Conductor Motor Sizing

[rmonroe]

We have a compressor with a single phase motor capacitor start 7.5 HP 208VAC. The run from the panel to the compressor is 250 ft. We are using #8 THHN to supply the motor with a 40 amp breaker. We are experiencing a 50 volt drop on start up. Do we need to up the wire size?

Thanks

rmonroe

 

[iwire]

We have a compressor with a single phase motor capacitor start 7.5 HP 208VAC. The run from the panel to the compressor is 250 ft. We are using #8 THHN to supply the motor with a 40 amp breaker. We are experiencing a 50 volt drop on start up. Do we need to up the wire size?

Thanks

rmonroe

It depends, is the motor starting or not?


The code book shows that motor to have a 44 amp minimum current. The start up current would be about 6 to 8 times that.


Forgetting start up current it would take 3 AWG to stay under 3% drop just steady state running.

If it was my job I would have bid it with at least 3, likely 1 AWG.

 

[rmonroe]

It depends, is the motor starting or not?


The code book shows that motor to have a 44 amp minimum current. The start up current would be about 6 to 8 times that.


Forgetting start up current it would take 3 AWG to stay under 3% drop just steady state running.

If it was my job I would have bid it with at least 3, likely 1 AWG.

No the motor does not start...it trips the breaker.

 

[tom baker]

If its 208 is three phase available? Three phase motors are smaller and easier to start
Compressors are hard starting if it starts loaded. Is there an unloader on the compressor?
Recommend a soft start or VFD

 

[GoldDigger]

Or bite the bullet and rewire.
See which is more economical and would give the longest motor life as a secondary factor.

As stated, the wires are marginal just for the running current at that distance.

If the motor does not turn at all but just hums until the breaker trips, then it is not developing enough torque to overcome the load.
(Is it a fast trip or a couple of seconds? Is the breaker sized to the maximum OCPD specified for the motor, or just for the MCA value?)

PS: A VFD or soft starter might work with the existing wiring, but a simple hard start kit is less likely to.
Tapatalk...

 

[iwire]

The options as I see it, larger conductors. 3 phase, higher voltage, relocate compressor closer to panel. Any one or more would help.

If it is a reciprocating compressor I do not think a VFD or soft start would overcome this much voltage drop. Those compressors really do not want to get moving even when properly unloaded. They have large flywheels combined with drag of the pistons in the bores.

 

[rmonroe]

My question is in regards the the wire size. Do you increase the wire one size after the first 100 ft and there after one size for every 50'? Where can I find this in the NEC?

Once again Thanks

rmonroe

 

[Smart $]

My question is in regards the the wire size. Do you increase the wire one size after the first 100 ft and there after one size for every 50'? Where can I find this in the NEC?

Once again Thanks

rmonroe

The NEC does not require, nor cover any voltage drop considerations for the case at hand. You will have to do at the very least whatever is necessary...

 

[GoldDigger]

My question is in regards the the wire size. Do you increase the wire one size after the first 100 ft and there after one size for every 50'? Where can I find this in the NEC?

Once again Thanks

rmonroe

Precisely what you do is first get a good estimate or measure of the motor starting current, then use formula, tables, or online calculator to determine just what the voltage drop is and what size wire is needed for a particular maximum drop.
This is an area where a calculation beats a rule of thumb any day or night.
In your case, you can also run the calculation backwards and see what the motor current must be to get that voltage drop in your #8 wire. Since the motor is not starting, the normal starting current must be close to or less than 208/150 times that value. That is if your voltage drop measurement is valid. If the breaker is cutting out immediately, you may have a hard time accurately measuring the voltage drop at the motor.



BTW, table 430.248 indicates that a 7.5HP motor will have a full load current in the range of 44 amps. That is not going to go well with a 40A breaker.
Similarly, Table 430.251(A) suggests a locked rotor current in the neighborhood of 265A. Try that in your VD calculator to see what happens.

 

[petersonra]

My question is in regards the the wire size. Do you increase the wire one size after the first 100 ft and there after one size for every 50'? Where can I find this in the NEC?

Once again Thanks

rmonroe

you need to figure the voltage drop for different wire sizes and determine the maximum voltage drop you are willing to accept.

i might run 1/0 AL, just because I like 1/0 wire a lot.