Motor calculations 430.53 single branch circuit

Status
Not open for further replies.
M

Mustwin351

Senior Member
Location
Texas
I came across the following installation: An air compressor with two 480v 3 phase motors 7.5 hp, nameplate 9 amps with each motor having a motor starter on the wall fed from a single branch circuit fused at 30 amps.

I wanted to check that ground fault and short circuit rating was sized correctly but was having a hard time finding in the code how to exactly size the individual branch circuit.

Do you add the motors together (11 amps each per 430.250) and multiply by 1.75 for time delay fuses?

430.53(B) states If the branch circuit short circuit and ground fault protective device is selected not to exceed that allowed by 430.52 for the smallest rated motor, two or more motors, or one or more motors and other loads with each motor having individual overload protection shall be permitted to be connected to a branch circuit where it can be determined that the branch circuit short circuit and ground fault protective device will not open on the most severe normal conditions of service that might be encountered.

Any thoughts?
 
Is it 1 30 amp circuit feeding both motors ?
If your motor starter does not have SCGF protection, the 30 amp fuse would exceed the allowable protection for 1 individual moto (11 x 1,75).
The 30 amp circuit would be allowable as a feeder for both motors (11 x 1.75 + 11)
(If it's a unit compressor, they normally have nameplates that provide you with manufacturer recommended protection)
 
thanks augie.

Yes one circuit feeds both motors. Motor starters do not have SCGF protection. I don’t believe the compressor had a nameplate.
My thought was to size it as a feeder but I didn’t think it technically qualified as a feeder but I guess it does as the motor starter overloads are the final overcurrent device
 
You could use it as the feeder but you would still need to supply the individual motors with proper GFSC protection.
(oddity in the Code, a 30 amp breaker would be an allowable install since 30 amps is allowed for the motor SCGF protection (11 X 2.5)
 
augie47 said:
You could use it as the feeder but you would still need to supply the individual motors with proper GFSC protection.
(oddity in the Code, a 30 amp breaker would be an allowable install since 30 amps is allowed for the motor SCGF protection (11 X 2.5)
Click to expand...
But isn’t the GFSC provided to both motors through the individual branch circuit with 30 amp time delay fuses?
 
Mustwin351 said:
But isn’t the GFSC provided to both motors through the individual branch circuit with 30 amp time delay fuses?
Click to expand...
The max GFSC for (1) 7.5 hp using fuses is 20 amps (11 x 1.75) where with a breaker it's 11 x 2.5 or 30 amp.
I thinik there is a good reason to have individual GFSC but I'm simply giving Code numbers,.
 
augie47 said:
The max GFSC for (1) 7.5 hp using fuses is 20 amps (11 x 1.75) where with a breaker it's 11 x 2.5 or 30 amp.
I thinik there is a good reason to have individual GFSC but I'm simply giving Code numbers,.
Click to expand...
There’s my confusion as 430.53(B) states:

GFSC is selected not to exceed that allowed by 430.52 for the smallest rated motor, two or more motors, or one or more motors…

I would think you would in this scenario size the GFSC based on using two motors for the calculation and not the smallest one.
 
Mustwin351 said:
There’s my confusion as 430.53(B) states:

GFSC is selected not to exceed that allowed by 430.52 for the smallest rated motor, two or more motors, or one or more motors…

I would think you would in this scenario size the GFSC based on using two motors for the calculation and not the smallest one.
Click to expand...
If both the same size then pick one and call it the smallest one, or pick the other and call it the smallest one, same result either way.
 
Does each motor happen to have a motor circuit protector type overload that takes care of both SCGF and OL?

I can see this being similar to some lift station pumping stations where they alternate pumps, yet will start both if the first one is not keeping up with demand. There will likely be at least two pressure switches if run by dry contact switching methods.
 
Dennis Alwon said:
You size the feeder for both motors then you run separate branch circuits for the motors.
Click to expand...
Since he says it includes an alternator controller, we might need more details and is possible it was intended to be supplied by a single circuit., individual motor protection possibly is included also.
 
david luchini said:
Or not...
Click to expand...
Ok.

NEC FLA for 7.5, 480, 3 phase is 11.0 X 2.5 = 27.5. next size up allowed so 30 amp breaker is still acceptable even for single motor.

Though something like this could be harder to start depending on whether there is any kind of head pressure release on each individual motor, otherwise seen many such size/same volts motors that don't need high torque at starting hold on 15 amp breakers.
 
I guess if it were my project to do the installation, if it came from manufacturer with a single terminal for supply conductors hopefully it also would be labeled for a single circuit supply specifications. If not something that is listed or follows listing standards, you either run the single circuit or do some field engineering on what was sent out. Can always try the 20 amp fuse and increase to 30 if it doesn't hold. Kind of sort of need to have #10 conductors though.
 
Dennis Alwon said:
You size the feeder for both motors then you run separate branch circuits for the motors.
Click to expand...
My thought is it should be sized as a feeder for GFSC since the circuit feeds two separate motor starters as opposed to sizing it for the smallest motor. But I don’t believe you then need to run individual branch circuits.
 
kwired said:
Does each motor happen to have a motor circuit protector type overload that takes care of both SCGF and OL?

I can see this being similar to some lift station pumping stations where they alternate pumps, yet will start both if the first one is not keeping up with demand. There will likely be at least two pressure switches if run by dry contact switching methods.
Click to expand...
No each motor is fed from its own motor starter.
 
Status
Not open for further replies.
Top