garage/shop wiring question

[Chris Friesen]

I'm purchasing a house with an attached garage, and will be adding a subpanel for use as a shop. The garage is already insulated and drywalled, so I'll be running conduit (most likely EMT).

The most-filled run will have three 20A 120V circuits for outlets wired with #12, and 2 30A circuits for 240V wired with #10. (One 240V for the dust collector, and one for everything else.)

I'm currently planning to run a single #10 ground wire (bare or green) to ground everything on this run. I think this meets code, but I'd like confirmation. (I'm in Canada so it's CEC rather than NEC, but I wanted to get a general idea if I'm in the ballpark.)

Assuming the above is okay, I'll have 11 wires in that conduit. Given that I'm allowed 11 #10 wires in a 3/4" conduit, does that sound like a reasonable conduit fill? Max spacing between conduit openings will be about 10 ft.

Since this is a one-man shop I'll only ever be using two tools at once, maybe three if someone is helping. I'm working on the assumption that my load diversity is enough that I can use the 70% ampacity derate for

10 conductors in a conduit. Is this reasonable, or do I have to go down to the 50% derate? Should I go with two 1/2" conduits instead of a single 3/4"?

Finally, what's the best way to pull this given that the various wires are going to different places, and one of the 240V circuits is going right to the end? Do I pull one section at a time with all the wires for that section?

Thanks,

Chris

[Lew Hodgett]

Chris Friesen wrote: >

No it won't.

Running multiple circuits in a common conduit requires some very serious derating of the allowable current a conductor can handle.

As a result, one (1) circuit per conduit is the lowest cost way to do the installation.

Don't feel alone, lots of folks see the table of max conductors per conduit and think they can save some money, but alas, not so.

Lew

[Dick Keats]

[Swingman]

I believe what Lew is stating is that multiple conductors in the same conduit reduce the allowable ampacity of the circuit to a point of diminishing returns that is quickly reached. IIRC, this reduction in ampacity is a heat related issue ... but it's been a while.

[Doug Miller]

Please cite the section of the Code you're basing this on.

[John Sellers]

Notes for Table 310-16. More than three current-carrying conductors in a raceway requires derating of ampacity.

This has been part of the NEC for a long time.

[Stoutman]

I'm definitely not an electrician, but I remember something from college about the "right hand rule" and magnetic fields. If two conductors are laying next to each other there magnetic fields will appose each other. Even thought we are talking about AC currents they will still be in the same phase, meaning there magnetic fields will always be opposing each other. I guess this will lead to a small amperage drop?

Not sure??

[Mike Marlow]

I'll expose myself a bit here - a conduit is not a raceway is it? I always thought that section of the code referred to running the likes of romex through a hole in a floor joist, not to wire in a conduit.

[Chris Friesen]

Here's my reasoning.

Up to 6 conductors requires derating to 80%.

According to my simplified code book (I'm in Canada), 7-24 conductors require derating to 70%.

I saw some more complicated stuff based on the NEC that gave a sliding scale for the derating based on load diversity (ie how many circuits are going to be used at the same time): if the load diversity is 50%, 7-24 conductors can also be used derated to 70%, but if the load is not diverse then this requires derating to 50%.

Since I'm not likely to be using more than six of the 10 conductors at a time, I should be okay even under the load diversity calculations.

Given that the derate can be done from the full ampacity of the conductors, #12 T90 starts at 30A, and #10 T90 starts at 40A. Thus, even derated to 70% I can still run #12 with a 20A breaker and #10 with a 30A breaker.

Chris

[Edwin Pawlowski]

I'm not so sure you have a good idea. Forget codes, allowed or not, is it sensible to pull that many wires and have them stop at different branches? You may avoid a lot of frustration and aggravation by running two or three conduits for the system.

[Nova]

Conduit: Round cross section electrical raceway of metal or plastic.

Electrical Raceway: Conduit that houses electrical cables giving them support.

[cdc]

The Canadian code (BC) requires derating whenever there are cables in contact with each other for distances exceeding 24". The specific derating requirements are:

1 - 3 conductors 100% load 4 - 6 conductors 80% load 7 - 24 conductors 70% load 25 -42 conductors 60% load 43 or more 50% load

Cables supplying general lighting or standard plug outlets are derated to

80% under another rule so you can run 2 3-wire or 3 2-wire cables together when running lighting or standard plug outlet circuits.

[Lew Hodgett]

Sounds to me like you are trying to avoid running pipe since just maybe your pipe bending skills are not quite as advanced as your wood working skills.

NBD, bending pipe is an art form.

Use plastic conduit and the problem is solved; however, let's pursue the idea of a bunch of circuits in a common pipe.

First things first, pipe is cheap, copper is not.

The economics of derating copper conductors will bite you in the pocket book, big time.

Pulling several conductors will require a bucket of wire-eze and probably a power take-up, not just a simple fish tape, to pull that wad of wires thru a pipe.

Trust me, you don't want to go there.

If this is a typical 2-car attached garage, I question the need of installing any conduit at all.

There are other ways that are not only simple, but low cost and legal.

Lew

[Mike M]

I won't go into code at this point but as an installer I would stick to no more than a boat in 1/2" and two boats in 3/4" pipe. A boat being three hots and a nuetral for a three phase circuit. If you have single phase and distances aren't too long and wire is thhn then go up to 4 hots and two nuetrals but use some monkey cum to pull the wires. Anything more go 3/4" pipe. If you have more than 180 degrees in bends back off on the wire fill. Invariably some one will come along and go can you get another circuit in that pipe.

Mike M

[Chris Friesen]

Heh...my pipe bending skills are nonexistant. Guess I'll need some extra for practice.

That's just it...from the numbers I saw, and based on the CEC, I can run up to 24 conductors (of #12 and #10) with no real derating at all since for those sizes THHN wire is basically already derated to 70% by the breaker limitations.

Okay, fair enough. I don't have any experience pulling power cables, just some mic cables for a sound system.

Yes, basic two car attached. Already insulated, with vapour barrier and drywall.

I'd love to hear your suggestions.

Chris

[Lew Hodgett]

Had a similar situation in a house I once owned.

Consider mounting a surface mount load center sub panel in the garage on a wall where it will be easy to run the 2P-60A feeder from the main panel.

Below this new sub panel, mount a 4x4 piece of wireway gutter that is connected to the sub panel by chase nipples, lock nuts and thread protectors.

Mix and match as required receptacles that get mounted in the gutter.

Since all the connecting wires between receptacles and branch c'krs are short and in a large cross section, the normal conduit fill, conductor derating rules do not apply.

Connect extension cords as req'd for the various loads.

Know anybody who is an electrician?

If what I have just given you is confusing, they may be able to explain it.

Lew

Lew

[Chris Friesen]

So far so good.

I had considered something like this but had dismissed it as more expensive...looks to be about $6 a foot for the gutter online.

Chris

[Lew Hodgett]

I haven't priced sheet metal for a while, but you only need 3-4 ft.

Less than $25 doesn't seem to be to bad.

Find a Hoffman distributor and get a price on Lay In, Type 1, Wireway.

Don't know where you are in Canada, but Hoffman has sales offices all across Canada.

One of them can direct you to a distributor.

Lew

[Chris Friesen]

Guess I misunderstood. I thought you meant running the gutter all the way around the three walls of the garage--which is more like 70 feet.

Did you mean put all the receptacles right below the panel and have extension cords running across the shop? What about lighting circuits?

Good to know the technical term for the stuff, anyway.

Chris

[Lew Hodgett]

Naw.

Yep.

My guess is you would only have 3-4 at most.

2P-30, Table Saw 2P-40, Air Compressor 2P-30, Dust Collector.

2-4, 1P-20, hand tools, drill press, etc.

If you make them say 25 ft, you could run them against the wall and keep them out of the way.

Use the plug/receptacle as a way to disconnect them when you are finished.

What kind of lighting do you have now?

You only need one(1).

Is this a garage with a finished or an open ceiling?

Lew