Circuit amperage question

If Jenny opens a ceiling box and finds ANY Rubber wire, it's time to check a lot closer. That rubber insulation is long-lasting but it was never meant to go forever, you need an honest assessment of the condition in all the junction boxes.

It may be in good condition in the conduits, but if it's been cooked brittle in the light boxes and crackles when you bend it, that's the time to schedule a rewire BEFORE you have a fire or a failure. If it checks good now, check it again every 10 years or so.

And if it's a 20A circuit there's enough excess capacity you can safely put a few table lamps or other small fixtures on that line with the window air conditioner - but not the coffeepot or toaster. This is the time to get a P-Touch or Dymo Labeler and identify all the circuit numbers on the wall plates, and add up the fixed loads.

And definitely not two space heaters, like I found at a rental house one time. (Worked when they were both set to Low, but kick one heater to High and *trip*.) One 18A continuous load to a circuit, not two.

In any house with Rubber or TW Vinyl (first generation 1950's-60's) a Green wire is probably NOT a ground, but a hot. I get the occasional call of "I added/extended a circuit and turned the power back on, and now there's a hard short" - they grounded the Green wire like they thought they should...

And if the house is still Knob & Tube wiring it's Way Overdue for a rewire. That's a fire waiting to happen, especially considering that the insulation has probably fallen off all the wire. You don't even go up in the attic without turning off the power to the house.

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I take these comments as solid arguments, based on facts, Roy.

For those of us that have seen what's done when someone is too cheap, a bad worker, in over their head, in a hurry, thinks they are smarter than the NFPA/NEC, or otherwise unqualified, even the smallest assumptions will result in potentially hazardous conditions.

The obvious solution to the original question would be to run a new circuit with only one receptacle on it, period. I'd go with 12ga and a 15A breaker, and a 20A rated receptacle (for durability/reliability). This would probably be about a $35 DIY-done-right project (DIY-DR), not for everyone, though. Doing it properly is the only way to know absolutely. The results are a safe place to sleep.

Old receptacles are generally crap.. made well, but worn out (the friction that's felt when plugging/unplugging a 3-wire plug is usually the ground pin, not the other two). New generic receptacles are crap, so buy the better grade, always (without exception for motor and other significant loads).

If the manufacturer recommends a 15A circuit, I don't see where a 20A circuit is good enough (this is merely the "it'll get it to run" philosophy). My last window unit (about 12k BTU) stated a separate 15A circuit. Subjecting the compressor motor to momentary delayed overcurrent conditions is going to be hard on that motor, maybe even lead to failure. One example would be a brief power failure while the compressor is running/cooling cycle, and when the power returns, there is an overload condition for the compressor motor. Most consumer units won't have any overload protection devices, as they would raise the cost of manufacturing the units. If a 20A breaker "waits" briefly, for a 20A overload, bad juju.

The Navy electrician story may be factual, or just an anecdotal seller's comment. Was he honorably discharged, or bad attitude, BCD/DD for negligence resulting in a major loss? Just because he may have gotten the tattoos doesn't mean squat. Nothin' wrong with squids though, in general.

WB ......... metalworking projects

That violates the NEC. A 15A circuit requires 15A receps. A 20A circuit may feed 15A or 20A receptacles.

I trust that you're correct, Ned, and it sounds reasonable. I should've recommended quality heavy duty receptacles. I'd be very concerned (alarmed) if it were a 20A receptacle on a 14ga circuit.

Marking the receptacle coverplate as 15A maximum would me my SOP.

BTW, aside from the suggested separate circuit issue, many receptacles aren't rated for the same feedthrough capacity that they can pass individually (as in a separate circuit). The pass-thru capacity may be lower, and marked on receptacles' strap (in small print).

WB ......... metalworking projects

aaah guys - the receptacle is irrelevant - the circuit is protected by the breaker - you can put a 100 amp receptacle on a 14 gauge circuit and use a

15 amp breaker - it will be safe - it won't power a 100 amp device, but it will safely deliver the 15 amps via the 100 amp receptacle (now, why you want to do that might be a different question)

The code is designed to protect the plant, not the equipment

No violation - you can under-breaker a circuit if you want, if the wires and receptacle can handle 20A you can use a 15A breaker as equipment protection. It's silly on a short run, but legal - consider that people use a larger wire gauge than the breaker (sometimes several sizes larger) all the time for voltage drop on long wire runs.

But you can't put a 20A breaker on that circuit and over-protect it if the entire circuit can't safely carry that much current. I.E. there's a wire gauge change to 14-gauge in mid-run.

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Larger wire is obviously OK, but *not* a 20A receptacle on a 15A circuit. I don't have the latest version of the NEC, but it's in table

210-24 in the 2002 edition.

20A receptacle means one with contacts like this ( | |- ) that'll accept a 20A plug with blades like this ( - | ).

Yup, the $.59 receptacles are pretty sketchy. You only have to spend $2-$3 to get something pretty good. I have a stash of new hospital grade receps that I found cheap at a salvage place for use in places where they get especially heavy use.

Not if you care to conform to the NEC. See my reply to Bruce and

which says, in part:

************************************************** Q: Can a 15A receptacle be installed on a 20A branch circuit? A: Yes, per the 2005 NEC Article 210.21, a 15A receptacle can be installed on a 20A branch circuit so long as it is not the only receptacle on that circuit. A single receptacle on a 20A branch circuit must be 20A rated. Q: Can a 20A receptacle be installed on a 15A branch circuit? A: No. According to the 2005 NEC Article 210.23 "in no case shall the load exceed the branch-circuit ampere rating." **************************************************

That is incorrect.

It is permissible to put a 15A breaker on a circuit that is wired with 12AWG copper conductors.

It is *not* permissible to put a 20A-rated receptacle on that circuit. The 15A breaker makes it a 15A circuit, for which the permissible receptacle rating is "not over 15". [2005 NEC, Table 210.21(B)(3)]

That IS the code - stupid as it is. It would make sense not to allow a

20 amp receptacle on a circuit WIRED as a 15 amp circuit, perhaps, but if a 20 amp circuit(wire and receptacle) is protected at 15 amps, what harm can be caused? No more than having multiple 15 amp receptacles on a 15 amp circuit, for sure.

aaah, ned, the second quote does NOT say you cannot put a 20 or 100 amp plug on a circuit fused for 15 amps (or one amp for that matter), it says the "load" cannot exceed the rating. Not having the NEC handy, I can't look up the definition of load, but in normal usage, one would differentiate between the rating of a receptacle and that of the load.

think about it this way - I plug my cell phone charger into a 15 amp receptacle, but the load is only .01 watt - clearly that's permitted.

Yep, that's great stuff, the hospital/institutional/medical grade hardware. I try to find and buy any of the better grade gear any time that I encounter it. Electrical, utility carts, door hardware, cabinetry and lighting, plumbing fixtures.

I'll generally buy Hubbell and better grade stuff for most anything, but the cost of those better grades are in the range of "more than one would normally want to pay for", not even being a particularly cheap buyer.

WB ......... metalworking projects

That's true -- but Article 210.21(B)(3) **does** say that. The page Ned cited gives the right answer, but for the wrong reason.

Article 210.21(B)(3) says "receptacle ratings shall conform to the values listed in Table 210.21(B)(3)" -- which says that the permissible receptacle rating for a 15A circuit is "not over 15".

Yes, but you're plugging that into a 15A receptacle, too. The point is that a

20A receptacle on a 15A circuit allows plugging in a single load that exceeds the capacity of the circuit and is therefore prohibited.

Sure it does, it says, "Can a 20A receptacle be installed on a 15A branch circuit?" And answers that qusetion, "No."

I agree the explanation is lacking, but it is just an FAQ, and the answer is correct. If you had the NEC handy you could read the applicable section.

This page paraphrases the NEC in more detail.

***************************************************If a branch circuit supplies two or more receptacles:

The total cord-and-plug connected load must not exceed 80 percent of the receptacle rating [210.21(B)(2)].

Receptacles must have an ampere rating that complies with the values listed in Table 210.21(B)(3) (Figure210?44).

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And here's the table:

************************************************** Table 210.21(B)(3) Receptacle Ratings Circuit Rating Receptacle Rating 15A 15A 20A 15 or 20A 30A 30A 40A 40 or 50A 50A 50A **************************************************

Meanwhile the customer, Jenny3k, is long gone.

Why was knob and tubing brought up when she clearly said that the Navy guy used conduit ?

It's because people like to discuss...show off.

Hey, it's a chat room I know....but still humorous.