Electrical Code Question

Reading the post below about how code allows using 1 side of a 220 breaker to run a 110 circuit got me to thinking.
So answer this code question...?


How can electric code allow you to put a 15 amp switch/outlet on a 20 amp breaker as long as you use a 20 amp (12 gauge) wire??

BUT....

It is not code to put a 20 amp switch/outlet on a 15 amp breaker.


That seems backwards. Seems to me the breaker should be the smallest amp rating of the circuit so it protects everything in the circuit.

If there is one device on a 20 amp breaker must be a 20 amp device. More than one and you can use 15 amp devices. There is almost nothing different in the devices except the plug layout. There are few appliances that come with a 20 amp plug,, window AC comes to mind. The main idea is to protect the wire with the right size breaker.

The primary purpose of a circuit breaker is to protect the circuit's wiring. So the wiring must have ampacity equal to or greater than the value of the circuit breaker.

However, it's understood that in most cases the total load on the circuit will be greater than its greatest single load. So if you put 15 amp outlets on a 20 amp circuit, it's still possible to overload the circuit and trip the breaker.

Personally, I always put 20 amp outlets on 20 amp circuits; that way I know which outlets are on 20 amp circuits even though I don't have any appliances with 20 amp plugs.

OK I kinda understand that.

I can see a 20 amp breaker on a 20 amp wire with say 4 - 15 amp plugs.
I plug a 5 amp devise into each plug.
So now some of the wire and the breaker are seeing 20 amps but each 15 amp plug is only seeing 5 amps.

You can connect multiple 15 amp outlets to a 20 amp circuit breaker, but you can not connect a single 15 amp outlet to a 20 amp circuit breaker. The outlet is constructed to handle 20 amps, same screw terminals, etc, as a 20 amp. But it is configured to accept a 15 amp plug to limit power to the appliance.


For lights, a 15 amp switch is OK, again, you would not be switching more lights that a 15 amp switch can handle. However if it were me wiring my house though, I would use a 20 amp switch.

The NEC requires that when a single device is installed on a circuit it must be rated for the ampacity of the circuit. The device you commonly have in your home that everyone calls a receptacle is actually a duplex receptacle. This is TWO receptacles mounted on a single yoke, not a single device. Therefore you are allowed to install one duplex receptacle on a 20 amp circuit and meet code requirements. However good practice would be to use a 20 amp duplex as it is possible to be using only one outlet of the device.

John, good question I vaguely remember it being raised way back when I attended NEC Training Seminars. Best I recall it was explained (By Mike Holt or Joe Mc Partland NEC board members and experts) we were to: First calculate the maximum continuous current,,,,,,,,,Then size conductors to have a minimum ampacity of 125% greater,,,,,,,,,,,,,THEN WE SIZED THE OVER CURRENT PROTECTION DEVICE TO PROTECT THE CONDUCTORS (not so much the devices). Other factors we took into account were diversity and using multiple devices on a single branch circuit. In that situation there might be a 20 amp branch circuit using 12 Gauge 20 amp rated wire protected by a 20 amp circuit breaker while if there were say four devices each may only draw 3 or 4 amps SO MORE THEN ONE 15 AMP RECEPTACLES WERE STILL PERMITTED on that 20 amp branch circuit.

As a practical matter when I practiced power distribution I may have specified 120 Volt 15 Amp NEMA 5-15R Duplex Receptacles on 20 amp branch circuits in residential BUTTTTTTTTTT in Commercial and Industrial applications if I used a 20 amp branch circuit I only used 20 amp NEMA 5-20R receptacles or switches.

As always, where fire and life safety is concerned consult current practicing professional engineers (NOT me I'm retired) and electricians, the NEC and any local authority and don't bet your home or life on lay opinions on a tractor forum.

John T

Not really understanding your question. The breaker size is determined by the wire size. If you have a 15 amp breaker that normally means the wire is 14 gauge which is really too small to use for anything but lights even though it's done everyday and to code. If you put a 15 amp outlet on 15amp circuit chances are if it was overloaded the outlet would burn up first being a safer failure. If you were to use a 20amp outlet on a 15amp circuit chances are the wire would fail before the outlet if it was overloaded. It would mean the difference between a spark inside an outlet box opposed to inside of a wall.

The best I can remember from NEC and arc flash training breakers protect the wire. Do you know a Vince May from over your way?

John,
I look at the rating of the outlet (in particular) not as how much load it can handle, but at what devices it ALLOWS to be connected to it.

Using the logic of a higher rated plug is better on a lower rated circuit, then we could put a 30 amp plug on a 15 amp circuit. How about a 50 amp plug on a 15 amp circuit?
Does that make the light go on? It would let you plug a stove into a circuit that was using 14 ga wire.

By only allowing a plug of equal or LOWER rating on a circuit, you a assuring that no one device can overload that circuit.

I understand the confusion on not allowing a 20 amp SWITCH on a 15 amp circuit,
but I believe that the definition of a switch is much broader than your typical wall switch. I believe that it includes devices that also allow things to be connected to them (think of a hot tub switch). Again, you wouldn't want to put a switch that allowed a 30 amp device to be connected to it, on a circuit that had a lower rating.

I hope I added to your understanding, just trying to help.
Keith Williams

Can't help you with an answer but what I want to point out is the wealth of information on this website for something technical like your question. I scratch my head when someone asks stuff about medical conditions, legal issues with your neighbor or the local bylaws, etc ..... but stuff like yours brings out the best in the YT expertise category, well done boys !!!

"breakers protect the wire."

EXACTLY I know a few Mays near Bloomington, but no Vince ???

John T

He teaches 70E classes( arc flash). Been in the electrical field for alot of years. Think he is from the elliotsville area. Good guy and real smart, I enjoy his classes.