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Old 09-01-2009, 12:24 PM   #16
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Originally Posted by micromind View Post
That's right; #14 wire connected to a 40 amp breaker is completely code compliant, provided the motor is hard-wired.
At what insulation temp. rating? Two conductors plus ground?
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Old 09-01-2009, 12:26 PM   #17
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In a circuit that feeds anything except a motor, A/C unit, or welder, the wire size must match the breaker rating. The wire can be larger, but not smaller. In your case, if the wire is #14, the breaker must be a 15. If it is #12, the breaker can be either a 15 or a 20.

A 15 amp duplex can be used with #14 (or larger) wire and a 15 amp breaker, or it can be used with #12 (or larger) wire and a 20 amp breaker.

A 15 amp single receptacle can be used only with a 15 amp breaker, and #14 (or larger) wire.

Rob
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Old 09-01-2009, 12:27 PM   #18
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Please do not let some people try to scare you. Someone put a 20 amp breaker in because they knew no better, or someone put it in due to the 15 amp breaker tripping. Simple as that. When you move in, double check the cable (wire) and see for yourself what size it is. Color alone does not indicate size. Home inspectors are not electricians. If it is #14, replace the breaker with a 15 amp. (safe for DIY'ers if main breaker is off). If the breaker was up sized for the previous owners equipment you may have no issue with it being 15 amp. Maybe you don't have a treadmill.

Chances are slim you would need to upgrade the wire. And the chances of the wire being somehow damaged is pure bunk. In fact you may have to do nothing.
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Old 09-01-2009, 12:38 PM   #19
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Originally Posted by micromind View Post
In a circuit that feeds anything except a motor, A/C unit, or welder, the wire size must match the breaker rating. The wire can be larger, but not smaller. In your case, if the wire is #14, the breaker must be a 15. If it is #12, the breaker can be either a 15 or a 20.

A 15 amp duplex can be used with #14 (or larger) wire and a 15 amp breaker, or it can be used with #12 (or larger) wire and a 20 amp breaker.

A 15 amp single receptacle can be used only with a 15 amp breaker, and #14 (or larger) wire.

Rob
40A should give a temp. rise above ambient of (40/15)^2 = 7x that of 15A. What kind of insulation does this and how long will it last?
Insulation life halves for each 8 C to 10 C rise above ambient.
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Old 09-01-2009, 12:47 PM   #20
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Yoy, the temperature rating of wire used to connect a motor must be at least 75C per 110.14(1)(a)(4).

For motors, the 75C column is always used in table 310.16, with two exceptions; de-rating for more than 3 wires in a raceway, and NM cable must be sized to the 60C column.

The 15, 20, and 30 amp limitations for #14, #12, and #10 wires do not apply to motors per 240.4(D), and table 240.4(G).

It seems completely ridiculous to me that you have to look in 4 different parts of the codebook to get the proper wire size for a motor, but that's how it is. I can't even imagine the idiocy of the political fights.....

Rob
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Old 09-01-2009, 01:20 PM   #21
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I don't mean to run this thread off in a different direction, but I think that a few things are important to understand about motors.

First, all motors are required by code to have some type of overload protection. The overload in a typical motor (built-in or separate) is a lot like a circuit breaker, except it is far more sensitive, and it doesn't trip quickly during a short circuit or ground fault.

This overload will also protect the wire feeding the motor. Much more accurately than a circuit breaker in a panel. It provides no short circuit protection though.

All motors draw more current to start than to run. Usually about 6 times more.

If the breaker were matched to the wire size, there's a good chance it would trip on starting. Since the breaker now provides only short-circuit and ground fault protection, it can be much larger than the wire. The wire is protected by the motor overload device.

In the case of our 1HP motor, the #14 wire will never see more than about 16 amps continuously. If the motor is overloaded, the overload device will trip long before the wire overheats. A #14 wire with 75C insulation can carry 20 amps continuously and not be damaged.

This motor will likely draw about 90 amps during starting. This current will not damage the wire, because starting only lasts a few seconds. If the motor fails to start, the overload will trip before the wire overheats. 90 amps might trip a 15 amp breaker though. So the code allows a larger breaker to be used.

The purpose of the breaker is to trip in the event of a short circuit, not an overload. In this case, the 40 amp breaker will see 40 amps (or more) only for a few seconds during starting.

The reason why receptacles are not allowed to be fed with smaller wire and larger breakers even if a motor is connected to them is because there's no way to guarantee that something else will not be plugged in.

I realize that the idea of using larger breakers with smaller wire is confusing, but its existence is valid. If you think understanding this is difficult, try explaining it to an inspector with a ten-pound-badge.

Rob

P.S. JVs post above is exactly correct. Since the circuit in question involves a receptacle, if the wire is #14, use a 15 amp breaker. If it's #12 use a 20.

Last edited by micromind; 09-01-2009 at 01:24 PM. Reason: Added P.S.
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Old 09-01-2009, 01:48 PM   #22
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To further muddy the water:
I finally got a scrap piece of #12 copper Romex, put a thermistor under the outer jacket and ran 10A through it.

It must have taken 15 minutes for the temperature to more or less settle to ~3 C above ambient, so 40A should have given me a 48 C rise above ambient.

The plot thickens!
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Old 09-01-2009, 07:15 PM   #23
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Bingo! We have a winner!

#14 wire is .064" dia. [1/16th of an inch], #12 is .081" dia. [5/64ths]. You guys on the metric system?

I am on metric system anyway here the the converson of mm˛'s is

14AWG = 2.5mm˛
12AWG = 4.0mm˛

Merci,Marc
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Old 09-02-2009, 10:45 AM   #24
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I heard back from some insulation people. They said:
"For the #14 conductors at 10 amps in an ambient temperature of 25C, the conductor temperature would be 33C
[so at 20A it'd be 57 C]

NM cable has a life of 40 years. I have seen some that has been installed for more than 50 years and still looks good.

I haven't seen [a definition] for end-of-life for . . .insulation" [so how did they come up with 40 yrs?]

http://www.electrician2.com/articles/ampacity.htm

Last edited by Yoyizit; 09-02-2009 at 10:49 AM.
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Old 09-02-2009, 01:18 PM   #25
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So just to get this if the receptacle is a duplex (I'm almost certain that it is), then the wiring should be ok? I will check it when we get in the house.

I'm gussing the breaker should still be changed back to 15amp as well as the receptacle (if it is a 20amp).

Is the changing of the breaker something I should be attempting myself, or would it be better to have an electrician do it? I don't recall seeing a shutoff ahead of the panel as Scuba_Dave had asked, but I will chack when we get posession. Is it unsafe to attempt this without shutting of the power ahead of the panel itself?
Are you comfortable working on electric?
Keep in mind that without being able to shut the whole panel off (main breaker in the panel or shut off ahead of the panel) there will be live power in the panel
Having worked with electric since I was a kid I'm comfortable working in the panel. But many people are not

The breaker needs to be changed back to a 15a IF the wire is only rated #14 gauge
If the outlet is a true 20a outlet then it should be changed back to a 15a IMO
I have changed out breakers while a panel is hot, but that's me

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Old 09-02-2009, 04:53 PM   #26
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Is the panel still hot if I turn the main switch on the panel off?
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Old 09-02-2009, 05:10 PM   #27
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My panel (Square D) turning the main breaker off means its all off
If it is a MAIN breaker then Yes everything is dead where the breakers go

BUT the MAIN feed lines are still hot that lead to the main breaker
Post a pic of the panel (once moved in), then we will be able to tell for sure
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Old 09-02-2009, 05:13 PM   #28
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My panel (Square D) turning the main breaker off means its all off
If it is a MAIN breaker then Yes everything is dead where the breakers go

BUT the MAIN feed lines are still hot that lead to the main breaker
Post a pic of the panel (once moved in), then we will be able to tell for sure

Alright, I'll post a pic when I am able to get into the House. Thanks everyone!
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Old 09-03-2009, 01:09 PM   #29
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Turn off the panel main breaker. This kills everything except the wires and lugs that feed the main breaker. Then just snap out the 20 amp breaker if required and replace. You do not need a 20 amp rated receptacle on a 20 amp breaker/circuit unless it a single receptacle. (see below). The type that has 'two receptacles in one' are called duplex receptacles. (see below). You can use this type on 15 or 20 amp circuits. Scuba Dave shows a duplex above. It's a 20 amp duplex receptacle. You do not need this type in most every circumstance. They cost more money and are not always required.

The one on the left is the single type and the one on the right is a duplex.
Note: They are shown upside down.

Last edited by J. V.; 10-09-2009 at 01:19 PM.
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