicy PF there). And....that is a single phase unit.

If the machine is really a 210 (also single phase), not a 250, then Joe's numbers would fit.

Something isn't lining up here, either the exact model, the name plate on it (get a photo?), or some of the specs are being confused.
 
I called Miller twice and unfortunately heir techs did not know how to interpret the name plate. Both said use a wire rated for 74A and a circuit breaker rated for 110A.
I am assuming their suggestion is based on 92 x .78 = 72A (71.76)

None of them knew anything about the 11.4KW rating

I will size the circuit conductors at 60% duty cycle 4AWG.

Thanks everyone for your replies.
 
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