Motor Conductor Sizing

[ksvalentine]

At work they have a 208V 55A, 7.5 HP motor they have wired with a 6/4 SOOW cable. When calculating the conductor size, I looked at 400.5(A) table, and it appears as though they should have used 4/4 SOOW. I'm not sure if I'm right, but maybe someone out there could tell me why one or the other would be correct.

Thank you in advance.

 

[petersonra]

At work they have a 208V 55A, 7.5 HP motor they have wired with a 6/4 SOOW cable. When calculating the conductor size, I looked at 400.5(A) table, and it appears as though they should have used 4/4 SOOW. I'm not sure if I'm right, but maybe someone out there could tell me why one or the other would be correct.

Thank you in advance.

The first question I would ask is can you "legally" wire this motor with cord in the first place.

According to my Square D slide rule chart, a 7.5 HP 3 phase 208V motor is about 25 A and requires only #8 conductors. 400.5(A) seems to agree with #8 as it says SOOW #8 is good for 35 amps.

 

[ksvalentine]

The first question I would ask is can you "legally" wire this motor with cord in the first place.

According to my Square D slide rule chart, a 7.5 HP 3 phase 208V motor is about 25 A and requires only #8 conductors. 400.5(A) seems to agree with #8 as it says SOOW #8 is good for 35 amps.

Yes, this is a military standard cable for equipment on a mobil unit. It is not being wired in a structure. The table I looked at has 6AWG for 45A, and 4AWG for 60A. Because the FLA is 55A, I thought the 4AWG would probably be the correct guage. I don't know if there are any other special rules that I'm not aware of yet. The length of of the cable is approximately 12 feet long.

 

[augie47]

Apparently you are talking about a single-phase motor. 430.248 lists the FLA at 44 requiring a 55 amp conductor as you state, and, also as you state 6/4 is not rated for 55 amps.
If in fact a cord is allowable under the rules of 400.7 & 400.8, and the motor is continuous duty per 430.22, a #6 would seem to be not to Code.

 

[ksvalentine]

Apparently you are talking about a single-phase motor. 430.248 lists the FLA at 44 requiring a 55 amp conductor as you state, and, also as you state 6/4 is not rated for 55 amps.
If in fact a cord is allowable under the rules of 400.7 & 400.8, and the motor is continuous duty per 430.22, a #6 would seem to be not to Code.

No, it's a 3 phase 208V motor.

Oh, Looking at table 430.250 shows a 7.5 hp motor, 208V and it is a squirrel cage type.

 

[raider1]

No, it's a 3 phase 208V motor.

A 3 phase 208 volt motor should have an ampacity of around 25 amps according to Table 430.250. Where are you getting an ampacity of 55 amps?

Chris

 

[ksvalentine]

A 3 phase 208 volt motor should have an ampacity of around 25 amps according to Table 430.250. Where are you getting an ampacity of 55 amps?

Chris

The amps are on the name plate.

This motor is being used for powering a hydrolic lift.

 

[ksvalentine]

The amps are on the name plate.

This motor is being used for powering a hydrolic lift.

Also, it's 400Hz output frequency.

 

[timt72]

Looks like 430.22 should apply , which gives us 125% or 58.8 amps . this will require A #4 according to column A
Tim

 

[ksvalentine]

Looks like 430.22 should apply , which gives us 125% or 58.8 amps . this will require A #4 according to column A
Tim

Thanks Tim.:grin:

 

[ksvalentine]

Thank you All

Thank you All

I appreciate the concentrated effort by all of you. I'm going to talk to the electrical team and discuss this with them, and point out the code that reflects these findings.

Geat forum!

 

[timt72]

upon further investigation "they may have used Table 430.22E I do not know the application of your motor but this could also have to do with the size of the wire Good luck just watch how far you stick your neck out.
Tim

 

[petersonra]

Yes, this is a military standard cable for equipment on a mobil unit. It is not being wired in a structure. The table I looked at has 6AWG for 45A, and 4AWG for 60A. Because the FLA is 55A, I thought the 4AWG would probably be the correct guage. I don't know if there are any other special rules that I'm not aware of yet. The length of of the cable is approximately 12 feet long.

So what we are talking about is basically a power cord. I am not real sure that the NEC is a good place to receive guidance on such a thing.

I would be looking at whatever specification says you can/should wire it this way in the first place.

 

[ksvalentine]

So what we are talking about is basically a power cord. I am not real sure that the NEC is a good place to receive guidance on such a thing.

I would be looking at whatever specification says you can/should wire it this way in the first place.

The SOOW cable is being run from a main control panel, (all equipment mounted on and a part of the mobile unit). The control panel receives power from external source. Max system voltage for main control panel, 120V, 100Amp. Everything is hard wired to the mobile unit.

So far we have realized the 6AWG is not correct, but the 4AWG would be right. It appears to fall under 430.22(e), as pointed out by someone else on this thread. It is a 5min on /2hour off, 8 hour duty cycle. This was a custom ordered electric motor, not a standard 7.5 hp.

I believe they are going with a THHW, 90?C, in pvc, to keep the diameter of the the cable at a minimum. A 55A*.85 is 46.75A, giving us 6AWG. Afterall, 4AWG is pretty stout. (4/4 that is).

Thanx all!

 

[S'mise]

The amps are on the name plate.

This motor is being used for powering a hydrolic lift.

I'm sure someone cought this but I didn't see it mentioned;
Name plate (FLA) is for calculatiing overloads. Is the (FLC) motor not listed in the tables?

(I don't have a copy with me)

 

[petersonra]

The SOOW cable is being run from a main control panel, (all equipment mounted on and a part of the mobile unit). The control panel receives power from external source. Max system voltage for main control panel, 120V, 100Amp. Everything is hard wired to the mobile unit.

So far we have realized the 6AWG is not correct, but the 4AWG would be right. It appears to fall under 430.22(e), as pointed out by someone else on this thread. It is a 5min on /2hour off, 8 hour duty cycle. This was a custom ordered electric motor, not a standard 7.5 hp.

I believe they are going with a THHW, 90?C, in pvc, to keep the diameter of the the cable at a minimum. A 55A*.85 is 46.75A, giving us 6AWG. Afterall, 4AWG is pretty stout. (4/4 that is).

Thanx all!

I would say based on what you have written that it is not premises wiring, thus is outside the scope of the NEC.

I still don't see how you get to 55A with a 7.5 HP motor.

You mentioned it was 400 Hz. Often there are military standards that are used for these applications that are probably what you should be looking at rather than the NEC. And these standards often allow ampacities far exceeding the NEC.

The cable for this application more closely approximates NFPA79 coverage rather then NFPA70 (aka NEC). However, IIRC, a few years ago they changed NFPA79 so it is the same as NEC for conductor ampacities.

BTW, is the 4th conductor a ground? If it is by chance a current carrying conductor you need to derate the cable.

 

[staticcontrol]

I work with motors all the time. One thing that is a FACT is that you're sizing the conductor completly wrong. The amps given on the name plate is not intended to size conductors. You have to look at the horsepower rating then reference that to the FLA table 430.250. 7.5 HP under 208v 3 phase is 24.2 amps. Next - 24.2amps x 125% is the amps you size your conductor too. You need Size #8 before any adjustments to ambient temperature and continuous load. Now the wire size will increase after you do those calculations. 3 elements to motor power are....conductor size,overload protection and circuit breaker selection.

You have some more calculations to do and now you know the steps.