How does the Receptacle , the wiring, CB and code Relate?

[Jadnashua]

That is the way it should be.

And the reason the plugs have different blade contacts.

Good Luck.

40A circuit, 50A receptacle is code compliant...not a great idea to then plug in an actual 50A device, which may not trip the breaker, as it may be only drawing 40A. Unlike where they prohibit putting a 20A receptacle on a 15A circuit, but you can install a 15A one on a 20A circuit...there, the receptacle does limit what can be plugged in, and both the wiring and receptacle should be fine. IOW, on a 40 or 50A circuit with a receptacle, both the plug and receptacle are exactly the same, so you CAN overload it (by not abiding by the 80% rule).

Many EVSE's are available in 32 and 40A versions, and if you buy one with a plug attached, BOTH have the same 50A one...only one of those should be installed on a 40A branch circuit...there's nothing to prevent the unknowing user to plug in something that is too large for the circuit, unlike a 15A receptacle installed on a 20A circuit. Good for one, good for all? Seems like a disconnect to me! A common connection point for many is to share the electric dryer socket, and it just may be a 40A circuit and they're trying to install a 40A device that will fit, but shouldn't be used.

Mine is installed correctly, so I'm not concerned about it, but still wonder about the wisdom of allowing a 50A receptacle on a 40A circuit. Obviously, if the load actually was a real 50A, the 40A CB would trip, probably fairly quickly. But, it would not trip quickly, if at all, on a 40A load, which had the same plug, but should not be installed (nothing obvious to the average user since it fits).

 

[DonL]

Mine is installed correctly, so I'm not concerned about it, but still wonder about the wisdom of allowing a 50A receptacle on a 40A circuit. Obviously, if the load actually was a real 50A, the 40A CB would trip, probably fairly quickly. But, it would not trip quickly, if at all, on a 40A load, which had the same plug, but should not be installed (nothing obvious to the average user since it fits).

I have never seen anything with a 50 amp plug that needed 50 amps continuous. Some big motors may when they start, But it is not continuous duty.

If that was the case, the connectors would most likely overheat. Using a over rated receptacle is good engineering practice in my book.

The breaker and wire size feeding the outlet is what is important for safety.

At least you know you did it right.

 

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[JWelectric]

Cord and plug is a disconnect.


If I take a fused disconnect and want to install 20 amp fuses I must use a 30 amp disconnect as this is the smallest made. What I can’t do is install 35 am fuses in that 30 amp disconnect, I must go to a 60 amp disconnect for those 35 amp fuses.


Most homes will have 15 amp receptacles installed in the kitchen which requires 20 amp circuits. This receptacle is limited to a 12 amp load although it is protected by a 20 amp overcurrent device. If this was a single device yoke and the only one on the circuit then it would be required to be rated at 20 amps but due to the fact that there are more than one on the circuit then 15 amp receptacles are allowed.


A 50 amp receptacle on a 40 amp circuit is still a 40 amp circuit and no different than a 50 amp fuse in a 60 amp disconnect. There is in most cases only one receptacle on this circuit therefore we are not allowed to use 40 amp receptacles on a 50 amp overcurrent device but we are allowed a 50 amp receptacle on a 40 amp overcurrent device.

 

[DonL]

Cord and plug is a disconnect.


If I take a fused disconnect and want to install 20 amp fuses I must use a 30 amp disconnect as this is the smallest made. What I can’t do is install 35 am fuses in that 30 amp disconnect, I must go to a 60 amp disconnect for those 35 amp fuses.


Most homes will have 15 amp receptacles installed in the kitchen which requires 20 amp circuits. This receptacle is limited to a 12 amp load although it is protected by a 20 amp overcurrent device. If this was a single device yoke and the only one on the circuit then it would be required to be rated at 20 amps but due to the fact that there are more than one on the circuit then 15 amp receptacles are allowed.


A 50 amp receptacle on a 40 amp circuit is still a 40 amp circuit and no different than a 50 amp fuse in a 60 amp disconnect. There is in most cases only one receptacle on this circuit therefore we are not allowed to use 40 amp receptacles on a 50 amp overcurrent device but we are allowed a 50 amp receptacle on a 40 amp overcurrent device.

That was well put.

Thank You.

 

[Jadnashua]

Problem comes, many people will look at the receptacle, see that the new thing they want to install will fit, and then just plug it in. Say it was a 40A circuit...a 32A device like an EVSE with the 80% rule works just fine. A 40A device might work, but all of the margin provided for the 80% rule is not lost. Yes, I understand that one SHOULD look at the actual circuit, but many people, seeing a receptacle, and the matching plug, will just try to plug the thing in. They won't allow you to do that with 20A receptacles on a 15A circuit. If you are hard-wiring something, you tend to look much more closely, but not with a plug.