Motor Calculation

[Seven-Delta-FortyOne]

Got specs for some equipment that I have to supply wiring and disconnect for.

Manufacturer sent us the following: "Booster pump 3HP 230V/3 Phase @ 9.6 Amps (230V 1 Phase @ 34 Amps)" and Backwash pump 5HP 230V/3 Phase @ 15.2A ( 230V 1 Phase @ 54A)"

I believe those figures to be wrong. I've calc'd it many times and I have 17A @ 1 Phase for the 3HP and 28A @230V for the 5HP. I have assumed a PF of .80 on my calcs.

Have I done my calculations wrong?

I believe they did the single phase calcs based on 115V, not 230. But before I question an engineer, I want to make very sure.

 

[Strathead]

I agree that the EE based his calcs on 115V, but your calculations shouldn't matter either. Bot wire size and MOCP are based on the table values in 430, not on a calculated value. The only thing you use the nameplate value for is overload.

 

[Seven-Delta-FortyOne]

I agree that the EE based his calcs on 115V, but your calculations shouldn't matter either. Bot wire size and MOCP are based on the table values in 430, not on a calculated value. The only thing you use the nameplate value for is overload.

Understood. But we don't have the motors yet. They have options on what to supply us, either 3 phase or single phase. 3 phase service is not available at this location, so I ordered single phase.

I have to do the submittals for the wiring, conduit sizes, etc., and when I saw their options for single phase motor alternatives, it didn't seem right. So I'm going off 430.250 table to design the wiring, conduits, and OCPD.

 

[kwired]

Fairly uncommon to see general purpose single phase motors capable of connecting to 115 volt supply over 2HP, also can't recall ever seeing any definite purpose motors over 2HP that can be connected 115 volts. Guess can't hurt to contact them and verify.

 

[Seven-Delta-FortyOne]

Correction: 430.248

 

[Seven-Delta-FortyOne]

Fairly uncommon to see general purpose single phase motors capable of connecting to 115 volt supply over 2HP, also can't recall ever seeing any definite purpose motors over 2HP that can be connected 115 volts. Guess can't hurt to contact them and verify.

They listed them as single phase 230V, but the amps calc was off.

 

[Jraef]

So I'm going off 430.250 (248) table to design the wiring, conduits, and OCPD.

This is what you are supposed to do anyway, as Strathead said earlier. Calculations and nameplate data are irrelevant at this point, so long as you know the voltage, phase and HP values. You don't need the nameplate data until you have to pick the OL heater settings or elements (provided they are needed).

To me, it's obvious that they picked the values out of the wrong column, I would MAYBE write up a clarification request alluding to that and give them a graceful way of admitting their error, but I would not act on those numbers, because, again, they don't actually matter for the task at hand.

 

[Julius Right]

It seems the VLL [phase-to-phase voltage] it is actually 230 V[and not phase-to-neutral voltage] the p.f [cosfi]͌≈0.7 and efficiency 0.83-0.88%