# matching up electrical ampacity

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• posted on April 29, 2005, 1:41 am
Hi everyone, quick question here, as sometimes when reading I get confused. I want to make sure I am correct here.
If I have a 15 amp circuit breaker, all outlets, switches, etc. must be rated at 15 amps or higher for use on that breaker, correct? So, if I wanted to, could I also used outlets, switches, etc. rated for 20 amps on the 15 amp breaker?
I think however the reverse is NOT true. That being if I have a 20 amp breaker, I CAN NOT use 15 amp switches, outlets, etc. on that breaker?
So in the end, the switch, outlets, etc. must be equally rated or rated higher than the ampacity of the breaker they are on? (I.E. if it is a 15 amp breaker, all switches and outlets must be rated at 15 amps or higher. If it is a 20 amp breaker, all switches and outlets must be rated at 20 amps or higher).
Thanks for any help and clarification.

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<%-name%>
• posted on April 29, 2005, 2:48 am

No. 15A or *lower* for receptacles, believe it or not, according to the Code. 15A or higher for switches.

Switches yes, outlets no.

Code specifically permits the use of 15A rated receptacles on a 20A circuit, but *not* a 15A switch.

Generally, receptacles must be equally rated, period. Exceptions are: <15A outlet on 15A circuit 15A outlet on 20A circuit 50A outlet on 40A circuit are all permitted.
Code says switches must be used "within their ratings" so a 30A rated switch is perfectly fine on a 15A circuit.

No. If it's a 15A circuit, receptacles must be rated 15A or *lower*: Code specifically says "not over 15". Switches over 15A are OK.

No. If it's a 20A breaker, receptacles may be rated either 15A or 20A. Period. Switches, 20A or higher.
-- Regards, Doug Miller (alphageek at milmac dot com)
Nobody ever left footprints in the sands of time by sitting on his butt. And who wants to leave buttprints in the sands of time?

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<%-name%>
• posted on April 29, 2005, 3:11 am
Doug Miller wrote:

Doug I'm afraid that you are wrong about the switches. "Within there rating" means that they must be rated for the load they will switch. Switches do not have to be capable of carrying or controlling the entire ampacity of the circuit in which they are installed. -- Tom H

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<%-name%>
• posted on April 29, 2005, 4:22 am

Can you document that? Why are switches any different than wire? (If you can use a 15a switch on a 20a circuit to feed a 5a light fixture, why can't you use #14 wire from the switch to the light?)

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<%-name%>
• posted on April 29, 2005, 5:42 pm
toller wrote:

Just look at the ratings stamped into the strap or yoke of the switches that are installed in your home. A very common rating is 10A@120VAC. Most common snap switches are not rated for fifteen amperes. The reason that it is acceptable is that there is a very low likelihood of the switch being subjected to the full amperage of the circuit. Also keep in mind that switches are not rated for the load they can carry but rather for the load they can make and break repeatedly. Switches can control higher AC currents than they can DC. Since switches are contained in a listed switch or outlet box and they are in series with the load they are very unlikely to carry the circuits ampacity and still less likely to have to break that ampacity. The obvious exception is a switch used to control a receptacle outlet. The inspector is correct to insist that those be rated for the ampacity of the circuit or the intended ampacity of that receptacles pattern whichever is less.
A very interesting trivia Question is what is the meaning of the T that is stamped into the strap of some switches.
As to your question about wire the answer is that the wire is rated for what it can carry without damage to it's insulation rather than what it can "control." Switches are rated for what they can control. They can carry more current than they can control without damage. -- Tom H

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<%-name%>
• posted on April 29, 2005, 2:56 am
Eric and Megan Swope wrote:

Yes and No. The outlets must be rated to carry the current of the circuit if they are wired feed through. The outlet pattern must comply with a code table that allows either 15 or 20 ampere pattern receptacles on a 20 amp circuit but only 15 ampere pattern on a 15 amp circuit. Switches need only be rated for the load they will control. Lately I have not seen any fifteen ampere rated duplex receptacles so the feed through issue is moot.
In general you can not install a receptacle that is meant to connect a larger load than the circuit will carry. The exception is that you can serve a fifty ampere receptacle outlet from a forty ampere circuit because there is no forty ampere patterned receptacle.
If this does not answer your question please let me know and I will listen louder. -- Tom H

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<%-name%>
• posted on April 29, 2005, 3:14 am
I believe I have it understood from the both of you now. I will type up what I have interpreted below, and if I am still incorrect, please feel free to let me know.
15 amp breaker = outlets must be rated 15 amps or less. switches must be rated 15 amps or higher
20 amp breaker = outlets must be rated 20 amps or less (so I could use 15 amp outlets here) switches must be 20 amps or greater.
So in the end, outlets can be equal to or less than the breaker's ampacity, and switches can be equal to or greater than the breaker's ampacity.
wrote in message

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<%-name%>
• posted on April 29, 2005, 3:45 am
In continuing my question, I know all my 15 and 20 amp breakers in my house have 15 amp outlets on them, so they should be ok. However, I know I replaced some switches, and this is in my pre-electrical knowledge days, the days when taking out a switch and replacing it meant only matching up wires and screw positions. With that said, I have some 15 amp rated switches on 20 amp circuits. They have been there for more than 2 years since I replaced them, and no problems so far, blown switches, fires, etc. Will having a 15 amp rated switch on a 20 amp circuit cause a problem, if it hasn't by now?

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<%-name%>
• posted on April 29, 2005, 4:39 am

Oddly enough, you are not supposed to put a 20a outlet on a 15a circuit. That is to prevent you from plugging a 20a device into it. It is perfectly acceptable to have 15a outlets on 20a circuits; the only exception being where it is the only outlet on the circuit. Then it must be 20a. I suppose that is because you wouldn't have a 20a circuit for one outlet unless you wanted to plug a 20a device into it. There is no difference between a 20a outlet and a 15a outlet except the holes. They have different ratings/holes only to stop you from plugging a 20a device into a 15a circuit. Since 20a devices are pretty rare, you should just use 15a, as your electrician did.
I am tempted to say that there is also no difference between 15a and 20a switches, but I don't actually know that. It would be really unusual to have anything over 15a controlled by a switch; and if you did, you would probably want a commercial grade 20a switch on it anyhow. (I put a 30a switch on my water heater, but that is a bit different.)
Does this make sense?

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<%-name%>
• posted on April 29, 2005, 10:20 am
It does, yes, so bottom line, I am alright having 15 amp rated switches installed on a circuit breaker that is 20 amps?

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<%-name%>
• posted on April 29, 2005, 6:21 pm
To make more explicit - the normal duplex outlet is 2 outlets and a 15A duplex outlet can be installed on a 20A circuit. (As I remember, a 15A duplex receptacle is rated for 20A total combined current from both outlets, but only 15A max from each single.)
Bud--
toller wrote:

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<%-name%>
• posted on April 29, 2005, 11:32 pm
wrote:

Yes, if you have more than one receptacle(duplex recept is two).

No, 15amp duplex is only rated for 15amps total, and max 12 amps on one. Meaning you can have 12 amps on one, and only allowed ot have 3amps on the other.
Becareful, reference the Codes for accurate information.
tom

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<%-name%>
• posted on April 30, 2005, 7:54 pm
The Real Tom wrote:
> wrote: > > >>To make more explicit - the normal duplex outlet is 2 outlets and a 15A >>duplex outlet can be installed on a 20A circuit.
>Yes, if you have more than one receptacle(duplex recept is two).
That, of course, is exactly the point that I was making.
>> (As I remember, a 15A >>duplex receptacle is rated for 20A total combined current from both >>outlets, but only 15A max from each single.) > > > No, 15amp duplex is only rated for 15amps total, and max 12 amps on > one. Meaning you can have 12 amps on one, and only allowed ot have > 3amps on the other. > > Becareful, reference the Codes for accurate information. > > tom
In 20 years as a master electrician I remember reading a 15A duplex receptacle on a 20A circuit can supply a total of 20A but only 15A from either of the outlets. But I could be wrong, unlike, from your post, you.
I can give you 2 plausability arguments:
1. A duplex outlet is designed for "wire-through" installation - you can connect a supply wire to one screw and a down-stream load to the other terminal. Since a 15A receptacle can be installed on a 20A circuit it is reasonable to expect this wire-through connection is safe for 20A pass-through current. Not designing it this way would be dangerous. As I remember, the sockets that the plug connects to individually connect to the side strap with the screws. It would be difficult to build a receptacle that would not supply a total of 20A.
2. There is nothing to prevent 2 plugs in a 15A duplex outlet with a total of 20A total load. It is entirely reasonable to expect that this will happen. It is reasonable to expect that the UL standard for receptacles would require a 15A receptacle to supply a total of 20A. Not having this rating would be dangerous.
12A on one???? You can only have 12A continuous (over 3 hours) on a single. No problem with 15A up to 3 hours. Be careful applying the code for accurate information.
Bud--

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<%-name%>
• posted on May 1, 2005, 2:32 pm
I have a similar question involving 30 A circuits for a workshop shed.
I have a surplus 30 A circuit breaker and some 3 wire (red, black, white, bare ground) 30 A wire also surplus. I want to add a heavy duty pair of circuits for power tools in a shed in my back yard. I will be adding a subpanel to the shed and would like to have an edison circuit(s) to supply the recepticles. The recepticles would be 20 A type, all wiring would be 30 A and I would install a second 30 A breaker next to the first in the subpanel.
If this is not allowed what could 30 A 120 V circuit in a workshop be used for?
I realize that this is sort of change of topic in this thread but the people answering seem well informed. I don't trust my local "electrical inspector" to follow the electrical code. His day job is an electrical contractor and he did some bad work for me.

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<%-name%>
• posted on May 1, 2005, 5:02 pm
Bond R. Milton wrote:

You need to take the time to learn the basic wiring techniques and materials as well as the rules that govern the installations in your area.
What type of wire do you have?
Is the thirty amp breaker double or single pole?
A thirty amp branch circuit may only supply thirty ampere receptacles. If you use the thirty ampere circuit as a feeder to supply a panel in your shed then you could have two twenty ampere receptacle circuits, a fifteen ampere lighting circuit, and a fifteen ampere heat cool receptacle for a heater or air conditioner in that panel. You will need to build a grounding electrode system in the shed to connect the Equipment Grounding Conductors to earth at that end of the circuit. -- Tom H -- Tom H

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<%-name%>
• posted on May 1, 2005, 5:21 pm

Install a 4 breaker sub-panel in the garage, fed from the 30amp circuit breaker in the house.
Now you can install 15 or 20 amp breakers as needed for the garage, and not have to run inside to the house to reset them, unless you overloaded the whole thing.
Two advantages, not as far to go to reset a breaker, and the ability to put lights on separate circuits, so they don't go off with an outlet that trips a breaker.

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<%-name%>
• posted on May 2, 2005, 1:21 am

Code violation. 20A-rated receptacles are not permitted on a 30A circuit. May be a second violation, too, depending on whether your receptacles are listed for use with 10ga wire.

30A-rated tools?
-- Regards, Doug Miller (alphageek at milmac dot com)
Nobody ever left footprints in the sands of time by sitting on his butt. And who wants to leave buttprints in the sands of time?

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<%-name%>
• posted on April 29, 2005, 3:13 pm

Are the switches controlling a 20A outlet, or just some lights? In the former case, yeah, you could have a problem (read: fire hazard). Imagine, say, a 20A load plugged into the outlet, and you switch it on via the 15A switch. It's probably going to be a slow overheat sort of failure. If you're lucky, something shorts while the switch dies, the breaker trips, and you have a Homer Simpson (d'oh!) moment. If you ain't lucky, your obit has a notation involving the words "smoke inhalation".
IMHO, if the breaker is capable of letting the current through, the downstream wire, switches, and outlets should be capable of handling said current, even if code allows a few shortcuts along the way. The difference is cost is minimal at the onesy-twosy level.

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<%-name%>
• posted on April 29, 2005, 11:30 am
Eric and Megan Swope wrote:

I think the confusion about the outlets is not too hard to understand if you consider that an outlet rating is not just how much current it can safely carry, but also the configuration of the slots for a plug. A 15A outlet will not let a plug designed to handle 20A to plug into it. A 20A outlet will. So if you put a 20A outlet on a 15A line, you could plug in a device that requires 20A on that line.
--
Joseph Meehan

Dia duit

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<%-name%>
• posted on April 29, 2005, 1:29 pm
Pa. code is now saying 15 amp circuits to all rooms. Receptacles and lights for that room on the same circuit, direct feed. No junction boxes. Exceptions are kitchens and dining rooms. They can be 20 amp, but no junction boxes. The best our electrical shop can figure is they are trying to do away with extension cord problems for one. Most extension cords are 15 amp. Before, people would buy an extension, plug it into a 20 amp receptacle and octopus everything into it. FIRE in the making. Now on a 15 amp breaker, so it trips before fire. Junction boxes??? No extra connections that can become loose? Don't make it easy for the not so knowledgable DIYer?? If you don't know your wire size, use a 15 amp receptacle. Unless an old home, the circuits are probably not pushed to the amp limit anyway.