Disconnect

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Bryan-79

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Nampa idaho
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Electrician
My boss is having me put up a 60 amp disconnect on 61 amp motor because he said the hp rating is okay. The motor is 40 hp and the disconnect is rated for 50 hp. I was trained to only use 80% of the amp rating so on a 60 amp disconnect 80% is 48 amps. Can't find in the nec 80% rule for disconnects. Any help would be appreciated, thanks.
 
infinity

infinity

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New Jersey
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Is this 480 volts and 40 HP? T430.250 lists the FLC at 52 amps.

Welcome to the Forum. :)
 
infinity

infinity

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New Jersey
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Do you think that this is applicable? 52*115%=59 amps or the exception?

430.110 Ampere Rating and Interrupting Capacity.
(A) General. The disconnecting means for motor circuits rated 1000 volts, nominal, or less shall have an ampere rating not less than 115 percent of the full-load current rating of the motor.
Exception: A listed unfused motor-circuit switch having a horsepower rating not less than the motor horsepower shall be permitted to have an ampere rating less than 115 percent of the full-load current rating of the motor.
Click to expand...
 
Dennis Alwon

Dennis Alwon

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Chapel Hill, NC
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Bryan-79 said:
So a 60 amp disconnect is okay?
Click to expand...
Does this help

430.110 Current Rating and Interrupting Capacity.

(A) General.

The disconnecting means for motor circuits rated 1000 volts, nominal, or less shall have a current rating not less than 115 percent of the full-load current rating of the motor.
Exception:
A listed unfused motor-circuit switch having a horsepower rating not less than the motor horsepower shall be permitted to have a current rating less than 115 percent of the full-load current rating of the motor.
Click to expand...
 
Jraef

Jraef

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infinity said:
Do you have a photo of the nameplate? The NEC lists a 40 HP at 480 volts as 52 amps.
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Could be a low speed / high pole count motor. I just ran across one that was 24 poles, 260 RPM ( 300 synch), the FLA was way above normal even for the overly conservative NEC charts, which are all based on 4 pole U-frame motors (that nobody uses any longer).
 
infinity

infinity

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Bryan-79 said:
No photo, but I will get one tomorrow
Click to expand...
Sounds good. Like Jraef stated NEC table values are generally conservative with the actually nameplate FLA being slightly lower. I think that a 40 HP, 400 volt motor may have an FLA near 61 amps.
 
Last edited:
T

TX+ MASTER#4544

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Location
Texas
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electrical Code instructor and mentor
Jraef said:
That's because there is no such thing.



I'm guessing this is a Square D 60A NF disconnect. This issue has come up before, it's apparently a quirk of their product.
Here's what I thought back then.
Click to expand...
2023 NEC
The 80 % can't be found for the disconnect.

It's the reciprocal of 125% which is what the NEC uses for continuous duty.

Example: 1 / 1.25 = 0.8 or 80 %
Example : 1/ .80 = 1.25 %

Panelboard's and disconnects have that 80 % rating.

When we size our continuous loads the NEC requires 125% of the load, so that satisfies the '80 %' rule.

Make sense? Sure does, 'cause NEC says so.

Oh, I can't forget to say.......
Thanks for reading.
Comments accepted.
TX+MASTER #4544
 
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