Electrical Wire & Breaker Sizes

[benjamming]

I'm going to purchase a Champion air compressor that comes with a Baldor ODP 7.5 HP 1/60/230V motor. Typical motor FLA for this size appears to be ~ 31A. 50A circuit breaker with 6 AWG wire is the path I'm going down. I'm going to use THHN b/c it's cheaper than NMB-6. I'll only have to pull the wire ~ 10' through the garage wall prior to entering the crawl space & basement. My house currently has 10 AWG with a 30A breaker for this service. Existing CBs are Square D but I can get Siemens better. Does all this sound right to everyone? Thanks.

 

[tom slick]

6 awg for a 10' 50A circuit sounds correct to me. are you going to run any conduit for the THHN?

 

[benjamming]

Actually it is ~ 110' total run. I was thinking about installation when I mentioned only 10' to pull behind the wall.

I don't have any plans for conduit. If THHN requires conduit, then I need to go to a different wire that doesn't require conduit. The crawl space is about 5' tall & the basement is 9' tall so easy access there but the garage wall had drywall so that would be a pain.

 

[tom slick]

by code single wire has to be sheathed somehow.

For a 50A circuit that is 110' long, actually 220' when you do calcs, you need 4 awg wire minimum.

 

[andyd]

Originally Posted By: tom slick
by code single wire has to be sheathed somehow.

For a 50A circuit that is 110' long, actually 220' when you do calcs, you need 4 awg wire minimum.
Just curious, How do you arrive at 220'?

 

[tom slick]

Ignore my recommendation of 4 awg, there was an error in my calc. you need a min of 6 awg

when calc'ing wire size you need to calculate round trip.

the formula is CMA= (KxIxL)/ VD
K is 12 for copper constant
I is amperage 50
L is total circuit length 110x2 220
VD is voltage drop, which is 3% max so 220x.03 6.6
CMA is 20000

you then look up the CMA on a chart to determine wire size.

when going by online calculators or "code" rule of thumb they have simplified the information to just the distance between source and load. it is usually based on a 100' run.

 

[chas3]

here is a nice little page with the info you need.

http://www.cerrowire.com/default.aspx?id=8

under technical info you can use the voltage drop table to figure out what you need as far as gauge for the 110'. you will need a minimum of #6 at 230v for 100' for the 3% voltage drop. since you are going 110' you should go to #4, or figure out exactly how far you are going. since this is a long run, you really should just ignore the ampacity chart. the conduit fill calculator says that 1/2" should be enough for two current conductors. as stated by someone else, you need to put the thhn in a conduit, either pvc or emt.

 

[eljefino]

Also consider "service entrance" wire, it's usually aluminum but pretty cheap and won't need sheathing. How the PO hooked my stove up.

Siemens vs square D code conformity depends on whats accepted for your box, it will have a list on the label inside.

Sounds like you already have a subpanel in the garage? Might consider maintaining the original line, so if you blow your main compressor breaker at the main panel you still have lights.

 

[brianl703]

Originally Posted By: eljefino
Also consider "service entrance" wire, it's usually aluminum but pretty cheap and won't need sheathing. How the PO hooked my stove up.


Just make sure to put the anti oxidization compound on it, and sand it with emery cloth after you apply the paste.

 

[mechtech2]

If you use aluminum, get a much larger diameter than if it were copper!

 

[tom slick]

Originally Posted By: mechtech2
If you use aluminum, get a much larger diameter than if it were copper!


That was the mistake I made previously. Al wire would require 4 awg to maintain a 3% voltage drop.

 

[benjamming]

Thanks for all the info & sorry I haven't responded sooner.
There is not a panel in the garage, only in the basement where the underground service comes in.
I'll look into the idea of service entrance wire.

 

[benjamming]

What about Nomex?

 

[tom slick]

Originally Posted By: benjamming
What about Nomex?


you need a fireproof suit?

 

[benjamming]

Originally Posted By: tom slick
Originally Posted By: benjamming
What about Nomex?


you need a fireproof suit?

Romex

 

[tom slick]

that makes more sense, unless you are doing some special wiring.

the wire size will be the same for romex (NMB cable) but it is often not code compliant to be exposed below 8 feet from the floor and cannot be run in conduit except under a specific circumstance.