Motor Leads for Large Motors

[Codemaster]

I am sizing motor leads to a 300hp motor. Per NEC, I need ampacity of 361A x 1.25 or 451A. The installation is in a locale that gets very hot. Temperatures over 110 deg. F are not uncommon. Using derate of .82 for 75 deg. C conductors at 105 - 113 deg F. ambient and code requirement of sizing motor leads for 75 deg. C conductor gives me an ampacity of 550 at 30 deg. C. That means my motor leads will be 1250 kcmil. Of course that won't fit the motor lead termination box. The soft start (by a major manufacturer) is wired internally with 350kcmil AWM (Appliance Wiring Material) with 105 deg. C insulation. Of course AWM is not mentioned in NEC and it is doubtful that the lugs are rated at 105 deg. C in the soft start enclosure.

Does anyone have any suggestions for NEC allowed smaller conductors or for terminating at the motor? Another option is two sets of parallel 300kcmil. How would you terminate that at the motor, remove motor termination box and replace with jbox?

 

[busbar]

Codemaster—review [99NEC] 430-10, -11, -12, 13. and reject the motor for noncompliance as needed. {Ehhm, 300 is not a “large motor” to many.}

AWM is an abbreviation for appliance-wiring material. A somewhat parallel rating may be MTW…machine-tool wire. For raceways, you need cable marked with NEC building-wire labeling.

thread 238-17855
thread 238-22703

 

[electricpete]

Is there a higher temperature grade insulation available which will permit smaller conductor? (if terminations are qualified for the higher temperature as well).

 

[electricpete]

I should hastily add that I was thinking out loud. Of course you need to assure that any solution you pick is safe and meets code requirements.

 

[dpc]

Just a couple of additional points:

If your ambient is really that high, make sure your motor is rated for above normal ambient. Otherwise, you will have to derate the motor for any ambient above 40 deg C (for NEMA motor).

Use 90 deg C insulation and de-rate for temperature based on the 90 deg C ampacity. This is OK, even for 75 deg terminations, provided the de-rated ampacity does not exceed the 75 deg rating. i don't have my NEC in front of me, but I believe this is acceptable.

The wiring inside a starter or other device will almost always be smaller than your field wiring. Sometimes it is much smaller. The magic UL label keeps it from overheating. Of course, if your starter is subjected to 115 deg F temps, you may have problems.

 

[Codemaster]

Thanks everyone for the suggestions.

I know that 300hp is not "large" for everyone, however I usually try to use 4160V, not 480V, for anything over 200 to 250hp.

I also used an incorrect term. "Derate" (or adjustment) is the term the NEC uses for accounting for more than 3 current carrying conductors in a conduit. "Correction" is the term the NEC uses for accounting for ambient temperature.

At first I did not think that I could start out "correcting" with the 90 deg. C ampacity, since this corrected ampacity is the amps that will cause the insulation temperature to reach the maximum of 90 deg. C. and of course you would not want to do this if the motor leads are required to be sized with a rating at 75 deg. C. However a closer look at 110-14(c) that the higher rating can be used for adjustment, correction, or both. Since the motor leads supplied with the motor are required to be rated at 125 deg. C (why the code requires sizing the condouctors to the motor at 75 deg. C when the leads are rated at 125 deg. C is beyond me), the code ampacity calculations are conservative in my view anyway, and the code allows it (110-14(c)), then I will do it. Of course now this brings up the question, for the 90 deg. C conductors, should I "correct" with the 90 deg factor (.87) or the 75 deg factor (.82) if the NEC does not want to exceed the 75 deg. ampacity? If I use the 75 deg factor (.82) then I am right back where I started when I first asked the question. If I use the 90 deg. factor (.87) then I can get down to 900kcmil or still parallel 300kcmil.

One thing that I never see mentioned is that most common wire used is THHN/THWN. THHN is rated 90 deg. C in dry and damp locations. My motor is going outdoors (which is a wet location), so I either can't use the 90 deg. C ampacity for THHN/THWN or I must use XHHW-2 (which is better anyway for pulling) and then be able to use the 90 deg. C ampacity before adjustment or correction.

I still would like to hear from anyone else if they have any more ideas. I like the comment that the UL label keeps the soft start from overheating. I am attempting to contact the manufacturer to see what codes they designed the soft start enclosure to meet. I assume it is some UL code.

 

[dpc]

I believe that you can use 90 deg C ampacity as the basis for the temperature correction **provided** that it is not applied above the corresponding 75 deg ampacity.

I know that this is allowed for derating required by more than three conductors in a conduit, and in principle, I don't see any difference. I suggest you contact your state electrical inspector and ask the question. Let us know what you find out.

The reason that you must be concerned about 75 deg ampacity is because of the terminations at the other end of the cable. I'm not aware that the NEC specifically requires 75 C rating be used for motor conductors. If you have terminations rated for 90 C conductors, then you can use the full 90 C ampacity.

Also, your motor terminations must be rated for the max conductor temperature being applied.

Hope that helps.

 

[jbartos]

Suggestion: In some instances a special room is built for the motor and its controls accommodation, which may have an air conditioning unit. The VFD may have its air conditioning unit. However, the conductors with higher conductor temperature Tc rating exist, e.g. Silicone SA conductor is rated for up to 200°C, Z and ZW conductors are rated for up to 150°C, etc. Also, the motors are built for higher temperature environment. If the motor has an Insulation rating NEMA Class H (with insulating material Class 220). This motor is permitted to continuously run at 220°C that consists of ambient temp 40°C and the motor temperature rise 180°C. If the motor does not generate temperature rise 180°C, then the motor ambient temperature may be higher but not high enough to cause the motor to exceed its the NEMA Class H 220°C, which is the motor absolute limit for the motor temperature.