15 Amp vs. 20 Amp

[Bud]

15A receptacles are rated 20A for both halves. The UL standard tests them at 150% of rating, which I presume means 1/2 a 15A duples receptacle is tested at 22.5A. Posts in other threads have said that 15 & 20A receptacles are identical, including sockets for 20A plugs behind the 15A plastic face.

As HorneTD has said in a number of threads, the NEC requires the switch has to be large enough for the load. If a switch is connected to a light fixture, it would be a little difficult to lamp it at over 1200W, the rating for a 10A switch. Going through my box of salvaged switches I found several with a 10A 120V rating. The NEC is a pragmatic code; if there are problems the code is changed to deal with them.

Bud--

[CL (dnoyeB) Gilbert]

And this is why we have Murphy's Law. You don't size the switch to the intended load. You size the switch so it can't cause a fire. If that fixture endures a resistive short, and its passing 15A, not enough to blow the breaker, but too much for the switch, you have a serious problem. So I guess NEC needs an updating wrt/ Murphy's Law.

[CL (dnoyeB) Gilbert]

Good point.

[Percival P. Cassidy]

On 08/05/05 10:09 am Chris Lewis tossed the following ingredients into the ever-growing pot of cybersoup:

Huh!? If you are switching a device or devices that pull 15A while operating, it's extremely unlikely that it/they will pull less than that at switch-on time. ISTM that the switching capacity should be higher than the "running" capacity.

Perce

[Rick]

A look at the Leviton and Cooper product lineups show all residential switches to be rated at 15 or 20 A. From the Leviton technical reference, AC/DC switches can be used on motor loads that are 50% of the switch rating (80% for AC only switches) . To qualify as a motor rated switch, an AC switch is tested at 6X the full load motor HP rating current, 10X for DC, and goes through 50 make/break cycles in addition to the normal overload endurance and heating tests

Your example of a 15A resistive short is 1800 watts. I would imagine governing bodies view the chance of a 15A switch made of fire retardant materials, installed in a box, and running a few amps over it's rating starting a fire is insignificant compared to the short itself.

[Chris Lewis]

According to CL (dnoyeB) Gilbert :

Note that switches have (at least) two ratings. Interrupt rating and passthru.

Some switches may indeed have a 10A max _switch_ rating, but they're still rated 15A (or 20A) continuous. The switching rating is based on arc erosion.

_Most_ breakers have a relatively low switch rating. You're not supposed to use breakers as a power switch unless they're rated for it.

[Chris Lewis]

According to Percival P. Cassidy :

The continuous rating is based around how much current can go through closed contacts without overheating.

This is radically different, and usually considerably _higher_ than you can do thousands of "make/break" cycles with.

The fact that a switch has a switch rating of 10A, with a continous current rating of 20A doesn't mean that it's deliberately designed for a circuit where this will happen all the time.

Not at all.

It just means that you limit the stuff you switch by it to 10A, but if you have a resistive short in the switched stuff that only passes

20A, the switch can safely pass the maximum current that the breaker will let through.