motor circuit reject.

[stew]

Just had an inspector tag a garage I wired that included a circuit for a compressor motor. The motor nameplate amps are 24 with no hp shown on the nameplate. I installed a 60 amp breaker in the sub with # 10 wire and he tagged it as" # 10 wire on 60 amp breaker". No code reference of course. Says " must be no more than 30 amp breaker for # 10 wire".. I guess i must show him my calcs eh and also how to read the code book?

[ March 16, 2005, 07:02 PM: Message edited by: stew ]

 

[hardworkingstiff]

Re: motor circuit reject.

Curious Stew,

Why don't you practice on me. What code references are you using to justify the 60 amp breaker?

 

[bphgravity]

Re: motor circuit reject.

430.52

 

[alberto]

Re: motor circuit reject.

hello stew, i think i can help you whit this information,
conductor size: 1.25 x 24 = 30 (use 8 AWG at 60?C)or you can install 10 AWG;

short circuit and ground fault protection: 2.4 x 2.5 = 60 amp (circuit breaker);

overload protection: 24 x (1.15 to 1.25) = 30 amp (it could be fuses);

 

[stew]

Re: motor circuit reject.

430.52 for breaker sizing and 430.22(A) for wire size.My clac were of course the same as the other getelman here . Not sure why he would want to use a # 8 however because a 10 is all that is required. Thhn 10 awg is good for 35 amps. 8 is good for 50.Motor has integral overload protection so none is required externally.

[ March 16, 2005, 09:01 PM: Message edited by: stew ]

 

[alberto]

Re: motor circuit reject.

if you know the rate of the terminations (e.g. lugs, etc.),
if i wouldn't have that information, i would install a conductor 8 AWG at 60?C.
if your motor has an over load protection for 30 amp, i dont see any problem with your installation.

 

[stew]

Re: motor circuit reject.

thanks for your response albeto but i was actually being facetious here. The inspector is the one who needs the guidance here . I already know the proper method. I guess I will also have to teach hiom how to read

 

[wmhwdh00]

Re: motor circuit reject.

In ref to the 35 amp wire and the 60 amp breaker. Wouldn't the wire burn up before the breaker tripped in an overcurrent condition????

 

[petersonra]

Re: motor circuit reject.

Originally posted by wmhwdh00:
In ref to the 35 amp wire and the 60 amp breaker. Wouldn't the wire burn up before the breaker tripped in an overcurrent condition????

The wiring is protected by the motor overloads. The CB is for short cirucit protection.

 

[stew]

Re: motor circuit reject.

good call bob,right dead on>

 

[stew]

Re: motor circuit reject.

Talked to this inspector by phone today and he now agrees somewhat reluctatantly that maybe I just mite have done a correct calc. Then he gets hung up on overload protection I guess as a way to sorta save face a bit. However the motor is Klixon protected integral with the unit. Says hes never see an "integral" overload on this large of a motor. The motor is 24 amps which makes it fall between 17 of a 3hp and 28 of a 5 hp from the tables. In any event Ive seen integral kloxon in motors as large as 10 hp single phase. He says "I waill look at this a bit closer when I do the final" Anyhow we are now communicating in the same language I think.

 

[sparky59]

Re: motor circuit reject.

Even though the motor has integral overload protection. Why would you use a 60 amp breaker on #10 wire when a 30 amp breaker is sufficent for the load. And would be safer in the future if the circuit was used for another application that didn't have integral protection?

 

[electricman2]

Re: motor circuit reject.

Originally posted by sparky59:
Even though the motor has integral overload protection. Why would you use a 60 amp breaker on #10 wire when a 30 amp breaker is sufficent for the load. And would be safer in the future if the circuit was used for another application that didn't have integral protection?

The original post gave the nameplate amps as 24. A 30A breaker probably could not carry the current required to start the motor and bring it up to speed without tripping. Breakers used as ground fault and short circuit protecton for motor circuits are sized to carry this starting current.

 

[jwelectric]

Re: motor circuit reject.

I think that the inspector may have been right with the red tag but not for the reason given. I will stay with this motor being a Continuous Rated Motor and forget the fact that it could fall under intermittent duty.

430.6 (A) (1) tells me to use the tables in 430.147 ? 150 (?02 code) and tables 430.247 ? 250 (?05 code) when sizing the conductors and over current for motors. When the name plate doesn?t have the Horse Power marked then we use the marked amperage to find the HP from the tables.
Looking at the tables I find that there is a 24 amp 2 horse power motor at 120 volts then you are right, but if this a 240 volt motor we have a problem.
At 230 volts single phase I find a 3 horse at 17 amps and a 5 horse at 28 amps. At 230 volts three phase there is a 10 hp motor at 28 amps.

The bottom line is that when the horse power is not marked on the motor according to 430.6 for the branch circuit conductors and the over current has to be calculated from the tables. Not knowing the voltages or the number of phase conductors I can?t give you a conductor nor overcurrent size.

Give us some voltages and phase information and we will get the job done

 

[iwire]

Re: motor circuit reject.

Originally posted by sparky59:
Why would you use a 60 amp breaker on #10 wire when a 30 amp breaker is sufficent for the load. And would be safer in the future if the circuit was used for another application that didn't have integral protection?

What size EGC do you run with a 60 amp circuit?

It is the same concept, the 10 AWG will open the breaker under fault conditions.

As far as what happens with the circuit in the future that would be the next installers responsibility to make sure the circuit is suitable for the changes they may want to make.

Would you run 200 amp SE for a 100 amp service in case they may increase the service size in the future?

 

[bob]

Re: motor circuit reject.



[ March 26, 2005, 11:04 PM: Message edited by: bob ]

 

[Brinkmann]

Re: motor circuit reject.

As a guess, the inspector may be looking at the Appendix? The tables for "Ampactities of Two or Three Insulated Conductors Rated......."

On the bottom of the table it says "unless otherwise specifically permitted elsewhere in the code....." then says the current shall not exceed 15 amps for # 14, 20 amps for # 12 and 30 amps for # 10 copper.

It's my understanding that 430-21 would be the "unless otherwise" and sizing would be as stated above ( 125 % of motor current).

Am I on the right boat or have I fell into the water??

Thanks

 

[sparky59]

Re: motor circuit reject.

Even though this installation is apparently allowed. #10 wire protected by a 60 amp breaker would have to be in metal conduit on one of my jobs.