Wire Size

A

Alwayslearningelec

Senior Member
Location
NJ
Occupation
Estimator
I know I'm sure I've asked this before, shame on me.

So if the breaker is 150A but the combo starter at the exhaust fan is 200A and fan HP is 60HP at 480v. what is the order/ process of sizing.

I don't see a FLA in the EF schedule but I believe you start by calculating the motor load(amps), then you size the OCPD? But there are two, the starter and the breaker.

The wire should be sized to the 200A starter/disco or 150A breaker?

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Answer C: None of the above. 96.25A.

It should be sized to the MCA, which is based on the motor's FLA. Motor circuits are unique in that the upstream overcurrent protection is only required to provide short circuit protection and needs to be sized to allow the motor to start. The circuit's overload protection is provided separately.
 
Like D0nut says.

60HP motor is 77FLA per table 430.250.

Conductors are sized at 125% (or higher) of the FLA. So, conductors must exceed 96.25A rated.
Breakers are sized at 250% (or lower) of the FLA. NEC 430.52. So, breaker must not exceed 192.5A, use table 430.6 which is 175A. 150A breaker is more than acceptable.
 
In your case the 200 amp breaker is serving more as a disconnect with the 150 providing SCGF protection. As noted the conductor ampacity must be 96 or greater
 
steve66 said:
For motors, I always use my Square D slide rule data calculator. Many of the supply houses gave them away, and you can still find them on ebay. (The back side has 3 phase motors.)

There is also an online version:

Motor Data Calculator | Schneider Electric

d2g0qnfxuxm8zr.cloudfront.net d2g0qnfxuxm8zr.cloudfront.net

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I miss those, they are hard to come by now. At one time I owned one from every manufacturer I had to deal with, including Clark Control and Arrow Hart. The only ones I can find now are AB, Sq.D and Siemens/Furnas. No idea what happened to the rest, but likely got swept up in one of my wife’s “junk purge” events…
 
Jraef said:
I miss those, they are hard to come by now. At one time I owned one from every manufacturer I had to deal with, including Clark Control and Arrow Hart. The only ones I can find now are AB, Sq.D and Siemens/Furnas. No idea what happened to the rest, but likely got swept up in one of my wife’s “junk purge” events…
Click to expand...
They must still make them. There is one on Amazon for $12 based on the 2023 NEC.
 
d0nut said:
Answer C: None of the above. 96.25A.

It should be sized to the MCA, which is based on the motor's FLA. Motor circuits are unique in that the upstream overcurrent protection is only required to provide short circuit protection and needs to be sized to allow the motor to start. The circuit's overload protection is provided separately.
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LOL, thanks.
 
d0nut said:
Answer C: None of the above. 96.25A.

It should be sized to the MCA, which is based on the motor's FLA. Motor circuits are unique in that the upstream overcurrent protection is only required to provide short circuit protection and needs to be sized to allow the motor to start. The circuit's overload protection is provided separately.
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I'm pretty sure none of the above is D. :giggle:
 
Alwayslearningelec said:
This is the data for the motors I inquired in my post. I have to size at 125% of MCA, correct?


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Sometimes, I wish you would better explain why you are asking certain questions. The table you gave us, in a vacuum could cause us or you to misunderstand answers. First off, ON this web site you can find comprehensive information on how to size motor branch circuits, overcurrent and overload. All you need for 1 and 2 are the horsepower # of phases and the voltage of the motor.

A cut off table like you just presented is not clear. It has to be taken in perspective of what the Engineer that drew it was intending to convey.
 
infinity said:
For single motors the conductors are sizes based on 125% of the FLC value in T430.250. Do those FLC values match what's in the table?
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Here is the complete schedule.

1. FLC is NOT listed only FLA. You state it's to be sized off FLC and NOT FLA.
2. For some reason HP is not indicated.


1762345230454.png
 
For the fans you would use the HP rating and the voltage to find the FLC from the table. Multiply that by 125% and that is the minimum conductor size.
 
Since EF-4 has a VFD, the rules change slightly and maybe not enough to matter for your application. The conductors to the VFD are sized not less than 125% of the rated input current. The conductors on the output of the VFD are sized to 125% of the motor's full load current. See 430.122 in the 2023 NEC.
 
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