15a switch on 20a circuit

[Mike03a3]

Thjs was in another thread that already had way too many twists and turns, but I did want to bring it up, so I broought it here:

Necdigest ?, February 2007

Compiled by Jeff Sargent and the NFPA Electrical Engineering Team

Q. Can I use 14-2 wire for the switch leg of a light fixture where the branch circuit is protected by a 20-amp circuit breaker? If this is against Code, why can I use 15-amp switches and receptacles on 20-ampere circuits?




A. The conductors from a switch location to the lighting outlet it controls are considered branch circuit conductors, not tap conductors connected to branch circuit conductors. These conductors are subject to the general overcurrent protection requirements of Section 210.20(B), which points to Section 240.4 for the specific overcurrent protection requirement. Section 240.4(D) specifies that 14 AWG copper conductors are to be protected by an overcurrent protective device with a rating or setting no higher than 15 amperes. And Table 210.24, which summarizes the requirements for branch circuits with two or more outlets or receptacles, specifies that the minimum conductor size for a 20-ampere-rated branch circuit is 12 AWG......

If it was answered, it didn't make the original quote, but I'd like to understand the answer to the switch part of the question:

If this is against Code, why can I use 15-amp switches and receptacles on 20-ampere circuits?

I understand that all UL listed receptacles are actually investigated for use in 20a circuits, and that 15 and 20a versions of any given grade actually use the same internals, differing only in slot configuration. That all makes sense.

What I don't understand is why a 15a rated switch can be used to switch a 20a circuit.

 

[roger]

IMO, the rules of 404.14 makes it our responsibility to use switches for the loads served.

A 20 amp circuit may feed numerous 60 watt switched loads, why would we need to use 20 amp switches for each .5 amp load?

404.14 Rating and Use of Snap Switches
Snap switches shall be used within their ratings and as indicated in 404.14(A) through 404.14(D).

If we are indeed switching a 20 amp load with a 15 amp rated switch we are in violation of 404.14

Roger

 

[Dennis Alwon]

The 15 amp switch is on a circuit that has 20 amp wire. The switch itself is usually carrying minimal voltage as Roger has said. That is why it can be 15 amp rated. Now if you had lots of lights that were controlled by this switch and the load total was 16 amps then you would need to use a 20 amp switch.

The same is true for receptacles. We have many 15 amp recep. on a 20 amp circuit but usually each plug has little to no amps on it. If you were to plug a heater or a large window a/c that drew current higher than 15 amps into those receptacles, then you may have trouble with that plug after some time.

 

[Mike03a3]

Dennis & Roger

I appreciate the feedback, and understand the logic. However, if that 15a switch is only switching a 150w ceiling light attached to our 20a circuit, the code still requires 12awg wire from the switch to the outlet.

We've had many threads over the issue of the wire size from the switch to the outlet, and the consistant agreement has been that #12 is required on a 20a circuit. Yet the code allows a 15a switch in the middle?

(I guess this is where Charlie reminds me the code doesn't say what I think it says, or what I want it to say; it says what it says.)

But it says the switch must be rated for it's use. Why doesn't that mean 20a if the circuit being switched is rated for 20a? We have no way of knowing that there won't be 20a worth of appliances plugged into a switched duplex.

 

[celtic]

Wasn't there a thread here where someone indictated that the internal components on a 15A recept. and a 20A recept. were the SAME? The only difference being the external NEMA config. ?

Would the same be true (if the above is true) for 15/20A switches?

 

[winnie]

Trying to focus on the switch question, and not discuss receptacles ....

Code requires two current related things of the branch circuit conductors. 1) That they have sufficient ampacity for the load served. 2) That they be suitably protected by the OCPD feeding the circuit.

But code only requires one current related thing of switches: 1) That they be rated for the load served.

The code as written is clear, so we don't need to invoke Charlie's law. The question is: what is the logic behind these somewhat different requirements?

My best guess: there has not been a problem identified with this practise, so there have been no suggestions to change these respective code requirements. But I could be wrong on both counts.

-Jon

 

[charlie b]

I think the answer is simple and straight forward, which means I do not understand the question.

Each set of (load PLUS the switch that controls the load) is in parallel with each other set. The switch only sees the current that will be passing through the load it serves. If that load is under 15 amps, you can use a 15 amp switch to control it.

But that current will be added to the current passing through the other sets of (load PLUS switch). The total current seen by the OCPD is the sum of the currents in the parallel paths. So the OCPD must be rated for the entire load (in this case, 20 amps). But the switches don?t need to be rated for the entire 20 amps. As has already been said, if a given switch serves a load that exceeds 15 amps, then that switch must be rated for more than 15 amps.

 

[charlie b]

However, if that 15a switch is only switching a 150w ceiling light attached to our 20a circuit, the code still requires 12awg wire from the switch to the outlet.

The thing we are trying to avoid is allowing a conductor to pass a current that exceeds its ampacity. The 20 amp breaker can only protect a conductor of #12 AWG or larger. Perhaps the load is only 150 watts (1.25 amps). But if there were a ?high impedance failure? within the luminaire or elsewhere, such that the ?fault current? were to be 20 amps, and if that current were to flow through a #14 switch leg, then that current would be able to flow forever. The breaker would not trip. That could lead to a fire.

I think the bottom line is that the requirement for using #12 for switch legs has to do with a possible high impedance fault, and is not related to the amount of load we intentionally install on that circuit.

 

[iwire]

I think the bottom line is that the requirement for using #12 for switch legs has to do with a possible high impedance fault, and is not related to the amount of load we intentionally install on that circuit.[/SIZE][/FONT]

That sounds great.....but 14 AWG NM is actually rated 20 amps and for some loads we can use 20 AMP OCP. :smile:

My best guess is that Jon's answer was closest to the mark.

there has not been a problem identified with this practise, so there have been no suggestions to change these respective code requirements.

 

[augie47]

switch

switch

for what it's worth, I found both answers informative (now, if I wasn;t too old to remeber them). In reading Jon's answer, I thought it hit the nail on the head. In addition, charlieb's input on high impedance failure brought to light an aspect I had never considered in mixing of wire sizes.
I am continually amazed by the knowledge demonstrated here.

 

[LarryFine]

We've had many threads over the issue of the wire size from the switch to the outlet, and the consistant agreement has been that #12 is required on a 20a circuit. Yet the code allows a 15a switch in the middle?

Maybe it's because the device is mounted in a prophylactic device box, and the cabling is concealed within the structure.

Wiring methods are as much for protecting the environment from the wiring as they are for protecting the wiring from the environment. Remember, the F in NFPA stands for Fire.

Wasn't there a thread here where someone indictated that the internal components on a 15A recept. and a 20A recept. were the SAME? The only difference being the external NEMA config. ?

I have said that many times, for devices of a given grade.