Wire sizing

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Steve528

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I'm attempting to wire a 40a circuit from a residence to a detached garage. The circuit is 75' long and will be enclosed in RNC for a 35' distance. It appears that I should use a #8 wire for ampacity and voltage drop constraints at 240v. However I want to provide two 20a 120v circuits in the garage.

The hot wires for this circuit will be connected to opposite legs of the incoming service, and each to a separate 20a breaker in the garage. I want to be clear that the neutral wire is correctly sized in the event of unbalanced 120v loads, which will be the normal case. I think it won't be an issue since I'm connecting to opposite legs of the incoming service, but I want to be sure.

Thanks for your help.
 
Re: Wire sizing

It would not be a 40A circuit if you are using 20A O/C protection.
If you are simply running (2) 20A 120V.branch circuits to directly supply outlets in garage, #10 hots, #10 neut. & #10 equip. ground should easily suffice.
However, if you plan on #8 wire anyway,consider a 40A 2-pole bkr. in service panel with #8 hots, #8 neut. & #10 equip ground as a 40A feeder to a small load center-type panel in the garage.
In either case, a full size neutral is best bet,
& is usually required.
Also, disconnect means requirements of Article 225 may very well come into play at the detached garage.
 
Re: Wire sizing

Thanks for the reply.

I will have a 40a two-pole breaker in the service panel. I would like this installation to be capable of handling a 40a 240v receptacle in the garage if there is a future need for it, that's why I don't want to use a smaller wire.

By the way I've received some misleading information on what type of wire I can use for the underground portion in PVC conduit. I thought I needed moisture resistant wire. I've been told I can use regular NM since it's in conduit. Any thoughts?
 
Re: Wire sizing

I've been told I can use regular NM since it's in conduit. Any thoughts?
Click to expand...
Type NM is not listed for wet locations such as underground raceway. I would use #6 USE aluminum direct buried. JMO
 
Re: Wire sizing

We just installed a 60A service to an out-building using THHN #4 Hots #4 Neut. #8 EGC buried in NMC to a grounded panel. The ground and Neutral were kept separate and the wiring was protected by the 2 pole 60A breaker in the main panel. Local plugs and lights were protected by breakers in the out-building panel.

The grounding of the out-building panel was via a 5/8" local ground rod.

Seemed to pass no problem.
 
Re: Wire sizing

pixmxr, I believe what you are describing is a "FEEDER" not a "SERVICE".

I guess you connected the local ground rod to the equipment ground bar in the out-building panel. If so, it sounds fine.

Has anyone ever seen any THHN in last ten years or so that was not THWN rated also ?
 
Re: Wire sizing

Sorry Guy, really my work is better than my posting. Yes it is THWN, I'm just in the habit of asking for the older THHN when I go to the parts counter.

As for the ground rod, yep, it's tied to the ground bar in the out-building.

Rained like crazy here lastnight and the lights still worked, always a good sign.

DAve
 
Re: Wire sizing

I don't know how large this garage is going to be nor have I done any preliminary calculations but 40A sounds a little light if one is trying to plan for some future demands. It is quite common for a garage to use more than just a few lights and a power tool once in a while. Seems today's garages tend to collect a lot of other heavier duty items such as welders, air compressors, etc. Have you considered the possibility of not having enough power if these types of items are added to the load? If it were my garage, I would much rather pay the extra few dollars now than redo it later for a much higher cost.

Bob
 
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