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I am putting in the electrical runs for my home shop and got some l15-30r (3-phase 240v receptacles) because I want to run 30 amp 3 phase circuits for the maximum size machine possible. All of my three phase machines are 20 amp, so l15-20p. I got some l15-30r receptacles. They are incompatible!!! Whoda thunk it. I assumed that it would be possible to plug a 20 amp (l15-20p) into a 30 amp receptacle, since that has more than enough current capacity. I would prefer not to downgrade the runs to 20 amps. The other solution is to lop off the l15-20p connectors to put l15-30p connectors on. This should be perfectly safe, since the machine will not draw that many amps. Would that violate the NEC or common sense?

Thanks.

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  • Why are you breakering them at 30A? Do they have a history of tripping the breaker at 20A? Apr 1, 2020 at 16:53
  • I suspect you misunderstood the question. I am putting these runs in myself, with proper wire size and breaker. Apr 1, 2020 at 16:55
  • Scott , proper size for a 20 amp device for over current protection is 20 amps you could put in # 8 wire a 20 amp breaker and a 30 amp receptacle would still violate code.
    – Ed Beal
    Apr 1, 2020 at 17:10
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    Scott, we've been answering questions like this for a very long time. We know what we're asking, and throwing tantrums because you don't understand why we're asking is not constructive. Apr 1, 2020 at 17:15
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    @ScottFranco No, I was fishing for an exception in Code that would work in your favor. One that you have no idea even exists (except my answer mentions it, so now you know lol). The reason you went that way, with cheek instead of helpfulness, is that you assume in any misunderstanding, the other person is the idiot. Surely you must see where this assumption can backfire and actually can make you the idiot... In future when you get a straight question, just go with it :) Apr 1, 2020 at 17:39

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Don't be fooled by the downward compatibility of 20A receps to take 15A plugs. That is a special exception in Code. In all other cases, socket size must exactly match breaker size.

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To run through the table quickly: the 15A exception supports ancient sockets. The 20A exception is to allow dirt-common NEMA 5-15 receps (plural) on 20A circuits, because that's only 33% over socket size and the benefit is obvious! The 40A exception is because 40A sockets are not made. But other than that, the socket size must exactly match the breaker.

What's the rationale? Consider what happened if you fed a string of 15A sockets with a 60A breaker. That seems clever. But consider the 15A appliance that is in trouble. It's internally shorting, but the resistance of its power cord is preventing more than 60A from flowing (it has melted off its insulation and is glowing cherry red). The breaker utterly fails to prevent the fire. An extreme example, but that's the concept. 15-20A is considered close enough (33% more) but 20-30A is 50% more and a great deal more power in an absolute sense. Code is written in blood and ash: rules get in the book by patterns emerging in fire inspection reports.

That is why sockets are keyed to be incompatible. (except 15-20A).

Believe me, we all know the pain of buying the wrong thing out of inexperience. So you just have to eat the loss and learn the lesson! "Harper's Rule: Buy the wire last" could probably be expanded to "learn, then buy".

The exception. There is a rule that allows up-breakering certain motor loads that whose nameplate has certain numbers, and have a history of tripping the correct sized breaker. Circuits dedicated to such tools can up-breaker (without even upsizing the wire, although upsizing wire is always allowed). If you fall into that exception, you could be good to go. That is to say, if your tool qualifies for the exception I'm fine with the idea of putting a -30 plug on it and simply using it on a normal 30A circuit.


The recovery. As a general rule, you should not be plugging appliances normally intended for 20A circuits into 30A circuits. If you wired your shop based on that misconception, the easiest face-saver is to slap a subpanel on the 30A feed and 15-20A breakers in the sub. Remember neutral and ground must be rigorously separated in a sub, so if the cable has no neutral, you can't support 120V anything in the sub.

Edit: To embellish on a subpanel. A subpanel is just a mini-main panel. If it's in the same building it does not need a main breaker. You then can support any number of circuits up to 30A - and yes, you can have one or more 30A breakers inside a panel fed by a 30A feed breaker! Classic example is expanding a 4-wire dryer feed to also support an EVSE; that's exactly how you do that. I would fit one 30A and one 20A breaker to feed my 30A downline and 15/20A downline.

You can use any fit 3-phase panel such as Eaton CH, Eaton BR or Siemens. In a subpanel, neutral and ground must be rigidly separate and you must remove the neutral-ground equipotential bond. If you're wiring for NEMA 15 I gather you are not bringing neutral; if so I strongly recommend getting an accessory ground bar and making a show of not using the neutral bar for anything at all. So it is obvious to "the next guy" that neutral does not exist in this panel and ground should not be misused for that purpose.

240V 3-phase can vary with center grounding, wild-leg grounding or corner grounding. If it's corner-grounded, ask about cheaper panel options.

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  • I like this answer, thank you. Apr 1, 2020 at 17:15
  • Someone told me to designate answers as being the correct one for the question, but I don't see a way to do that? Apr 1, 2020 at 17:17
  • So I understand the issue, but also realize that this basically SUCKS it in real life. I want to wire my shop to be flexable, but now I either have to run two completely separate circuits, one 20 amp and one 30 amp, or give up and derate the entire shop down to 20 amps (for these 3 phase runs, but I suspect the single phase 240v runs have the same issue). Apr 1, 2020 at 17:25
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    @ScottFranco You may want to re-read my section "The recovery". Subpanel and Bob's your uncle. You know, I could stand to edit to make that more clear. Also if you don't like the cost of receps and plugs, nothing prohibits hardwiring... just go to a real electrical supply with the cord diameter and buy a proper strain relief for the knockout hole. $1-5 typically. Apr 1, 2020 at 17:43
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NEC 210.21(B)(3) is the section and table that coordinates breaker and receptacle selection.

The common sense thing is if a 15A tool is on a 30A circuit and develops fault that draws double the current it won't trip. This could be a difference between smoke and fire.

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  • Interesting point. However the counter to this exists. A 20 amp circuit with 20 amp plugs (those "winking" plugs) will accept a 15 amp standard plug. So your same issue exists here, and that is quite legal. Apr 1, 2020 at 17:02
  • The table is interesting. So a 20 amp circuit will take 15 amp or 20 amp receptacles, and 40 amp will take 40 or 50, but a 30 amp circuit will only take 30 amp receptacles. Apr 1, 2020 at 17:05
  • There code is written with some compromises. If they started to require 10A breakers for convenience receptacles the public would just go crazy. They had to pick some minimum. Also the "40 or 50" reference is due to NEMA not designating a design for 40A. Apr 1, 2020 at 17:11
  • You might also review how long a 3 x 15A failure would take a 30A breaker to trip. I could take up to 5 minutes. lh3.googleusercontent.com/… Apr 1, 2020 at 17:15
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Would plugging a 20 amp device into a 30 amp receptacle violate the NEC , yes it would. You said your devices are 20 amp but they probably have 20 amp plugs not a 20 amp rating. If the devices are not motor loads 422.11 not exceed 150%. What size wire was run ? If 12 awg again allowing a 30 amp receptacle would allow a larger load than the wiring method allows. There are allowances for motor loads to have a larger breaker for starting purposes but receptacle size is fixed for safety code has only 1 place using a larger receptacle is allowed it is on a 40 amp circuit a 40 or 50 amp receptacle is allowed. Table 120.21.B.3. Oops I transposed 210.21.B.3 But this violates as my comment above repeats after your comment.

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  • Again, please READ THE QUESTION. I am putting the runs in, with proper wire size and breaker. Apr 1, 2020 at 17:01
  • I did read the question the wire is not the issue it’s the rating of the device. I noticed no sparks referenced the same table you should look at it! You did not say what kind of machines these were but plugging a 20 amp device into a 30 amp receptacle would violate code.
    – Ed Beal
    Apr 1, 2020 at 17:06
  • Quote: "What size wire was run ?" again, the wire size is installed by me and is proper. The key point here is RECEPTACLE vs PLUG. Of COURSE the receptacle is l15-30R (R as in RECEPTACLE). I was discussing the plug. I am not trying to be argumentative, I am trying to prevent this conversation from gutter balling into a discussion of wire sizes and breakers. Apr 1, 2020 at 17:09
  • " but receptacle size is fixed for safety" - reread question. PLUG NOT RECEPTACLE. I was very careful to state that. Apr 1, 2020 at 17:13
  • @ScottFranco gutter balling... you should probably read this. Apr 1, 2020 at 17:41

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