Motor Disconnects

Dennis Alwon

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Motor nameplate is 30 amps
Tables show 40 amps
Max breaker- inverse time can be 250% of 30 amps = 75 amps

Can I use a 60 amp disconnect with 70 amp breaker in it.

The disconnect needs to be 115% larger than the fla so 60 amp covers that and the breaker is only for short circuit and ground fault. The only thing I can come up with is 110.3

Is there an actual danger in doing this. This is totally hypothetical...
 
There's a good chance the label on the 2 space panel specifies a maximum breaker size.
 
I think that the code requires you to protect the bus at or below it's rating. If it is only rated at 60 amps, it needs a 60 A or less rated ocpd somewhere upstream.
 
petersonra said:
I think that the code requires you to protect the bus at or below it's rating. If it is only rated at 60 amps, it needs a 60 A or less rated ocpd somewhere upstream.
Click to expand...

But the buss is protected as the overload will keep the buss from seeing more than 38 amps or so.
 
AFAIK max inverse time breaker can be 2.5 times the NEC current which you said was 40 amps.

Many 2 space breaker enclosures are rated 70 amps max breaker. The ones I'm thinking of also have aluminum bus and don't seem to last in this situation. But I am often seeing them on items that take time to accelerate the load and those seem to fail more than if not as high of inertia load being driven. Those long time accelerating loads also need max allowed breaker size more often than other loads with same HP motor.

Side note, they need to develop better starting capacitor terminals for those motors driving those long time acceleration loads as well. Seem to need to replace those a lot more than on the lower inertia loads. Almosgt always burns the "rivet" that holds the terminal to whatever is behind the insulating plate on top of the capacitor.
 
kwired said:
AFAIK max inverse time breaker can be 2.5 times the NEC current which you said was 40 amps.

Many 2 space breaker enclosures are rated 70 amps max breaker. The ones I'm thinking of also have aluminum bus and don't seem to last in this situation. But I am often seeing them on items that take time to accelerate the load and those seem to fail more than if not as high of inertia load being driven. Those long time accelerating loads also need max allowed breaker size more often than other loads with same HP motor.

Side note, they need to develop better starting capacitor terminals for those motors driving those long time acceleration loads as well. Seem to need to replace those a lot more than on the lower inertia loads. Almosgt always burns the "rivet" that holds the terminal to whatever is behind the insulating plate on top of the capacitor.
Click to expand...

Okay, but this doesn't answer the question and remember it is hypothetical. The point is not whether they make a 70 amp disconnect it is for argument's sake a 60 amp rated disconnect. Assume the hp is fine for the disconnect.

I believe the max 250% is based on the nameplate not the Nec table. The fla is 30 amps
 
Dennis Alwon said:
Okay, but this doesn't answer the question and remember it is hypothetical. The point is not whether they make a 70 amp disconnect it is for argument's sake a 60 amp rated disconnect. Assume the hp is fine for the disconnect.

I believe the max 250% is based on the nameplate not the Nec table. The fla is 30 amps
Click to expand...
Well when you say 60 amp disconnect I think of safety switches, which you know they go from 60 amp frame to 100 amp frame.

Individual breaker enclosures kind of varies with ratings. I've seen ratings of 30, 40, 60, 70 in smaller units. They all physically would accept a 125 amp breaker but may not have the wire bending space for 125 amp conductor if you wanted to go that route for whatever reason.

Not sure if such things are classified as a panelboard or if they are just considered a breaker enclosure. With such miniature breakers you need a bus to connect them to - though there is unit breakers with lugs on both ends I don't recall ever seeing an enclosure specifically for those until you get into the 200 amp frame sizes or maybe into over 250 volt ratings.
 
david said:
430.6(A) (1)
Table Values
Shall be used to determine branch circuit short circuit and ground fault protection

Sent from my SM-S901U using Tapatalk
Click to expand...

That's interesting because if the values in 430.52 are not enough for the motor starting then..
Wonder why that is different


The rating of an inverse time circuit breaker shall be permitted to be increased but shall in no case exceed 400 percent for full-load currents of 100 amperes or less or 300 percent for full-load currents greater than 100 amperes.
Click to expand...
 
Since it is 250% of 40 amps we can then use a 100 amp overcurrent protective device . Can that be used in a 60 amp disconnect .....hahaha
 
I would say you'd be fine as long as the disconnect was rated for the horsepower of the motor it serves.
If we went solely by what the max OCPD could be for motor startup, we'd have some very large disconnects installed for some very small motors.


JAP>
 
Dennis Alwon said:
That's interesting because if the values in 430.52 are not enough for the motor starting then..
Wonder why that is different. Not following you there.
Dennis Alwon said:
Since it is 250% of 40 amps we can then use a 100 amp overcurrent protective device . Can that be used in a 60 amp disconnect .....hahaha
Click to expand...
Click to expand...
The code is MAXIMUM. As long as the breaker holds for start up, you can use anything 100 amps or smaller. If a 60 amp breaker holds it is allowed per code. As you implied the disconnect needs to be rated for the horsepower.
 
Last edited by a moderator:
petersonra said:
Are motor overloads allowed to be used to protect a panelboard bus?
Click to expand...
Panelboard bus is required to be protected by an overcurrent device not greater than the rating of the panelboard, located within or on the supply side of the panelboard.
Dennis Alwon said:
IDK, but it is indirectly do that. I don't see why it can't.
Click to expand...
What kind of disconnect are you envisioning that you would mount a circuit breaker inside it?
 
david luchini said:
Panelboard bus is required to be protected by an overcurrent device not greater than the rating of the panelboard, located within or on the supply side of the panelboard.

What kind of disconnect are you envisioning that you would mount a circuit breaker inside it?
Click to expand...

So is a disconnect considered a panelboard if it only has one dp breaker

1715699810495.png
 
You must log in or register to reply here.
Top