I removed 2 spot welders that were 480 volts single phase. Need to reinstall them in another location. One is 50 and the other is 150 KVA at 50% duty cycle. My question is, do you calculate it as single phase or 3 phase. Technically your pulling 2 legs from a 3 phase panel. Yet I removed 250 amp fuses and 4/0 wire at 90 degree which is rated for 260 amps for the 150 KVA. Yet if you calculate it as single phase you need at least 312.5 amps. KVA x 1000 divided by supply voltage. Divide that by 1.732 for 3 phase. Is there a different formula, or am I correct that the wire and fuses are undersized.
Calculating single phase KVA
- Thread starter Jon1544
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- Lockport, IL
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- Retired Electrical Engineer
I don't know what impact "duty cycle" has on branch circuit or fuse sizing. But if the two welders each take 2 legs from a 3-phase system, you calculate each as single phase. I agree that 150 KVA at 480V single phase corresponds to 312.5 amps. You don't bring the 1.732 into the equations until you start counting up loads on the upstream panel. These two welders will draw 200 KVA, which equates to about 240 amps on the 3 phase panel. But you need to recognize that there will be an unbalanced loading condition (unless there are other significant loads to help with the overall balance).
Eddie702
Licensed Electrician
- Location
- Western Massachusetts
- Occupation
- Electrician
Everything mentioned above sound correct to me. The welders however are nothing more than transformers basically. Ifthey never use full output they will never need full input. Suppose they should be wired to the code though.
Maybe talk to the welder MFG. 50% duty cycle will allow the wires to cool and see if there is any additional info on the nameplate
Maybe talk to the welder MFG. 50% duty cycle will allow the wires to cool and see if there is any additional info on the nameplate
Eddie702
Licensed Electrician
- Location
- Western Massachusetts
- Occupation
- Electrician
Actually a duty cycle of 50% will allow wire to be 71% of the nameplate according to table 630.11A for a non motor generator. 150 kva = 221.9 amps. 50 kva= 74 amps
My calculations tell me that your circuit breaker size of 600A is correct!
The line currents I got from two, single-phase welders (50kVA and 150kVA at 50% duty cycle, actually a 50 and 106 effective kVA ) are:
Ia = 107.7A;
Ib = 287.4A; and
Ic = 220.8A.
You will have to take 287.4A as your highest current and choose a 200% CB, which is 287 X 2 ~ 600A.
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