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Thread: Electrical question, conduit, boxes

  1. #1
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    Electrical question, conduit, boxes

    I want to run some 110volt and 220 volt lines in my shop. I just installed the subpanel and wanted some basic info. on running conduit. How many no. 12 THHN lines can you run in EMT 1/2"? I want four 110 outlet boxes and three 220 lines on one wall. The 220 would all be one one circuit and same with the 110. How would you run the wires for this? Keep the two voltages separate or run all in one conduit?

    I've heard people here say to wire quad boxes with two separate circuits in each box. Any info. to get me started on this project much appreciated. Alan in Md.
    Alan T. Thank God for every pain free day you live.

  2. #2
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    Alan, I did the same thing in my shop. We ran 4 wires a hot and neutral for the 110. And two hots for the 220. Then piggy back each set to each box.

  3. #3
    First, never run a 1/2" conduit. You will hate yourself later. Get a copy of Oct. 2005 Popular Woodworking and check out article on "Efficent Shop Wiring." This system uses a double pole breaker (220) with a neutral to furnish both 110 and 220 recepitals. This is permited by NEC, though most inspectors aren't familiar with it.

  4. #4
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    Alan,

    Annex C in the NEC is a set of tables that shows conduit fills for same size conductors. The short answer to your question is (9) THHN/THWN conductors in 1/2" EMT.

    You have to be a little careful about how many conductors you pull through a single conduit - more than a few conductors and you need to start derating the conductors. In your case (#12 THHN/THWN), 7 or more conductors in the same conduit and you won't be able to run 20-amp circuits. You don't include the Equipment Grounding Conductor when you're counting conductors.

    You can run all this in the same conduit and still have room for (1) 240v circuit and (2) 120v circuits. Personally, I'd run 3/4" conduit just to make the pulling easier. Don't forget to get pull lube.

    Remember that [general purpose 120v] garage or unfinished basement receptacles need to be GFI-protected.

    Rob
    Addy protocol - unlicensed, but experienced homeowner electrician.
    Last edited by Rob Russell; 07-17-2006 at 9:16 AM. Reason: Add "general purpose 120v" to last sentence.

  5. #5
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    Thanks guys and question for Rob

    Rob, Do you see anything in the NEC book about the blue flexible plastic tubing they sell in Hd and Lowes. It's called Carlon Flex Plus Blue ENT (nonmetallic tubing). I used the 3/4" size to pull 4 no. 6 cables about three feet from the main panel to the sub. Accoding to their site, you can only have 3 no. six for 3/4" . But if you don't count the ground then I'm within that spec. What do you think? Thanks.
    Alan T. Thank God for every pain free day you live.

  6. #6
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    Quote Originally Posted by Alan Tolchinsky
    Rob, Do you see anything in the NEC book about the blue flexible plastic tubing they sell in Hd and Lowes. It's called Carlon Flex Plus Blue ENT (nonmetallic tubing). I used the 3/4" size to pull 4 no. 6 cables about three feet from the main panel to the sub. Accoding to their site, you can only have 3 no. six for 3/4" . But if you don't count the ground then I'm within that spec. What do you think? Thanks.
    The number of conductors you can fit in a given size raceway (conduit) and what conductors you include when you're counting for derating based on "bundling" are 2 different things.

    If the Carlon website says you can only have (3) #6 in their 3/4" flex, then your current installation does not meet code. You would need 1" flex or - and I'll check the tables later this morning for an example - a different kind of conduit.

    Rob

  7. #7
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    Quote Originally Posted by Rob Russell
    Annex C in the NEC is a set of tables that shows conduit fills for same size conductors. The short answer to your question is (9) THHN/THWN conductors in 1/2" EMT.
    That said, I needed 7 #12 THHN conductors (including the ground) and pulled them through 1/2" conduit in our basement to feed a receptacle and light switch. Even though it was an almost straight piece about 6' long, it was a bear to get the wire pulled through it.

  8. #8
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    I too vote for at least 3/4". When you go to pull all your conductors for the various circuits, you'll be happier. You may even need to go 1". Code said I could use 3/4" when running my old subpanel, but when I went to puul wires (it was a long run) I couldn't do it! Too many turns, etc., even with pull boxes at every direction change.

    I'd check your local inspector when running different voltages in the same conduit. I was able to do it for mine. i was going to run 240 straight through my 120 boxes, but if i remember correct, the inspector made me pigtail EVERY conductor passing through EVERY box!

    Also remember to use different colored wire for each circuit and voltage running through the same conduit. Yourself and the next owner will thank you for it!

  9. #9
    Quote Originally Posted by Rob Russell
    Remember that garage or unfinished basement receptacles need to be GFI-protected.
    Not strictly true (i.e. special purpose 110v outlets don't) and 220v circuits generally don't. Anyhow, one should ask their electrical inspector to see what he/she will require.

    As for conduit, definately run 3/4" and the conduit probably won't be the limiting factor, box fill calculations will be. You will need that portion of the NEC that covers box fill calculations and you will need to calculate how many conducters you can have in a box with the number and type of recepticals you're planing on having. Hint; buy the largest outlet boxes you can with receptical covers that have volume information printed on them.

  10. #10
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    Quote Originally Posted by Steven Wilson
    Not strictly true (i.e. special purpose 110v outlets don't) and 220v circuits generally don't. Anyhow, one should ask their electrical inspector to see what he/she will require.
    True, I've got to remember to be more specific about that. I've editted that post to add "general purpose 120v" to that last sentence.

  11. #11
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    Alan,

    Back to your questions:

    ENT (Electrical Nonmetallic tubing) is definitely only rated for (3) #6 conductors through 3/4". You need to replace that either with 1" ENT or 3/4" EMT, FMC (Flexible Metal Conduit), or Schedule 40 PVC/HDPE. This all comes from that same Annex 3.

    Russ and Steven also make some valid points / interesting comments.

    First - on Russ' experience with his inspector in Chicago forcing him to pigtail connections in every box vs. alowing straight through runs - I suspect that's so you're guaranteed to have adequate conductor in each box in case you wanted to tap into that circuit there. I would have asked if I could have left a loop or 2 of wire in the box to that there was the required 6" for each side of the conductor in the box if cut. That way you could tap into the circuit in the future, but for now wouldn't have had to make as many connections.

    Steven's comment about box fill is also right on. Let's asume that:
    • you want to run (1) 20-amp 240v-only circuit and (2) 20-amp GFCI-protected circuits;
    • you will run a common Equipment Grounding Conductor (EGC); and,
    • you'd like to run this all through the same conduit.
    First, before we can look at the conduit and box fill, we need to know what size conductors you have to run. We have (6) current carrying conductors - (2) hots for the 240v circuit and (1) hot + (1) neutral for each of the 120v circuits. From Table 310.15(B)(2)(a) Adjustment Factors for More Than Three Current-Carrying Conductors in a Raceway or Cable, we see that running 4-6 current-carrying conductors in the same conduit forces a derating factor of 80%. We need a conductor that, when derated with the 80% factor, has an ampacity of at least 20 amps. #12 THHN/THWN has an ampacity of 25 amps when used with devices that have 75 degree rated terminals (as do our standard residential circuit breakers and receptacles. (If you look at the teeny-timy print in a circuit breakers, you'll see that the listing says 75 degrees). This means that we take the ampacity of 25 amps, multiply it by 80% and get a derated ampacity of 20 amps. That's fine for us to run a 20 amp circuit.



    Now we need to know what conduit to run. Based on the assumptions, we have 7 conductors to run (the 6 current-carrying conductors + 1 EGC). Let's assume that you're running EMT. From Annex C, Table C1, we see that we need a minimum of 1/2" EMT. As has I commented earlier and as has already been noted by others, more room in conduit makes the pulling much easier, so I'd go with at least 3/4" EMT.

    Now let's look at the boxes you'll need from a Box fill perspective. Start with article 314.16, Number of Conductors in Outlet, Device, and Junction Boxes, and Conduit Bodies, if you want to read about this. Let's assume the worst case and that you'll need to terminate every conductor in every box. If you don't, that's fine - go with the bigger boxes for the extra working room. Assume also that you'll have 1 box where you mount (2) GFI receptacles to feed your 120v circuits. This would give a need for [(6 conductors in) + (6 conductors out) + (1 EGC allowance) + (2 devices @ 2 conductors each)] = 17 conductors worth of volume allowance needed. #12 has a volume of 2.25 cubic inches, so that means you'd need 17 * 2.25 = a box with at least 38.25 cubic inches. The only box that you can use would be the large, 4 11/16" x 4 11/16" x 2 1/8" box which has a volume of 42 cubic inches. Personally, I'd use that anyway and I'd consider getting an extender ring just for the 1 box where you'd have the 2 GFI receptacles - just to have more working room.

    I'd be careful about using different color conductors for every circuit. You're restricted to certain colors for the neutral - "white or gray outer finish or by three continuous white stripes on other than green insulation along its entire length" (200.6). You can use white or gray at the termination, but that makes using the different colors a moot point in the middle of the runs. I suppose you could do something like purple or brown for the (2) 240v conductors with (black + white) and (red + white with red stripe) for the (4) 120v conductors. The EGC must be green or green with yellow stripe (250.119).


    Hope this helps. I've proofread it, but wouldn't be surprised if I have a typo or 2 in here.

    Rob

  12. #12
    Quote Originally Posted by Bruce Wrenn
    First, never run a 1/2" conduit. You will hate yourself later. Get a copy of Oct. 2005 Popular Woodworking and check out article on "Efficent Shop Wiring." This system uses a double pole breaker (220) with a neutral to furnish both 110 and 220 recepitals. This is permited by NEC, though most inspectors aren't familiar with it.
    Here in the Los Angeles area, they no longer permit two circuits from opposite sides of the transformer in the same outlet box. The reason is that the potential difference between the two hots is 240 volts, which is seen to be dangerous. Additionally, a person working on it would not expect to encounter 240 volts in a regular 120 volt convenience outlet box.

    Mike

  13. #13
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    Thanks a lot guys and Rob I appreciate all the info. you've given me. Our info. concerning the 3/4" tubing size seems to match. This new info. will go a long ways toward getting this installation right. I really appreciate it all.

    Alan
    Alan T. Thank God for every pain free day you live.

  14. #14
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    Alan, the more basic question is WHY CONDUIT? (Perhaps a concrete wall?) I may be jumping to conclusions, but if you installed your subpanel flush with the wall, just use good old Romex and save yourself a bunch of hassel. My shop walls are sheet rocked, and I have a 200 amp panel with a 100 amp sub panel, every thing is romex.
    Best Regards, Ken

  15. #15
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    Quote Originally Posted by P. Michael Henderson
    Here in the Los Angeles area, they no longer permit two circuits from opposite sides of the transformer in the same outlet box. The reason is that the potential difference between the two hots is 240 volts, which is seen to be dangerous. Additionally, a person working on it would not expect to encounter 240 volts in a regular 120 volt convenience outlet box.

    Mike
    Mike,

    This effectively says that that multiwire circuits are prohibited. Does this local modification to the code also force people to remove existing multiwire circuits if they are in working on them?

    Rob

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