compressor

Status
Not open for further replies.
M

mickeyrench

Senior Member
Location
edison, n.j.
i'm wiring a compressor rated at a total of 84 amps and the only breaker i have left availabe in the panel is a 100amp. i'm wondering if the 100a will hold on startup. unit is rated at fla 84a at 208 volt 3 phase. also at the unit a 120 volt recept is needed can that circuit be ran in the same conduit as the feed for the compressor.
 
read 430.52. what's the code letter on the motor? what type of motor running the compressor? @ 84a FLC, branch circuit ampacity will have to be 105 amps; that's all i can tell you for now.
 
For that specific motor on that specific breaker, trying it will be the best test.

And yes, they may share a conduit, with the usual sizing and derating issues.
 
Personally, I would not run a motor directly from a breaker. You are asking for breaker tripping problems if you do so. The 100 amp breaker should be of sufficient size for the 84amp FLA motor. Run a conduit to a fusible disconnect with NEC sized dual element time delay fuse. The fuses will compensate for the locked rotor start-up current of the motor by not blowing immediatly, whereas a circuit breaker will trip instantly upon the 2x-3x 84amps the motor will draw at start-up. You can run the receptical circuit in the same conduit with no problems provided you use 600v insulated wire for both the rec. circuit and the motor circuit(assuming the two circuits are different voltages). You may also want to install a contactor with a correctly sized motor overload on the load side of the fusible disconnect.
 
compressoe

compressoe

i guess i did'nt make it clear about the way i had planned to wire compressor.
there is a starter and a disconnect at the unit but i am concerned that the 100amp breaker would trip on start up. but if you say that if i put a fused disconnect with daul element fuses at the unit that would keep the cb from tripping . is that correct? thanks again
 
elvis_931 said:
Personally, I would not run a motor directly from a breaker. You are asking for breaker tripping problems if you do so. The 100 amp breaker should be of sufficient size for the 84amp FLA motor. Run a conduit to a fusible disconnect with NEC sized dual element time delay fuse. The fuses will compensate for the locked rotor start-up current of the motor by not blowing immediatly, whereas a circuit breaker will trip instantly upon the 2x-3x 84amps the motor will draw at start-up. You can run the receptical circuit in the same conduit with no problems provided you use 600v insulated wire for both the rec. circuit and the motor circuit(assuming the two circuits are different voltages). You may also want to install a contactor with a correctly sized motor overload on the load side of the fusible disconnect.
Click to expand...

Dual-element fuses do not compensate for the locked rotor current, they accomodate it. Putting fuses between a breaker and the motor does not overcome the sizing issues of breakers.

I have never seen a thermal magnetic circuit breaker trip instantly at 2-3x full load amps. Most circuit breakers can handle inrush currents of 6x and some are up to 10x. But many breakers sized at 125% cannot accomodate the accelerating current/time, this is why the NEC allows breakers to be sized higher.
 
breaker operating a heavy motor load

breaker operating a heavy motor load

This breaker seems to be sufficiently sized to withstand the starting current most breakers seem to have a couple (I was taught 3.5 second deleay for starting current) of second delay for startup. The more important problem that I see is that if this breaker is connected to a motor load which starts frequently and the breaker is a plug in style the buss bar connection could become permanently damaged over the course of a year by high starting current. For heavy motor starting loads I would reccomend bolt on breakers
 
can just over size your breaker and install a 100a time delay fuse to compensate for the locked rotor current?
 
Status
Not open for further replies.
Top