Shop wiring question Num 1256

Hey all: I'v got a detached garage/shop that has a separate 100 amp sub panel. I'm wiring from the sub panel to the individual outlets in the shop. I wanted to run 3 separate 20 amp circuits of 12AWG THHN stranded wire in a single piece of exposed 1/2" EMT (conduit), and figured I'd have to run 3 sets of 3 wires each that would likely put me over the 40% fill limit. I called the city inspector and he said that don't have to run 9 wires down the conduit, I can instead run 3 common's, two neutral's (1 for each phase, 1 of which will be shared by 2 circuits), and use the conduit as the ground. That would be 5 wires total run in the tube. From the information I've gotten to this point, this is wrong. I'm skeptical of 2 breakers sharing 1 neutral, and of the EMT providing the only equipment ground. However I'm just a 1 man shop so I'm only running 1 tool on 1 circuit at a time. Does anyone have a copy of the 2002 NEC that would clarify this? I have the Illustrated guide the NEC in front of me, but it assumes that you also have the actual NEC book to reference, so it's sort of like reading every other paragraph in a novel. Thank you for any advice: Dean
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Sounds like a multi-wire circuit. That should Be OK. I believe there should be a tie on the or a double pole breaker , and of course, each half of the breaker needs to catch oposite sides of the buss's. Post it over here http://www.selfhelpforums.com/forumdisplay.php?f  This guy(Warran) knows the code "By Heart" And Check out there Main Page. http://homewiringandmore.com /
Thanks, Tony D.

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A tie shouldn't be required if this is wired as the inspector suggested. A handle tie may be used for safety purposes if both of the circuits terminate in the same outlet box.
What the OP wants to do is run three 120v 20a circuits through the same conduit. Two of the three hots can share a single grounded conductor (aka neutral) if the two hots are split across the two 240v legs. The other hot should have a dedicated grounded conductor.
Note that the two current carrying conductors which share the single grounded conductor should be of equal length to the grounded conductor. Strangely enough, in this configuration, if both of the 120v circuits are equally loaded, the current flow in the grounded conductor will be zero; since the currents cancel, the worst case (with one side fully loaded and the other with nothing), the neutral will still only carry 20a.
EMT is suitable as a grounding conductor, but flexible metal conduit is not, and a separate grounding conductor must be pulled.
(hot leg - current carrying) (grounded conductor - neutral) (grounding conductor - ground)
scott

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Thank you for all the responses: I hadn't thought at all about the phase of 2 circuits sharing 1 neutral but I think I understand now. One phase is 180 degrees off of the other so if you were looking at 2 "sine waves" the peaks would be the reverse of one another? I'll go ahead and use the EMT as the ground as I have only 2 very manageable short runs. 3 circuits in one, 2 in the other. And to BRuce: I originally wanted to run 3/4" EMT but my suppliers (HD/Menards), wanted over twice the component cost of 1/2", (handi-boxes, face plates, connecters), so I went with 1/2". Again, thank you to all: Dean
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You should rethink using the conduit as grounding. Pull a ground wire in there. It *is* legal but there is something reassuring about that wire. You should have plenty of room in the conduit if the plan is the same.
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you are welcome and I do understand. if you ever need to add additional circuits the wire will be reusable.
the reason I recommend using the ground wire is that the "fit" of EMT is somewhat loose and you are relying on a steel screw for continuity, these screws rust over time and the ground becomes more resistive.
I've seen it both ways, from experience I always run the wire.
BRuce
dean b wrote:

--
---

BRuce


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>I can instead run 3 common's, two

That will work. I agree with the suggestion of using a 2 pole breaker for the multiwire circuit (shared neutral) I would also use different colors for the hots on that run (red/blue?) so you won't get them confused later. Since 9 #12 is max for 1/2" EMT you still have room for a green wire ground. I would certainly do that. You only need one. The reality is you can run 3 hots, 3 neutrals and one ground. That is still only 7.
Buy a bottle of pulling lub. You will appreciate it when you start pulling in the wires. It works better than all the makeshift solutions and it won't hurt the wire.
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run the ground wire
if it isn't already run, run 3/4"... a week from now you will want another circuit.
2 - 110 circuits on one neutral is not a problem, as long as the 2 hot wire are on different phases, otherwise you could overload the neutral.
BRuce
Greg wrote:

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He will start bumping up against derating problems if he gets more than 6 current carrying conductors in the pipe. As it is the 90c THHN will compensate for the 80% derating factor (4-6) and still allow him to stay with #12. This is one place where multiwire circuits do help. You don't derate a shared neutral. 310.15(B)(4).
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The city said you can do it, which means that 'edison' wiring *is* legal in your community.
There -may- be a 'difference in understanding' about "what kind of circuits" are involved.
You _can_ run 2 separate *120V* circuits (as long as they're on different 'hot' sides of the 240V supply), with a single, shared, 'neutral'.
Note: for 120V wiring in EMT, you only need 1 'hot' (black or other color), and 1 'neutral' (white) wire, the 'ground' ("green") _is_ handled through the conduit.
In 'Edison' wiring, two 120V circuits, *on*separate*phases* , share a common "return" (the neutral) which has to be only the same size as the largest hot wire. This works because the _maximum_ load in the neutral is when one hot is at capacity, and the other has -zero- load. Since the two hot leads are out-of-phase with each other, any load on the 2nd hot results in a *reduction* in the current flow in the neutral.
If you're talking about running 240V circuits, it's a whole nother ball-game. Gotta have 2 'hot' wires (typically 'black' and 'red') for each circuit. 'safety ground' is still on the EMT. "Neutral" is -not- needed, *unless* you have 240V equipment that has 'included' 120V components (like lights). Note: you -cannot- 'share' that neutral between multiple 240V circuits.
In _either_ the situation of 3 120V circuits, or 3 240V-without-120V-option circuits, you only need 6 wires to provide 'dedicated' wiring for each circuit. "safety ground" is supplied by the EMT.
The only case you'd require 9 wires is running 3 240V circuits _with_ 120V support on each circuit. In that situation, you've got 2 hots and a neutral (that cannot be shared across 240 circuits) for _each_ circuit, with 'safety ground' being handled by the EMT.
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