Motor Calc

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basjak

Member
Location
Las Vegas, NV
Hello,
New to forum. 27 years and first time here. Hope someone can assist a bit.

We have an air compressor nameplate rating at : 60 HP, 45KV @ 230 volts, 3PH. "We" sized our wiring and disconnect and fusing per "our" calculations. I believe "we" made a mistake and or the equipment installer did not disclose the correct information to us at the beginning. We have 120/208 volts, 3PH. We installed buck/boost xfmrs to correct voltage issue, sized per xfrmr manufacturer specs. So my question(s) are these:

What should the calculated amperage be at the 208 volt line side of the disconnect, prior to the xfmrs?
What should the calculated amperage be at the 230 volt load side of the xfmrs?

We did the 45K/230V x .95PF to get 119 amps. The motor is pulling 140 to 150 amps at this location. We are getting 180 amps at the line side disconnect.

Thanks for your assistance, Andy
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Many issues here.

It was probably 45kW, not kV. 45kW = 60HP.

Per the NEC, you do NOT size anything with regard to the circuit from the nameplate FLA, except the starter overload protection.

Officially, you size based on the nameplate HP, use table 430.150 to get a standardized FLC value, then size everything based on that. So you have a motor that says 60HP on the MOTOR nameplate, right?** If so, the NEC table says 169A FLC at a 208V supply. Period. From that, you size the conductors at 125% of 169A, so 211A, picked from the appropriate column for insulation and number of conductors etc., paying attention to distance for voltage drop, then adjust up accordingly, but never down.

You likely did not need to bother with the transformer(s), but if you already have it and it is 2-1/2-3X the kW rating of the motor, (so in this case 112.5kVA or larger or if in your case a set of 3 B-B xfmrs, hopefully you told the supplier it was for a a motor) then re-adjust based on the 230V FLC of 154A, x 1.25, so 192.5A basis.

I'm not sure where you are reading 190A, but if the disconnect you mention is on the 208V side, that would suggest you are losing a lot in the transformers, which might indicate they are over heating because they are undersized. If the disconnect you mention is on the 230V side, then that may indicate that your motor is rather severely overloaded.

Your disconnect must be rated for the HP, and at least 115% of the standardized FLC. Since disconnects are in large increments, a 100A is too small, so a 200A will be the next size up and that is fine.

**If the "60HP" is not on the actual motor nameplate, and it doesn't have one, and the "60HP" is just a sticker on the compressor, that is a marketing term, means nothing. When that is the case, you should see, in the instruction manual and (hopefully) on the machine nameplate, something to the tune of 'MOCP', short for Maximum Over Current Protection, which dictates your breaker size, and/or 'MCA', Minimum Circuit Amps, which dictates your minimum wire size.
 
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basjak

Member
Location
Las Vegas, NV
60KW

60KW

Thanks for that. Yes it is 60 KW.... We engineered a project with multiple motor loads from 5hp, 15hp, 30hp and this problem child 60 HP. We were given all the "specs" when designing. The BB xfmrs are more than adequate. Specs were sent to manufacturer. Believe they are 208 amp rated. They appear to be appropriate as heat is minimal. Never had a problem with motors in the past utilizing nameplates and adjusting from there. I don't like overkill and try to save the customer where I can and still provide a reiiable electrical system. This one particular motor got away from me. I should have insisted on the characteristics of the the unit it is installed in and more technical data. The company had issues providing for this one unit.

I appreciate your comments. Thank you very much.....Andy

Many issues here.

It was probably 45kW, not kV. 45kW = 60HP.

Per the NEC, you do NOT size anything with regard to the circuit from the nameplate FLA, except the starter overload protection.

Officially, you size based on the nameplate HP, use table 430.150 to get a standardized FLC value, then size everything based on that. So you have a motor that says 60HP on the MOTOR nameplate, right?** If so, the NEC table says 169A FLC at a 208V supply. Period. From that, you size the conductors at 125% of 169A, so 211A, picked from the appropriate column for insulation and number of conductors etc., paying attention to distance for voltage drop, then adjust up accordingly, but never down.

You likely did not need to bother with the transformer(s), but if you already have it and it is 2-1/2-3X the kW rating of the motor, (so in this case 112.5kVA or larger or if in your case a set of 3 B-B xfmrs, hopefully you told the supplier it was for a a motor) then re-adjust based on the 230V FLC of 154A, x 1.25, so 192.5A basis.

I'm not sure where you are reading 190A, but if the disconnect you mention is on the 208V side, that would suggest you are losing a lot in the transformers, which might indicate they are over heating because they are undersized. If the disconnect you mention is on the 230V side, then that may indicate that your motor is rather severely overloaded.

Your disconnect must be rated for the HP, and at least 115% of the standardized FLC. Since disconnects are in large increments, a 100A is too small, so a 200A will be the next size up and that is fine.

**If the "60HP" is not on the actual motor nameplate, and it doesn't have one, and the "60HP" is just a sticker on the compressor, that is a marketing term, means nothing. When that is the case, you should see, in the instruction manual and (hopefully) on the machine nameplate, something to the tune of 'MOCP', short for Maximum Over Current Protection, which dictates your breaker size, and/or 'MCA', Minimum Circuit Amps, which dictates your minimum wire size.
 
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