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My home has a 240V NEMA 10-30R outlet in the garage, fed by 6/3 NM-B WG wire. This run is on a 50AMP breaker and doesn't utilize the 4th ground wire in the 6/3 cable.

Shortly after moving in, I swapped a NEMA 10-30P plug onto my 240v table saw and have been using the outlet just fine since. That said, I've since picked up a few bigger and more expensive tools and don't want to push my luck.

What should I do to make this circuit proper for my home and tools? Can I continue to put 10-30P plugs on my tools and use the outlet freely? Should I put a subpanel in the garage?

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Your 50A breaker is unsafe with 30A tools. Just imagine something heavy and sharp falls and damages that #12 awg cord on a 30A circuit or a compressor motor locks up and 100A flows on that cord. How long before the insulation on that cord melts? If that breaker is within NEMA specs it could hold that load for over 90 seconds.

And NEMA 10 receptacles are only legal for use in existing installations for dryers and ranges when no equipment ground exists, no other application. You could argue that it doesn't really matter what the Code says, the wire you are using presumably as a isolated ground doesn't care what color the insulation is, but I wouldn't argue that with an insurance company lawyer.

Bottom line is you basically have three options:

  1. Change the receptacle to a NEMA 14-50 or 6-50 and only use equipment designed for 50A protection,

  2. Change the breaker to 30A and you can install multiple 14-30 and 6-30 receptacles using at least #10 wire as needed, or

  3. Keep the 50A breaker (or change to 60A), install a (minimum) 60A rated panel in the garage, and distribute 15, 20, and 30 amp circuits with properly sized wire and receptacles as needed. #6 NM-B is only good for 55A, but that isn't a recognized standard breaker size and you are allowed round up to the next size larger.

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  • " #6 NM-B is only good for 55A " That's only true for a given temperature rating on the insulation of copper wire, higher temperature rating allows more current. " but that isn't a recognized standard breaker size and you are allowed round up to the next size larger " I'm not so sure about that, I'd double check with electrical code, an inspector, or licensed electrician.
    – MacGuffin
    Jan 7, 2022 at 9:15
  • @MacGuffin #6 NM-B is the existing wire type in question, see 334.80 for 60°C limit. See 240.4(B) for application of next larger breaker size, and breaker sizes in 240.6. You can view the NEC for free at nfpa.org/codes-and-standards/all-codes-and-standards/… or just search stackexchange with the section numbers. Jan 7, 2022 at 17:13
  • Now that I realize I am dealing with a pro here I will humbly, and with trepidation, ask how one should size up the breakers in the panel fed by this 60 amp breaker. Should they add up to no more than 55 amps? (e.g. 40+15) No more than 60? (30+30) Within some "rule of thumb" beyond 60 amps? Perhaps 150%? (30+30+30 or 30+20+20+20) Or is it more complicated than that? Asking as that's a likely follow-up question.
    – MacGuffin
    Jan 7, 2022 at 18:02
  • @MacGuffin the magic words in Code are "sufficient for the load to be served". What are you planning to run at once?" Being the deciding question. Jan 7, 2022 at 23:56
  • @MacGuffin The NEC takes a reverse approach, it asks to identify loads, applies service and demand factors to identify minimum size feeder wire and required overcurrent protection. Realistically if not adding equipment that the instructions specify half the capacity of the panel you should be able to add as many receptacles and breakers as it takes to safely and conveniently use tools where you need them. Jan 10, 2022 at 15:01
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I'll pay you to convert to NEMA 14 (or a subpanel)

Not me personally. Home sellers are finding they get better offers when they say the garage is ready for level 2 EV charging. And yours is, but for the wrong socket. Install NEMA 14-50, write "EV Charging" on a Post-it note, and put it in the listing with a $1000-3000 increase in ask price. "That was easy"

But a subpanel will be much more versatile for you, so I suggest you hold out for that.

NEMA 14 is the way to go (or NEMA 6). I physically smash all NEMA 10 plugs and sockets. If you don't, they'll come back like the undead. They're insidious.*

#1 problem: Grounding to NEUTRAL is insane

Neutral is one of the three "hot" wires that normally handle current. If everything is going well, neutral is bonded a particular way and that makes it safer than the other two hot wires. Ha ha!

But I've had problems - twice! - that made neutral unsafe.

However when it goes wrong, "grounding" your equipment chassis to neutral will cause that chassis to become energized. This is the classic problem with NEMA 10 for ovens and ranges (permitted until NEC 1996), a simple break of the neutral wire assured the dryer chassis was energized.

However, a whole-house "lost neutral" can have the same effect. In this case, the house can't return neutral current via the neutral wire to the utility, so it tries to return it via the neutral-ground bond, the ground rods, and the dirt to everyone else's houses or the transformer. This works about as well as it sounds, which is to say "not very". I had one of those 2 years ago.

My other one was a lost neutral-ground bond in the panel (the green screw was there, but the thread was vaporized). An unrelated ground fault had pegged L1 to 0V and neutral to 120V. (we had our own transformer for that installation).

In your case your wall wiring has the 3rd pin on neutral (correct for NEMA 10) but your appliance has the chassis mis-wired to neutral (very wrong).

There is simply no excuse to use the wrong socket when the right one is $10.

And by the way, you could have a line of people around the block swearing they've never heard of anyone killed by a NEMA 10 receptacle. How would they know? It's not going to have a neon sign. NEMA 10 dryer fatalities are usually mis-reported as a miswired outlet, when they are correctly wired but experienced an ordinary wire break. THAT shouldn't make anything lethal, which is why NEMA 10 was banned in 1996. With welder accidents, the socket type is lost in details - example: here is a very detailed accident report that doesn't even mention socket type, although investigators had every opportunity to see the socket, and a faulty NEMA 10 circuit fits the facts.

Problem #2: 15-30A appliances are not certified to be protected by 50A breakers

NoSparksPlease does a great job of covering this, and so I won't.

But yeah, UL tests every appliance to make sure its failure modes will get an appropriate trip out of appropriately sized breaker. (It's also why UL allows appliances to use smaller, short wires inside a metal chassis).

That subpanel

NEC says that a feeder to a subpanel must be sufficient "for the load to be served". That means basically that there are no hard limits, as you only need to consider loads that will run simultaneously.

For each group of loads that will run together, you must provision the loads based on their nameplate data (not their circuit breaker). For instance a 240V motor might draw 3800VA (16A), nominally take a 20A breaker and #12 wire, yet UL may approve it for up to a 35A breaker to avoid nuisance trips from motor startup. That doesn't count for 35A, it counts for 16A.

Your ruling load will be a Tesla Model S charging at 44A (with 125% derate giving 55A, the thermal limit of your 6-3NM feeder).

As such, you may have ALL these breakers in the subpanel at the same time! Breaker spaces are laughably cheap, so really splurge on the number of spaces in your subpanel. We have a 24 space that is half full and we only have two 240V tools wired so far. 30 spaces is not excessive. When NEC 2020 is adopted in your state, 240 circuits will require GFCI protection, and that can only happen at the breaker, and they are only made in full-size, so the "circuits" number is useless.

Now, here's a spot of good news (especially if the NEC 2020 requirements hit you): 15A, 20A and 30A circuits can have any number of receptacles. (40-50A circuits cannot). And 15A plugs fit 20A receptacles.

So you only need 3 total circuits to cover all needs:

  • A 20A circuit with NEMA 6-15 or 6-20 receptacles (or 14-20 if you had an application for that)
  • A 30A circuit with any number of NEMA 6-30 and 14-30 receptacles
  • A 50A circuit with one NEMA 14-50 or 6-50 receptacle (14-50 being the universal donor) Remember to mark that one "EV charger" - ka-ching!

Yes, you can put a 50A branch circuit off a 50A (well, 55A) feeder.

Since they don't make 55A breakers, you round up to the next available breaker size, 60A... just don't plan on using the extra 5A.






* They're insidious. NEMA 10 was adopted as a "private socket" by the welder community and some others. I assume when they coined this family standard, NEMA 10 dryer sockets were common and most people plugged into those as a habit. That was when nobody cared about grounding, but it stuck.

Today, even more are spread because when a home-buyer says "hey my dryer is 3-prong", the home builder or seller just changes to NEMA 10 socket because it's their house (and not their appliance). But when the appliance store sells them a new dryer and finds the socket is 3-prong, they change to NEMA 10 plug because it's their appliance (and not their house). We'll never get rid of the beast! But you should.

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  • "Your ruling load will be a Tesla Model S charging at 44A (with 125% derate giving 55A, the thermal limit of your 6-3NM feeder)." How does this work? To my knowledge there's EVSE's that can provide up to 40A (e.g., Grizzl-E), and ones that can provide up to 48A (e.g., Clipper Creek HCS-60), but none that can provide up to 44A. Surely it can't be safe or legal to install a 48A EVSE on 6-3NM and rely on the homeowner to turn down the charge current on their Model S to 44A every time they use it. And what breaker would you use for that, since 55A ones don't exist? Jan 8, 2022 at 4:58
  • @Joseph EVSE ampacity is a soft setting so you simply tell the EVSE "draw no more than 44A" (assuming it gives that kind of fineness) and you're all set. NEC says you round up to the next available breaker value. Jan 8, 2022 at 17:17
  • I don't think any EVSE's give you that kind of fineness. All of the 48A ones I've seen will let you limit them to 40A if you want, but not 44A. Now some cars (including Teslas) will let you turn down the charge rate like that once you start charging, but that seems like it'd be really easy for the owner to forget one time. Jan 8, 2022 at 21:55
  • @JosephSible it’s not that soft. UL requires that there be a commissioning procedure when installing a wall mount EVSE.... via DIP switches or a special WiFi network which only exists during initial install. That is the point where max ampacity is set based on 80% of circuit. Nothing on any normal config will let you exceed that, once set. The J1772 spec says amps can be anything, and 42A is enumerated as an example (70% duty cycle). 43.8A should happen at 73%. If EVSE's don't let you, that's the EVSE maker's call, not the spec. Fortunately it is an open standard. Jan 9, 2022 at 1:36
  • In that case, I think 44A, though legal in theory, is impossible in practice, just because there's no EVSE's you can buy that offer any commissioning options between 40A and 48A. This kind of surprises me, exactly because that could take full advantage of 6-3NM, which is really common. Jan 9, 2022 at 1:45
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A 10-30 outlet should not be on a 50 amp breaker, it's designed only for 30 amps. That's the first thing that should be resolved.

As far as using a 10-30 outlet goes there is nothing inherently wrong with it, the plug is at least theoretically safe. The problem lies in the ambiguity of the middle contact, is it supposed to be a neutral or a ground? Answer, avoid the ambiguity with a properly wired modern outlet.

I'm no professional electrician but I can read the electrical code. A 50 amp breaker to a 30 amp outlet is an obvious violation. One fix is to replace the breaker. Another fix is to replace the outlet. Perhaps one might want to replace both, putting in a sub panel as you hinted at.

Again there is nothing inherently wrong with the 10-30 plug and outlet, people have used them safely for decades. The issue is that with the ambiguity of their application it is best to leave them in the 20th century. Any new install should not have a 10-30 plug or outlet but because of rules allowing for grandfathered replacements this rule has been abused.

I'd consider three options. Maybe four.

  1. Keep this as a 30 amp outlet by replacing the 50 amp breaker with a 30 amp breaker and replace the plug with a proper 30 amp plug.
  • If I thought I'd need a 4 pronged outlet then I'd put in a 4 pronged outlet and have an adapter for any 3 prong tools.
  • If I thought I'd only ever use 3 prong tools then put in a 3 prong outlet. With the neutral wire still in the box I know this decision is easily reversed.
  1. Keep this as a 50 amp circuit by replacing the outlet with a 14-50 outlet. The tools sound like they are 30 amps so having an adapter for those 30 amp tools should be reasonably safe. If in doubt then get adapter cords with it's own fuse, breaker, GFCI, or whatever makes you feel better.

  2. Put in a 60 amp sub panel. Check with your electrician friend that it's all up to code but it looks like a 6/3+G can handle 60 amps.

  • It appears you are running 30 amp tools so I'd wire in two outlets to match the tools and minimize needs for adapters. If some had an L14-30 and others a 6-30 then that's the outlets I'd put in.
  • Consider a 30 amp outlet for your big tools and 15 or 20 amp outlets for other tools.
  1. This option is a bit of a wildcard as it involves going even higher in the amps on the wire. If the wire is up to it then you might be able to put a 65, 70, or 75 amp breaker to a sub panel. If you have a need or desire to go bigger then this is your chance, because you are going to have to do something to rectify this so you may want to use this as a reason to expand.

I had a situation like this in my bother's garage. He had a rather hack job of an install done on a 14-50 plug that was apparently for a welder or a pizza oven in the "man cave". After he got married the outlet had to go, as did the hot tub. The disconnect panel for the hot tub found itself reused in the garage. The 50 amp breaker fed the GFCI breakers in the panel, one 20 amp and the other 30 amp. The 30 amp outlet was originally a 10-30 type for an old heater but when that failed the plug was swapped out for a 6-30 for a new heater. The 20 amp breaker originally fed a 6-20 outlet for another heater but when that failed a quartet of 5-20 outlets were put in place for heaters in winter and fans in summer. The heated garage became a place to entertain the kids after school all year around.

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    "The problem lies in the ambiguity of the middle contact, is it supposed to be a neutral or a ground?" Isn't it unambiguously supposed to be a neutral, and if you wanted a ground but not a neutral, then aren't you supposed to use a 6-30 instead? Jan 6, 2022 at 16:57
  • There is no ambiguity. The non-hot position of a 10-30 is neutral. 10-30s are not theoretically safe, they are actually dangerous. They have been outlawed for new work for many years now.
    – nobody
    Jan 7, 2022 at 0:30
  • " There is no ambiguity. " That would be true if everyone followed the rules on how to wire a 10-30 outlet. As that is not the case it is frighteningly common to see 10-30 outlets wired improperly, with the center conductor connected to ground. If people wanted a ground pin then they should use a 6-30 instead but people will reuse a 10-30 outlet out of their parts bin than buy the right part, or whatever. I will maintain that the outlet is not inherently unsafe, the reason they are not safe is primarily because people are not using them as intended. I also maintain they need to go.
    – MacGuffin
    Jan 7, 2022 at 2:05
  • I think you're being confused by seeing 10-30's wired with SE cable, in which the neutral wire is bare webbing and there's no ground wire. That was OK by code for as long as 10-30's themselves were, and does not mean that anything was wired improperly. The reason that code no longer allows 10-30's is that they're unsafe even when they're wired correctly, since appliances need to bootleg ground to use them, which is unsafe. Jan 7, 2022 at 4:48
  • " I think you're being confused by... " No, I am not being confused by this or that. I'm making a statement on the outlet being safe when used as intended. In your example " since appliances need to bootleg ground to use them, which is unsafe " that is not using them in a safe manner. Comments are not intended for extended conversation. You are not going to change my mind here. There is a way to use the 10-30 outlet that is safe, but with so many untrained people doing home repairs abusing this outlet style it needs to go because it is almost impossible to know if it was installed safely.
    – MacGuffin
    Jan 7, 2022 at 9:09

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