9 Answers
9

The plug specs are the maximum ratings given for how the plug is meant to be used in any given application. So a plug speced as 120V/30A is not designed to be used in applications over 120V and not to be loaded over 30A.

Now that said....those numbers do not directly have anything to do with the capabilities of the supply circuit or wiring. If the circuit was wired and deployed with the proper National Electrical Code guidelines then yes a 30A outlet would be wired with proper sized conductors capable of supplying 30A and the distribution panel being equipped with the proper sized circuit breaker rated at 30A.

However there is no guarantee that the distribution panel can always deliver the full 30A to this outlet. It is possible that the panel can support 100A total via its main breaker but there could be an active kitchen range drawing 50A on one circuit, an AC unit drawing 30A on another circuit leaving only 20A available. If you put the full 30A load on the outlet under consideration the main breaker would trip.

Regarding the voltages....outlets designed for 120V will generally be rated for supporting circuits at a nominal range of 110 to 120VAC. Similarly for outlets designed for 220 to 240VAC. The contact arrangements will be different for the two voltage ranges to prevent folks from attaching devices to the wrong voltage.

250V is the insulation rating of the L6-30 plug. It's the "never-exceed" voltage. Normally it's used for 240V single phase (delivered as split-phase).

However in some locales, mainly NYC, they supply houses with two legs of 208V 3-phase wye. So it is 120V phase to neutral, and 208 phase to phase. There is no separate receptacle design for "208 2-leg", so they use L6-30. It's on the manufacturer to certify the appliance for 208, and on the installer to confirm it is labeled for 208.

If a person wires 120V to L6-30, there is something wrong with their brain. I would say this violates the general NEC requirement for professional and workmanlike work (110.12). Also, the rule that receptacles must be non-interchangeable (406.8) is defeated if you put a non-standard voltage on a receptacle.

If you sell an appliance with an L5-30 plug, you may assume the circuit can supply 30A. If it can't, not your problem. Of course, your design must comply with NEC and UL, e.g. if it's a continuous load, you can only draw 24A.

An example of bumping up to hard receptacle limits: In America, 480V wiring is done with parts rated for 600V. The Canadians often bump that to 575V.

Well if you wanna talk about it that way, it's the never-exceed RMS AC voltage. But in architectural NEC context, AC implies RMS, so it's redundant. Safety PSA here for the passersby: This does NOT imply this device is suitable for DC voltage. It should not be used with DC.
– HarperMay 1 '16 at 0:15

if you come across a [250V receptacle], does it have to supply 240-250v, or can it supply 208, or even 120?

Simple answer

It is an absolute requirement for safety that, from the appearance of the receptacle, it should be immediately and unambiguously obvious what voltage is supplied.

In most cases, receptacles and plugs are designed so that a low voltage appliance cannot be plugged into a high voltage outlet (and usually vice versa too). Usually this is achieved by the position, shape and orientation of the prongs, sometimes (e.g. IEC) by color.

Does a receptacle have to supply the max current that it is rated for?

No.

Suppose you come across a L5-30R (120V, 30A receptacle) somewhere. By spec, can you assume that there will be 30 amps at it?

No.

Follow up: if you come across a L6-30R (250v, 30A), does it have to supply 240-250v, or can it supply 208, or even 120?

If they are following the NEC, then they should be using the correct 120/208V outlets if the supply is 208V. These are uncommon and thus more expensive, though, and so a lot of people ignore the recommendation to use the parts only for "their intended use".

As such you may find an L6-30R supplying 208V. You should not see them supplying 120V, but you can check with a multimeter easily enough.

Keep in mind that people will occasionally use outlets that are overrated, so you might find a 50A outlet that is using a 30A breaker and 30A rated wiring. Thus you can't count on the outlet to supply the maximum listed voltage and current, but you can generally count on it not supplying more than the listed maximum.

This breaks NEC rules (Thanks Tester101!): 551.72Distribution System. Receptacles rated at 50 amperes shall be supplied from a branch circuit of the voltage class and rating of the receptacle.

So in a properly installed 50A outlet you should find the rated amperage as long as no other items on the circuit are using that current.

I can't think of a reason why somebody would put a 50 ampere receptacle on a 30 ampere circuit, but maybe I'm missing something.
– Tester101Apr 29 '16 at 14:33

1

@Tester101 Mostly to save money on wiring. I've seen this especially with people who want to plug in and use their RV at home. The RV plug is 50A, the electrician says it'll be $$$, they balk, the electrician says, "Well, I can install the 50A plug and back it up with only 30A wiring and breaker, but you'll have to be careful with your power usage in the RV." and years later someone tries to plug in a large hot tub and keeps tripping the breaker only to find out the shortcut taken earlier.
– Adam DavisApr 29 '16 at 14:37

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551.72 Distribution System. Receptacles rated at 50 amperes shall be supplied from a branch circuit of the voltage class and rating of the receptacle.
– Tester101Apr 29 '16 at 14:53

Yes, in general it is safe to assume that an L5-30R will supply you up to 30A (nominal, 24A continuous) at ~120V. Give or take.

If you're still reading...

Suppose you come across a L5-30R (120V, 30A receptacle) somewhere. By spec, can you assume that there will be 30 amps at it?

The ratings of these outlets tells you about the safety limits of the outlet, not the actual characteristics of the supply.

Assuming that the supply is correctly configured - correct cabling, circuit breakers and so forth - then you can reasonably expect to be able to draw 30A from that outlet.

However there are a few factors that can limit your actual available current, most notably the impedance of the circuit from your outlet back to the point of supply.

The biggest impediment (if you will pardon the pun) to your maximum current draw may be the impedance of the supply circuit. The higher the impedance of the supply circuit the larger the voltage drops per amp of current draw. In extreme cases this can limit the current drawn by your appliance, effectively preventing you from reaching the rated current.

The line impedance would have to be fairly extreme to have a significant effect, but I've seen old buildings with long cable runs to a central distribution board where the voltage drop at 10A was upwards of 15 volts. At 30A on a (nominally) 125V circuit that impedance would drop your supply voltage to less than 80V, which could cause your appliance to fail. In most cases the appliance won't draw the full current because its performance will be limited by the voltage drop.

Long supply runs with large voltage drop are generally a thing of the past however.

A more rare case would be when the outlet is connected to a circuit breaker that is rated lower than the outlet's safe current. This probably falls under the definition of incorrect supply configuration though, just as providing 120V to a P6-30R would.

The outlet needs to be listed for the voltage being used. A 10-30r is listed for 125-250v 30Amp
This outlet would require a minimum of #10 wires you could use a smaller breaker but that would be silly in my opinion because the cost of the wire and outlet are most of the cost of this outlet.
The problem of using the wrong type of outlet comes up when another person plugs the proper device into the outlet and it damages the equipment (a brown out in this case that would draw excessive current).
If one were to reverse and use a 120 outlet in a 208 or 277 for a “lighting fixture” now a maintenance guy comes along and is working in the area plugs his drill or grinder into it and there goes the magic smoke.
This did happen at the last plant I worked at where the wrong type of outlet was used.
There is a proper outlet / plug combination for every voltage / current application that is legal to be cord connected.
The second case I know of was a friend purchased a house that the garage had 30a 480-3p outlets on a 240 single phase with 4 wires neutral and ground my friend damaged his welder and the plug / outlet because the neutral was tied to a hot leg. The short went through the welder primary windings and took out his control board. I helped him rewire the shop he did not know most houses did not have 480 but saw the outlets and thought wow this is the shop for me.
Sorry to get long winded but using the wrong voltage outlet is dangerous. Lower breaker size is fine as long as the wire is correctly sized for the outlet.

Does a receptacle have to supply the max current that it is rated for?

The answer is usually no.

A line of 20 amp outlets on a 20 amp circuit cannot all be expected to provide 20 amps—at the same time.

Likewise, a 50 amp dryer or range outlet on a dedicated circuit may well have been installed for plug ("pigtail") compatibility—not because it is meant to provide 50 amps. If it has #4 wire and a 50 amp breaker then it could. But more likely it will have #8 or #6 wire and, respectively, a 30 amp or 40 amp breaker.

It is allowed to have heavier-duty components in a circuit than the circuit breaker's protection. For example, a 30 amp breaker is nice and safe when protecting #8 wire and a 50 amp outlet. So, just because you see an outlet rated at 50 amps, you can't expect that much current. You have to check the breaker to see what the circuit is rated for. (If there is the slightest reason to suspect the breaker was improperly installed, then also check the wire gauge itself preferably both in the breaker panel and at the outlet.)

@Harper: Ah, that is not an aspect I had considered. So 30 amps is special. I know I have a 40 amp range outlet wired up to 30 somewhere (maybe my mom's house). So is that a Class A violation?
– wallykApr 29 '16 at 7:32

Apparently, because of the goofy zigzag way this code was written. Well, it was written in ash.
– HarperApr 29 '16 at 7:36

There is no requirement in the National Electrical Code for any particular receptacle configuration.

If I want to wire my whole house with twist-lock receptacles there is no prohibition of that.

There is no guarantee that the rating of the receptacle matches the rating of the circuit. It may be implied but not required. The code requires a minimum voltage/amperage rating but after that it is up to the designer/installer. So, a 120 volt 20 amp circuit could be put on a 250 volt 30 amp receptacle just as others pointed out that 208 volt circuits are routinely put on a 250 volt receptacles. Notice there there are no 208 volt receptacle/plugs listed on this chart. So, the connection of a lower voltage circuit to a higher rated receptacle is already a common practice.

Although, common practice is to match nominal voltage ratings of circuits and receptacles it is not a requirement by the NEC.

Here is what the code says about receptacle configurations:

406.8 Noninterchangeability. Receptacles, cord connectors, and attachment plugs shall be constructed such that receptacle or cord connectors do not accept an attachment plug with a different voltage or current rating from that for which the device is intended. However, a 20-ampere T-slot receptacle or cord connector shall be permitted to accept a 15-ampere attachment plug of the same voltage rating. Non–grounding-type receptacles and connectors shall not accept grounding-type attachment plugs.

So, traditional work practices do not guarantee any particular installation is the same as the last one.

As always, safety and common sense indicates that you should verify rather than assume.

I'm pretty sure somewhere in the NEC it says something about using devices according to their listing and labeling. Also, see 210.21(B)(3), which says that putting a 30 ampere receptacle on a 20 ampere circuit would be a violation.
– Tester101Apr 29 '16 at 12:54

True but using a 20 amp 250 volt receptacle on a 20 amp 120 volt circuit is not a violation.
– ArchonOSXApr 29 '16 at 16:19

Actually it is. NEC 2011, 406.4F says you can't use the same receptacle type for 2 different voltages on the same premises. 406.8 requires receptacles to be unable to accept plugs from devices meant for a different voltage... but it isn't particular as to scope. And the old standbys 110.3A8 and 110.12 which call for safe and competent work, and might argue against anything which makes an inspector go WTH???
– HarperApr 29 '16 at 18:54

No it isn't. I didn't say to use it for two different voltages in the same premises. As long as you are consistent with your receptacle configuration you can use any configuration that you like. As long as the voltage or current ratings are not exceeded.
– ArchonOSXApr 29 '16 at 23:42