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I own a duplex and the attached shed is divided in 1/2 for each side of the duplex. On the rear exterior are the 2 electrical panels, one for each side of the duplex. I was going to install hookups for a new electric washer/dryer on each side of the shed so the tenants have their own set. I need power running from each panel. I assume I need a 12/2 for the washer and a 10/3 for the dryer on 20amp / 30 amp circuits respectively. Since the power to each washer dryer set needs to come from each panel, I assume I'll need 4 circuits total (a 120 and 240 from each panel run to each side of the shed). Does that sound about right?

Additionally, this will be run through exterior conduit and the panels are right next to each other. Can the conduit run from each panel separately, join up for the long run of 30ft, and then divide again in the shed to go to their respective sides... or do I need to have separate conduits coming from each individual panel to each side of the shed?

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    Unless you're a licensed electrician, you cannot do your own work in a shared/rental dwelling. You're allowed to burn down your own house with sketchy electrical work, but not somebody else's, even if you own it and they're "just" renting it from you. Hire an electrician, it'll be cheaper in the long run...
    – FreeMan
    Feb 21 at 15:15
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    I think this may be state specific. I've done a decent amount of electrical work.. Mostly just replacing outlets, running cable to new outlets, moving outlet locations, changing breakers, changing out light fixtures and stuff like that. I am confident that I'm not going to burn down anybody's house. Feb 21 at 17:10
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    well, "check local laws" still applies. :D
    – FreeMan
    Feb 21 at 17:14
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    I just called the city and they said I can do my own work but I do need a permit it sounds like. Feb 21 at 17:58
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    Once again I'm not in Tacoma Washington, I'm in Phoenix AZ. That's an old username. I met with an electrician to get a quote for this job. If it's not too much, I may just have him do it instead of doing it myself but he said that they wouldn't even need a city permit to do a job like that over here. Feb 21 at 20:41

2 Answers 2

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Note that everything here assumes you are using individual wires, not cables. 12/2 and 10/3 usually refer to cables but also work as shorthand for "2 12 AWG wires plus ground" and "3 10 AWG wires plus ground". You can't run regular NM cable outdoors in conduit because it is a wet location. And lots of other complications as well.

Aside from the issue of identifying feeds from multiple panels, there are 2 issues with "lots of wires in one conduit":

  • Conduit Fill

This is easy. Checking with a conduit fill calculator it looks like 1" will be fine, though there are variances depending on which conduit type, how many ground wires (can be shared between circuits), etc.

  • Multiple Circuit Derate

This is the complicated part. I believe the neutral in the dryer circuits gets effectively skipped, which means 2 x 4 = 8 conductors. According to 310.15(C)(1):

  • 7-9 wires = 70% of 310.16 90 C column

That means 12 AWG = 30A x 0.7 = 21A (more than the 20A circuit you can actually install) and 10 AWG = 40A x 0.7 = 28A (less than the 30A circuit you can actually install, but I am pretty sure (Harper or someone please correct me if I am wrong) that as long as the dryer is actually rated for 28A or less (typical is ~ 24A) you are OK.

However, that means you can't add any additional circuits in the same conduit. One more wire gets you to 10 wires = 50% derate! That would mean bumping up wire sizes, which will increase cost considerably. That's really important if you need any additional circuits in each laundry area for any reason.

  • Wire Colors

In conduit, white and gray are always neutral. Green and bare are always ground. Everything else is always hot, switched hot or traveler. There is no functional difference between the two hots in a 240V circuit. Size helps distinguish wires between circuits. Ground can be shared at the larger size. So you need hot 12 (e.g., black), 12 white, 10 hot x 2 (same color ok), 10 white and a 10 ground.

If you run all 4 circuits in one conduit then mark the end of all the wires from each set with a different color - e.g., red on one, blue on the other. That way you can keep the hots and neutrals paired together correctly, which is critical. If they are not paired correctly then they will not work at all on a GFCI, but if you are in a place that still allows new 240V circuits without GFCI then you could have mismatched neutrals between panels and you wouldn't know it - and that would be a big problem.

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  • Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on Home Improvement Meta, or in Home Improvement Chat. Comments continuing discussion may be removed.
    – Michael Karas
    Feb 21 at 23:31
  • Interesting thanks. I'll have to follow up on that. Additionally, I was looking at wiring that would work for outdoors and someone mentioned THWN and this looks like it's sold in rolls of single strand. I need about 55 to 65 feet for each circuit. Since the original comment was "2 12 AWG wires plus ground" and "3 10 AWG wires plus ground" and clarified colors I'm guessing that I need 3 rolls of 12 ga wire (white/black/green) and 3 rolls of 10 ga wire (white/black-x2/green) since it's not bundled up like Romex or can you purchase it that way? Or is UF wire a better option? Feb 21 at 23:35
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    UF isn't wire, it is cable. It can go outside (good) but is not a good idea to run it through conduit. Feb 21 at 23:36
  • Cable/wire, I understand now. So UF is not good for conduit? I think it's more expensive anyways. I'm new to this forum so not sure what's up with the conversation being moved to chat. Reading the posts again, I think I was incorrect. It looks like I can use the larger ground for the smaller circuits so 5 rolls of wire would be needed. 3 10 gauge black/white/green and 2 12 gauge black/white. Does that sound about right? Feb 21 at 23:57
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    NO CABLE is good for conduit. The only practical time for running cable through conduit is for very short distances as physical protection - e.g., from a junction box up the ceiling. And in those instances there are often better/easier things such as a strip of plywood. Feb 22 at 0:04
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You can put power from two sources into a single junction box, run it through one conduit, and split it back out at another junction box elsewhere. When you do this, 312.8(A) would apply, meaning you need to apply a warning label compliant with 110.21(B) to indicate that the enclosure is energized from multiple sources. As an alternate that lets you skip the junction box, you could run conduit between the two electrical panels, apply the same labeling to one panel, and run the circuits from the other panel through it. Read 312.8 for the specific rules.

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  • If I understood that correctly, if I go the second route, I'm assuming that I need to have a short conduit run from panel to panel with the 2 two circuits from the first panel just passing through (not interacting with) the second panel, and then the other 2 circuits from the second panel join the first two and all 4 run through that conduit to the shed? Then I label the second panel? Does there need to be a junction box at the end if done this way? Additionally, is there a limitation on conduit size with this many/type of circuits? Feb 21 at 6:40
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    That's it exactly. Circuit from first panel in conduit to second panel, passes through second panel. Conduit from second panel to where the washers and dryers are. Either a junction box there that splits out to two conduits, or two outlets along the one conduit. Label any boxes that have two circuits going through them. You don't need to label somewhere where you just have a conduit body like a T as that's not a junction box. Standard conduit fill and multi circuit rules apply for derating. No limit on overall conduit size.
    – KMJ
    Feb 21 at 17:33
  • Once inside the shed I need to run the cables about 8 feet to their respective locations and hook up outlets for the washer/dryer. Do I need to use conduit inside the shed running to each outlet? Feb 21 at 17:55
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    You can use any appropriate wiring method, just transition with a junction box and label any junction box that is energized from multiple sources.
    – KMJ
    Feb 21 at 18:14

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