Motor Conductor Calculations

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ElectricMatt

Senior Member
Location
Waco, tx
To start, I already have installed as per my own calculations. However, the have been looking at some motor conductor/heater calculations and want a comparison. I will give the info below:

480v
3ph
75hp
Fla 80.9
SF 1.25


Please advise of wire and breaker size based on your calculations per the NEC.

Thank you for your responses.


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Jraef

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
The only calculation you need is to read the appropriate table in the NEC, in this case table 430.250 for Full-Load Current, Three-Phase Alternating-Current Motors. Find 75HP @ 460V and read the FLA. Then take that value x 1.25, that is your minimum conductor ampacity. The exception is when the motor nameplate FLA is HIGHER than these table values, in which case you use that basis. That's not the case here.

So 75HP in the table is 96A, x 1.25 = 120A minimum conductor ampacity. We don't know anything else about the installation (distance, other conductors in the conduit, etc.), so we can't tell you the actual conductor size.

Breaker size is per 430.52, assuming a standard Thermal-Mag breaker; no more than 250% of the 430.250 value, so 300A max, but you can use smaller if you like. If it's in a combination starter / MCC bucket with a factory assembled MCP (mag-only) breaker, it can be larger, depending on several other factors not in evidence.
 
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E

ElectricMatt

Senior Member
Location
Waco, tx
Thank y’all for the quick responses. That is exactly what I thought, but a co-worker was saying use the FLA and I did not have my code book handy.

It was a short run, with a thermal mag breaker, and a normal fan duty(fan motor) application. So I just wanted opinions on what size wire to use.

Sometimes I have a brain fart and I come here for clarification from people far more intelligent than myself.


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infinity

infinity

Moderator
Staff member
Location
New Jersey
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If you used the FLA given then your minimum size conductors would have been too small.
 
T

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
ElectricMatt said:
Thank y’all for the quick responses. That is exactly what I thought, but a co-worker was saying use the FLA and I did not have my code book handy.

It was a short run, with a thermal mag breaker, and a normal fan duty(fan motor) application. So I just wanted opinions on what size wire to use.

Sometimes I have a brain fart and I come here for clarification from people far more intelligent than myself.


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Click to expand...
You would use the nameplate FLA only for OL selection. As mentioned, the conductor and OCPD must use NEC values.
 
T

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
Jraef said:
The only calculation you need is to read the appropriate table in the NEC, in this case table 430.250 for Full-Load Current, Three-Phase Alternating-Current Motors. Find 75HP @ 460V and read the FLA. Then take that value x 1.25, that is your minimum conductor ampacity. The exception is when the motor nameplate FLA is HIGHER than these table values, in which case you use that basis. That's not the case here.

So 75HP in the table is 96A, x 1.25 = 120A minimum conductor ampacity. We don't know anything else about the installation (distance, other conductors in the conduit, etc.), so we can't tell you the actual conductor size.

Breaker size is per 430.52, assuming a standard Thermal-Mag breaker; no more than 250% of the 430.250 value, so 300A max, but you can use smaller if you like. If it's in a combination starter / MCC bucket with a factory assembled MCP (mag-only) breaker, it can be larger, depending on several other factors not in evidence.
Click to expand...
You must be sleeping... 96 X 250% = 240. Next size up is 250 amp breaker.
 
E

ElectricMatt

Senior Member
Location
Waco, tx
texie said:
To be more technically correct, I should have said the nameplate FLA and SF should be used for OL selection
Click to expand...

That is exactly what I thought. However, the guy working for me tends to be very good with remembering stuff like that so I was second guessing myself. Plus it was the end of a 15 hr day so I wasn’t thinking properly. Again thank you both for the quick and accurate responses. This is why I love this forum, tons of knowledge here.


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