21 amps on #12 wire?

Status
Not open for further replies.
S

sirdle

Member
Location
California
A 15hp 460V 3-phase motor has a FLA of 21A. The instantaneous-trip circuit breaker feeding this motor is set to trip at not more than 168A and has lugs rated at 90?C. Can #12 THHN be used to feed this motor? (The motor has proper OL heaters installed.)
 
Re: 21 amps on #12 wire?

Sorry I was thinking 21 amps when actually the conductor ampacity would need to be 26.25

You would need 10 AWG. unless you had 90 C equipment. ;)
 
Re: 21 amps on #12 wire?

Bob,

What equipment, apart from the breaker, would need to have the 90?C rating? Oh... I guess the starter would. Don't think I've ever seen a starter with 90?C lugs.

Wire nuts are rated at 105?C (I think) and the motor leads are not rated... Is that right?
 
Re: 21 amps on #12 wire?

Sirdle, don't even consider using any conductor at the 90?C rating. The only thing the 90?C rating is good for is derating. Also remember, the lugs are always attached to something and the overall rating will be 75?C max. :D
 
Re: 21 amps on #12 wire?

I don't believe that.

It might usually be true. But always?

Am I splitting hairs?

Edit: Mil Spec stuff for instance, the tolerances and ratings they can afford are pretty luxurious.

[ April 23, 2005, 04:44 PM: Message edited by: physis ]
 
Re: 21 amps on #12 wire?

OK Sam, you are right as soon as someone proves me wrong. I have eaten crow before . . . not too bad with a little salt. :D
 
Re: 21 amps on #12 wire?

No you're right Charlie. It's the military who are wrong, they don't even need those specs, (for everything they own) they just have the ability to demand them.

And you know what's funny, their stuff still breaks! :D

[ April 23, 2005, 05:03 PM: Message edited by: physis ]
 
Re: 21 amps on #12 wire?

Section 430.22(A) requires the motor branch-circuit conductor to be sized at 125 percent of Table 430.250, based on HP and voltage rating.
1.25 x 21A = 26.25A, therefore, No.l0 cu is the minimum size conductor allowed, provided the conductors and terminations are rated at 60 or 75 degrees C. Where an instantaneous trip breaker is used, it is allowed to be sized at 800 percent of Table 430.250. 8 x 21A = 168 amperes. According to Section 430.52(C)(3) the instantaneous trip breaker is required to be part of LISTED combination motor controller.
 
Re: 21 amps on #12 wire?

Am i missing something here? The current is 26.25 amps. The 75 deg rating of thhn is 30 so why would you use # 10? #10 is good for 35 amps at 75 c. What am I missing here?
 
Re: 21 amps on #12 wire?

Originally posted by stew:
Am i missing something here? The current is 26.25 amps. The 75 deg rating of thhn is 30 so why would you use # 10? #10 is good for 35 amps at 75 c. What am I missing here?
Click to expand...
The rating of 12 AWG THHN is 30 amps which is taken from the 90 C column of Table 310.16.

We can not use the 90 C rating as the equipment the conductors terminate to will not be rated more than 75 C. That being the case we must use the 75 C column of Table 310.16

12 AWG Copper @ 75 C is rated 25 amps, the motor requires a 26 amp conductor. ;)
 
Re: 21 amps on #12 wire?

And even though the original question specified that the LUGS were rated at 90C, several people have pointed out that all of the EQUIPMENT also needs to be rated at 90C. Very few pieces of equipment have this rating.
 
Re: 21 amps on #12 wire?

Why would a piece of equipmet have 90? C. rated terminals if using the terminal at 90? would damage or destroy it?
 
Re: 21 amps on #12 wire?

Sam, a good number of terminals have a 90?C rating. The equipment manufacturer would lose the ability to use a lot of good terminations if they had to be rated for 75?C. :D
 
Re: 21 amps on #12 wire?

The only terminations that are listed as 90 degrees C are located only on medium and high voltage equipment, not on equipment rated for 600 volts or less. Don't be fooled by terminals or lugs that are marked 90 degrees C. Only the lugs are rated at 90, not the equipment that you will install them on in the field. See the UL White Book.
 
Re: 21 amps on #12 wire?

Yep. Somethin's goofy with this. :roll:

Am I one of the only guys this seems all globbly gloopy to.
 
Re: 21 amps on #12 wire?

Sam, you are the only one in the world that is crazy, all the rest of us are OK. The guys in white coats will be there shortly. :D
 
Status
Not open for further replies.
Top