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A (built in 1987) house that we are under contract to buy has a 30-amp sub-panel in the attached garage. The sub-panel is currently populated by two 120V, 20amp circuits for receptacles in the garage. Upon inspection, I discovered that its electrical supply is as follows:

  • 30 amp breaker at main panel, with 8/3 NM-B with ground run to a box with a NEMA 10-30R dryer receptacle in the basement (yes, I'm aware that 10-30R should be replaced with a 14-30R)
  • Another 8/3 NM-B with ground running from that box to the garage sub-panel

So it certainly seems that the dryer receptacle and sub-panel share the same 30-amp supply at the main panel. I know that this isn't best practice and will include it in my inspection objections, but can anyone point me to a code reference that would forbid this installation? For example, if they exist, what are the NEC sections requiring a dryer receptacle or subpanel to be on dedicated circuits?

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  • Don't make them fix it. Make them concede you some money, and you fix it. First that'll mean it'll get fixed right (he has no interest in fixing it right). Second you could save/make some $ DIYing it.
    – Harper - Reinstate Monica
    Commented May 6, 2019 at 17:59

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If it goes to a subpanel, it's a feeder.

You cannot put a receptacle on a feeder.

What needs to happen is the dryer receptacle needs to be replaced with a blank junction box cover, and then a new 10/3 cable run from a dryer receptacle in a new box, to either main or sub panel, where it'll land on a 30A breaker. This means the 10/3 cable will be alongside the 8/3 feeder, either to the main panel or to the sub.

You can use 8/3 for this dryer run if you really want to, but I don't see a reason to do that. Dryers won't pull more than 30A (really: <24A) because every dryer manufacturer expects a 30A circuit, and sizes for it.

Once the entire 8/3 feeder is devoid of receptacles, it can be upgraded to a 40A breaker, because that's allowed on 8/3. In practical terms, that will support the dryer and 1 receptacle circuit on each leg of power (so 2 if balanced).

One more thing. If the subpanel came first and the dryer plug was an afterthought, it's possible this was spliced inline, and that probably means the wires in that box are way too short. You need 6" of length past the clamp, or 3" beyond the surface of the box. In that case, move the box so one end has enough length, and re-run the other end. Better, leave the "enough length" side as the dryer branch circuit, scrap the other end, and run a whole new feeder in 6AWG, bypassing the box entirely. 6AWG gives you 60A at the subpanel.

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  • Thanks Harper. This post essentially encapsulates what I would like to do; get a concession from the seller so I can fix it myself, and then use the money to install a 60A subpanel instead. I just thought it might be helpful to cite an NEC provision prohibiting the current installation, rather than coming off as just me being picky (in somewhat of a seller's market).
    – doctoris
    Commented May 6, 2019 at 18:23
  • This is basicly the same answer as I gave. Not sure why the down vote unless the receptacle is in between the main and sub that would not be code compliant.
    – Ed Beal
    Commented May 6, 2019 at 18:52
  • After rereading again I see the receptacle is not on the sub , I through it was so I will retract my answer but the coward that gave a down vote without a comment well not worth worrying about.+
    – Ed Beal
    Commented May 6, 2019 at 19:01
  • @EdBeal Well, he edited it, perhaps after you wrote your answer. That's why I took it to mean the dryer receptacle is indeed tapping the feeder.
    – Harper - Reinstate Monica
    Commented May 6, 2019 at 19:01
  • Actually, whether something can feed both loads and a subpanel at the same time is an open question in my book -- it seems the NEC simply assumes that it won't happen, ever, without saying it's verboten anywhere? (In fact, looking at the Art. 100 definitions of "branch circuit" and "feeder", I can't seem to read mutual exclusion into them; I'd love to hear the logic of your interpretation here!)
    – ThreePhaseEel
    Commented May 7, 2019 at 1:06
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The sub panel must be supplied by four wires and the grounds and neutrals need to be completely isolated in the sub panel!

The two 20A circuits on the sub panel being supplied by a (hopefully) single double pole 30A breaker don't scream holy shit there's a problem here but depending on the calculated load from them I'd estimate this wiring is undersized and needs upgraded. You also didn't specify the distance to the garage from the main panel, this is absolutely essential.

Also, you didn't specify was if the garage was attached or detached, that makes a major distinction with the NEC and subsequent local codes. If this garage is attached then we only need to ensure that the sub panel and it's feed is appropriately sized. If the garage is detached then we open a whole different can of worms where we're needing external grounding, blah blah, etcetera.

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    I've updated my post to reflect that the garage is attached and these are 8/3 NM-B with ground, and the sub-panel is wired correctly with isolated neutral and ground. I would estimate about 40 feet of wire from the main panel to the junction/receptacle, and another 40 feet to the sub-panel.
    – doctoris
    Commented May 6, 2019 at 17:57
  • The fact that this breaker is supplying both a sub panel and an appliance is a definite issue, either one or the other but not both. They likely did this as a half assed diy job where they wouldn't run the appliance and the sub panel at the same time to avoid tripping the breaker...
    – Bob the builder
    Commented May 6, 2019 at 18:04
  • I expect the subpanel came first, and then the owner decided to add the electric dryer plug. That may suggest the cable was cut/spliced to add the box, in which case the wires may be too short inside the box -- if so, one or both ends will need to be re-run, or the whole feeder. And if you're going to run the whole thing, leave the dryer connected to the old #8 (move the dryer box to collect the additional length you need) and re-run the feeder with #6, good for 60A.
    – Harper - Reinstate Monica
    Commented May 6, 2019 at 18:08

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