Wire Size?

[jc3l]

Here is my application: Motor Nameplate Info

1phase .75HP motor 10.6FLA 1.0SF 115V 1725RPM

Using a motor watchmem switch (contains overloads) to turn motor off/on
------------------------------------------------------------------------
According to Art. 430.6 (A)(1) refer to Tab 430.248 to get FLA for 1 phase motor
.75 HP = 13.8 FLA
------------------------------------------------------------------------
Then refer to Tab 430.52 for Inv Time Bkr for 250%

13.8 x 250% = 34.5Amp or 35Amp Bkr
-------------------------------------------------------------------------
Overload refer to 430.6(A)(2)
Motor Nameplate: 10.6 FLA
430.32(D)(1) --> (B)(1) --> (A)(1) = 115%

10.6 X 115% = 12.19 overload
--------------------------------------------------------------------------
Wire Size: < 50ft run THHN
If using 430.22(A) 13.8 x 125% = 17.25Amp which would equal 14AWG

However, the wire is only rated for 25 amp and is not to exceed 15 amp according to Tab 310.16 --> 240.4D
Does 310.1 says this does not apply to motor/circuits
--------------------------------------------------------------------------
My question is what is the proper wire size or is something else incorrect in the calculations. The Breaker size vs wire size concerns me??? Please explain and use NEC reference if possible. Thanks.

 

[Dennis Alwon]

I think you answered your own question. 240.4 allows the #14 to be used assuming 75C or better insulation.

The #14 wire will only protect for ground fault and short circuit so it is okay to fuse higher than the 25 amps it is rated for. The motors overload protection would take care of the overload.

So, does the motor have overload protection built in? If not then you would need to have them installed.

You are correct in using the Table rather than nameplate for your calculations.

I did not check all the calc. but you seem to have a good understanding-- more than I do I am sure.

 

[jc3l]

Another question

Another question

Based on above calculations I should also note that the motor watchmen switch w/overlaod is approx. 40 ft from the 35A breaker. Is size 14 wire still ok between the overload protection and the breaker? or do I need size 8 wire - which is way to big for a .75 HP motor? Learning & confused????
Any help is appreciated.

 

[augie47]

You are still o.k. The theory is that the overload device, no matter where it is in the circuit, will prevent the wire from an overload.
The breaker, although rated higher than the conductor ampacity is being used as a short-circuit/ground fault protector will trip if needed.