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NEC's Table 210.24 " Summary of Branch-Circuit Requirements" shows required condudcor and outlet requirements for different circuit ratings:

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Why is it permitted to put a 50 A receptacle on a 40 A circuit?

(Note: this question based on a comment in this question: Can different gauge wiring be used in the same circuit?. NEC screengrab from @maple_shaft's answer.)

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Hank
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  • 210.23(C) 40- and 50-Ampere Branch Circuits. A 40- or 50- ampere branch circuit shall be permitted to supply cooking appliances that are fastened in place in any occupancy. In other than dwelling units, such circuits shall be permitted to supply fixed lighting units with heavy-duty lampholders, infrared heating units, or other utilization equipment. – Jason Feb 21 '13 at 18:26
  • Because the only made one section to handle both? Maybe they got a little relaxed on the rule because it's based on safe load not physical maximum? <-- Guesses, I'm curious now too. – Jason Feb 21 '13 at 18:27

4 Answers4

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Table 210.24 is a summary of branch circuit requirements. If we look at 210.24, we'll see which sections this table summarizes.

National Electrical Code 2008

ARTICLE 210 Branch Circuits

210.24 Branch-Circuit Requirements — Summary. The requirements for circuits that have two or more outlets or receptacles, other than the receptacle circuits of 210.11(C)(1) and (C)(2), are summarized in Table 210.24. This table provides only a summary of minimum requirements. See 210.19, 210.20, and 210.21 for the specific requirements applying to branch circuits.

As we can see, this table summarizes section 19, 20, and 21 of article 210. Also note that these are the requirements for "circuits that have two or more outlets or receptacles".

In this particular case, we'll want to read through section 21(B).

210.21 Outlet Devices.
(A) Lampholders.
(B) Receptacles.
(1) Single Receptacle on an Individual Branch Circuit. A single receptacle installed on an individual branch circuit shall have an ampere rating not less than that of the branch circuit.
(2) Total Cord-and-Plug-Connected Load. Where connected to a branch circuit supplying two or more receptacles or outlets, a receptacle shall not supply a total cord-and-plug-connected load in excess of the maximum specified in Table 210.21(B)(2).
(3) Receptacle Ratings. Where connected to a branch circuit supplying two or more receptacles or outlets, receptacle ratings shall conform to the values listed in Table 210.21(B)(3), or where larger than 50 amperes, the receptacle rating shall not be less than the branch-circuit rating.
Exception No. 1:
Exception No. 2: The ampere rating of a receptacle installed for electric discharge lighting shall be permitted to be based on 410.62(C).
(4) Range Receptacle Rating. The ampere rating of a range receptacle shall be permitted to be based on a single range demand load as specified in Table 220.55.

First we're pointed to Table 210.21(B)(3).

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Which again tells us that we can have a 50A receptacle on a 40A circuit, so it would seem that it's allowed because it's allowed. However, if we look at the table again and read 210.21(B)(3) Exception number 2.

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They both seem to point us to 410.62(C).

410.62 Cord-Connected Lampholders and Luminaires.
(C) Electric-Discharge Luminaires.
(1) Cord-Connected Installation. A luminaire or a listed assembly shall be permitted to be cord connected if the following conditions apply: (1) The luminaire is located directly below the outlet or busway. (2) The flexible cord meets all the following: a. Is visible for its entire length outside the luminaire b. Is not subject to strain or physical damage c. Is terminated in a grounding-type attachment plug cap or busway plug, or is a part of a listed assembly incorporating a manufactured wiring system connector in accordance with 604.6(C), or has a luminaire assembly with a strain relief and canopy having a maximum 152 mm (6 in.) long section of raceway for attachment to an outlet box above a suspended ceiling
(2) Provided with Mogul-Base, Screw Shell Lampholders. Electric-discharge luminaires provided with mogulbase, screw shell lampholders shall be permitted to be connected to branch circuits of 50 amperes or less by cords complying with 240.5. Receptacles and attachment plugs shall be permitted to be of a lower ampere rating than the branch circuit but not less than 125 percent of the luminaire full-load current.
(3) Equipped with Flanged Surface Inlet. Electric discharge luminaires equipped with a flanged surface inlet shall be permitted to be supplied by cord pendants equipped with cord connectors. Inlets and connectors shall be permitted to be of a lower ampere rating than the branch circuit but not less than 125 percent of the luminaire load current.

If you look at 410.62(C)(2) & 410.62(C)(3), you'll see that it says:

Inlets and connectors shall be permitted to be of a lower ampere rating than the branch circuit but not less than 125 percent of the luminaire load current.

Say I have an electric discharge luminaire that is 35A. The first part says if I can find a 35A receptacle, I can use that. However, it also says that the connector cannot be less than 125% of the load. Which means I'll need a 43.75A receptacle. Since they don't exists, I'll have to use a 50A receptacle.

Also, they don't make 40A receptacles, so you'll have to use a 50A receptacle.

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Tester101
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  • You present a lot of analysis of Electric-Discharge Luminaires but I think the more common application of a 40AMP receptacle in the home is an oven, range or cooktop. – Philip Ngai Feb 22 '13 at 21:47
  • @PhilipNgai I'm not sure a receptacle for a range or cooktop would be covered here, as these are "The requirements for circuits that have two or more* outlets or receptacles*". Circuits for cooking appliances typically have only a single outlet or receptacle. – Tester101 Feb 23 '13 at 15:23
  • Good point. But most of us don't need 40AMP*220V = 8.8KW of lighting in our homes. :-) – Philip Ngai Feb 23 '13 at 16:55
  • @PhilipNgai National Electrical Code (NEC) applies to commercial, and industrial too, not just residential. – Tester101 Feb 24 '13 at 13:11
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The answer is simple.

Is it safe to use 12awg wire in a circuit with a 15A breaker? Yes, it is safe. In the event of a current overload, the breaker would interrupt the circuit when the current exceeds 15A, and the 12awg wire can safely handle the over current until the breaker activates.

So, even though a 15A breaker circuit requires only 14awg wire, in a pinch you can use 12awg wire. You'd be wasting money; but it is completely safe.

Similarly, it is safe to use a 50A receptacle in a 40A breaker circuit. In a pinch, if you cannot find a 40A receptacle (like if they don't make 40A receptacles), it is safe to use a 50A receptacle in a 40A breaker circuit.

What happens if someone connects a 50A oven to the 40A breaker circuit? They would soon find out when the 40A breaker trips.

Can the 8awg wiring (for 40A breaker circuit) handle the over current safely until the 40A breaker trips? Yes. The 8awg wire is rated for 40A at 60 degree C, and 50A at 75 degree C; it should handle the temporary overcurrent until the 40A breaker kicks in.

To summarize, for a 40A breaker circuit, 8awg wiring, it should be safe to use a 50A receptacle.

Is it legal? Check with your local building inspector. I surmise the inspector would not be able to find a 40A receptacle either.

Jon Kwong
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  • @FreeMan: if nothing else, this answer is much simpler to read and understand than the extensive quoting from the NEC with no tl;dr. – Martha Dec 01 '21 at 18:50
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    But it is wrong. The logic it presents would allow a 20A, 30A, or 50A receptacle on a 15A circuit because the receptacle “can handle it”. But code definitely does not allow those. Code almost never permits situations where an obvious overload is possible. The 40A circuit / 50A receptacle situation is special and this answer does not address that. – nobody Dec 02 '21 at 03:59
  • point well taken, @nobody – FreeMan Dec 02 '21 at 17:19
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This is explicitly allowed in the Code in Table 210.21(B)(3).

John
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    Yes, I see that it's allowed by code. My question is why? You're not allowed to put a 20 A receptacle on a 15 A circuit, for instance. – Hank Feb 22 '13 at 00:36
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    @HenryJackson My feeling is that there is no 40A plug/receptacle standard, and the idea is that all else being equal, the 40A breaker would trip if a device tried to draw 45A of current through this receptacle, then all things being equal, it was considered "ok". Just a guess, though – Aaron Feb 22 '13 at 17:27
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  1. Consider an analogy with a 200 pound man who needs a ladder (assume he is not carrying anything as he uses the ladder). You could provide him with a ladder rated to support 200 pounds or a ladder rated for 300 pounds. Either is fine. The breaker protects the 40AMP circuit, not the receptacle.

  2. As far as I know, there is a NEMA 5-30 and 5-50 but no 5-40. So you have to jump from a 30AMP receptacle to a 50AMP if you need more than 30AMPs.

Philip Ngai
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  • I don't understand your analogy. If a device tries to draw 50 A from a 40 A circuit, the breaker will trip. How is that like a 200 lb. person on a ladder rated for 300 pounds? – Hank Feb 22 '13 at 00:36
  • The 50AMP receptacle is the 300 pound rated ladder and the 40AMP appliance is the 200 pound man. – Philip Ngai Feb 22 '13 at 02:25
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    So I can install 50A receptacles on a 15A circuit, because that's like giving the 150 lbs man a ladder rated for 500 lbs? – Tester101 Feb 22 '13 at 12:22
  • Physically it would be fine, the NEMA 5-50 would never overheat or otherwise be overloaded but table 210.24 would not permit it. – Philip Ngai Feb 22 '13 at 17:16
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    @PhilipNgai receptacles and plugs are different shapes and sizes based on the load they handle, which is done to prevent plugging the wrong thing in the wrong place. If you have a 50A receptacle on a 15A circuit, you're praying the overcurrent device will react before the cable burns. It's just a bad analogy. – Tester101 Feb 24 '13 at 13:15
  • And yet a 50A receptacle on a 40A circuit is completely legal and safe. Explain that. – Philip Ngai Feb 24 '13 at 17:56
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    @PhilipNgai And yet a 50A receptacle on 15A circuit is not legal or safe. Explain that. – Tester101 Feb 27 '13 at 17:08
  • @Tester101: What would be the preferred way of wiring a device such as a lighting console which, depending upon usage, might draw less than 5 amps or more than 40? If one will want to use ten 500-watt lamps when putting on a show in a place with 50 amps available, and ten 100-watt lamps in a place with only 15 amps available, it would seem like it should be possible to use the same lighting board in either scenario. – supercat Jul 09 '14 at 18:58
  • This analogy still just doesn't make sense to me. OP is asking for a scenario where a 300-pound man uses a ladder rated for 200 pounds, not a 200-pound man using a ladder rated for 300-pounds. I think you have it wrong/backward, respectfully. – TylerH Dec 02 '21 at 22:22
  • My analogy is a current limited to 40 A by the breaker going through a 50 A receptacle (the 300 pound ladder). You can construct your analogies any way you prefer. – Philip Ngai Dec 04 '21 at 07:46
  • "If you have a 50A receptacle on a 15A circuit, you're praying the overcurrent device will react before the cable burns."

    That is how overcurrent devices are expected to work. Certainly an overcurrent device is more likely to protect the circuit than the receptacle is, even if the receptacle is perfectly sized for the circuit, 15 A in your example.

     – Philip Ngai Dec 04 '21 at 07:48