Conduit fill - mixed wire sizes?

[DanielVetpath]

I am going to run metal conduit (currently using 3/4" EMT, but could go larger for this if needed) to hook up my air compressor using 6 gauge THHN 240v. I want to run 120v to the compressor automatic drain using 12 gauge THHN. Can I run both circuits in the same conduit? I have found charts listing the number of allowable wires of the same size, but not mixed equivalents. Thanks for any input. Daniel

 

[nehog]

Why not run two hots, and a neutral (all 6 AWG) instead of a separate circuit? That will give you the 120 you need for the automatic drain, make life much easier, and make it so you only have one breaker supplying power to the compressor.

 

[Aceman]

You can fit all that in a 3/4 just fine. You can use #10 for the ground assuming your breaker is 60 amp or less.

Why not run two hots, and a neutral (all 6 AWG) instead of a separate circuit? That will give you the 120 you need for the automatic drain, make life much easier, and make it so you only have one breaker supplying power to the compressor.

The auto drains I've come across plug in. There's no way I'm putting a 15 amp recep on the 50-60 amp compressor breaker.

 

[Charles (in GA)]

Run red/black/white #6 and a #10 ground to a four breaker sub panel at the compressor. This will give you a disconnect at the compressor and a single pole breaker to power the drain. Possibly a little more costly, but more flexibility in what you do with it.

Charles

 

[Alchymist]

You can fit all that in a 3/4 just fine. You can use #10 for the ground assuming your breaker is 60 amp or less.



The auto drains I've come across plug in. There's no way I'm putting a 15 amp recep on the 50-60 amp compressor breaker.

Put one of these on the compressor to plug the drain into:
http://www.acehardwareoutlet.com/ProductDetails.aspx?SKU=30727

 

[buzz4041]

That must be a big motor on that compressor or a long way from the panel to need #6 feeding it. What is the HP on that bad boy ?

 

[mtne]

I keep wondering about that too..... I've got a 60gal 240v compressor and it just called for a 20A breaker. How big are these things ya'll use?

As for conduit fill calcs you'll need to reference NEC chapter 9 table 5 to get the various wire areas, then go back to chapter 9 table 4 to find out the available are of the conduit your using.

For example
2- 6awg thhn = 2*.0507=.1014
1-10awg thhn= .0211
2-12awg thhn= 2*.0133=.0266

.1014+.0211+.0266=.1491 inches squared.

3/4" emt @ 40%fill = .213

.1491 is less than .213 and thus would be allowable.

 

[Thruxton]

Try this: http://www.electrician2.com/calculators/rf_calculator.html

 

[DanielVetpath]

Compressor is 7.5 hp. Seller is recommending 75 amp breaker - also suggesting wire autodrain directly to compressor so that it turns on every time the compressor starts. Not quite sure how - he said run one white wire from the electric panel (off the neutral bus bar?) and use one leg of the 220 supply to the motor? Any suggestions?

 

[Alchymist]

Compressor is 7.5 hp. Seller is recommending 75 amp breaker - also suggesting wire autodrain directly to compressor so that it turns on every time the compressor starts. Not quite sure how - he said run one white wire from the electric panel (off the neutral bus bar?) and use one leg of the 220 supply to the motor? Any suggestions?

See post #5. Pick up neutral and one leg of 220 at the compressor, run to the box pictured, install 15 A fuse, plug drain into outlet. (Mount th box on the compressor if convenient).

 

[nehog]

Compressor is 7.5 hp. Seller is recommending 75 amp breaker - also suggesting wire autodrain directly to compressor so that it turns on every time the compressor starts. Not quite sure how - he said run one white wire from the electric panel (off the neutral bus bar?) and use one leg of the 220 supply to the motor? Any suggestions?

Exactly what I suggested in my reply to you above... Run 3 wires (plus ground, of course) two hot (black/red (or black with red stripe, or taped to mark it, whatever), one neutral (white).

To meet code the white needs to be the same AWG as the hot leads. This is the best and easiest solution to your setup. It worries me that you are considering two power sources to the compressor, that could lead to confusion at some point (such as shutting off the breaker for the drain thinking one is shutting off the compressor's breaker...)

 

[Norcal]

Another option is a control transformer to power the auto drain, I did that on my compressor to provide 120V to run a solenoid valve (manually operated) to drain the moisture from the tank, or at least I did have it that way as that compressor has been disconnected since the spring & finally the project has come far enough along to be able to get it back in service.

 

[Charles (in GA)]

Compressor is 7.5 hp. Seller is recommending 75 amp breaker

That motor is about 33 amp running, peak on startup is probably 200 amp or slightly less, but for just a millisecond or two. My 7.5 hp starts and runs fine on a 50 amp breaker.

Charles

 

[DanielVetpath]

Thanks for all the input. Charles - I had seen your advice in another thread and was thinking along those lines (#6 red and black, #10 ground; #12 white for autodrain; 50 - 60 amp breaker). It will only be a 10-12 foot fun of wire to the breaker box. I will get the machine home and likely ask more specific questions about which lugs to use, etc., to hook up the autodrain. Thanks again, Daniel

 

[nehog]

Thanks for all the input. Charles - I had seen your advice in another thread and was thinking along those lines (#6 red and black, #10 ground; #12 white for autodrain; 50 - 60 amp breaker)...

Don't think code will allow you to use a smaller AWG for neutral than the hot terminals... A failure could shunt higher currents than the 12 AWG could safely handle through it, creating a fire hazard. All three need be the same size.