Branch Circuit in Garage – Conduit type?
freestate1
| Posted in Construction Techniques on
I plan to run a 100′ 20A branch circuit to my garage. Wiring will be surface mounted in the garage, and I want to use some form of conduit for protection.
I was planning on sheathed 12-2 (“Romex) and 1/2” thin wall conduit. Will I have to de-rate the circuit?
Or should I use MC? How about non-metallic conduit? Any and al feedback appreciated. Thanks, guys.
Replies
I believe that "Romex" is not meant to be used in conduit. Why not use THHN inside some EMT conduit?
First of all, for a 100' run, you probably want to go to #10 or even #8 wire, to prevent excessive voltage drop at a 20 amp load.
If you're going to use conduit, PVC is probably best since it won't rust and is easy to assemble. You don't need Romex for the underground run in conduit, individual conductors (look for wire marked THHN or THWN) would be appropriate. Three #8 wires will fit in a 1/2" schedule 40 PVC conduit, but it will probably be easier to pull through a 3/4". Put a junction box at either end where you can transition to Romex inside the buildings.
You could also use direct bury multiconductor wire (marked UF) and then skip the conduit, except for a section at either end to protect it where it emerges from the ground and enters the buildings.
100 feet is a long way to dig... are you going to hire that out, or rent a trencher of some sort? Assuming you're just going across the yard, if the wire is in PVC conduit it should be 18 inches deep minimum; if it's direct bury it needs to be at least 24 inches deep.
I probably should have been clearer in my original post. The run is 100' from the panel to the outlet, and is all indoors. It's an attached garage so I don't need to bury any of it. About 50' will be run through the basement ceiling joists ("romex"), and 50' is in the garage. As I interpret it, code would allow me to run bare "Romex" along the interior walls of the finished garage if the area is deemed to not present a hazard from physical damage to the cable. But, I would prefer to run it in conduit so that an errant yard tool or something doesn't damage it.
I know that Romex is not intended for use in TWC, but understood that it can be used if derated as appropriate. Stuart, sounds like you recommend PVC and THHN/THWN for the garage portion.
There is no requirement for derating NM-B in conduit. But there are a couple of catcha's.
First it does not pull easy and requires extra large conduit. Typically it works fine for protecting a drop (straight line), but anything else it is just not worth it.
The other is if you use metal conduit then you need to make sure that it is grounded and you need a connected on the end so that the NM is not cut by the end of the conduit.
So it what is practical is to use PVC just as short sleave or to use full conduit system, either PVC or EMT, and individual wires.
With that distance I would think about runing a little larger cable to a sub-pannel in the garage.
Ah. I thought it was a separate garage some distance from the house. In that case, I'd go along with what Bill Hartman said as far as what kind of conduit to use. If the run is protected inside the house you could go with NM-B wire without a conduit. If it's exposed, or if your local code requires conduit, PVC or EMT will work. PVC would be the easiest choice along with individual THHN/THWN wires. Still, with that distance you probably should have larger wire - do a google search to find a voltage drop calculators online, and use it figure out what size is needed.