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Old 09-06-2007, 07:41 PM   #1
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Default 14 AWG allowable ampacity

Hi all,

With 14 AWG copper the max overcurrent protection is 15 amperes.
But its allowable ampacity is 20 amperes at 60 deg. C.

Now, besides it being code, why cant we have a 20A breaker on 14 AWG wiring if the allowable ampacity states it? Im taking this from table 310.16.

thanks for answering!

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Old 09-06-2007, 08:03 PM   #2
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Nope. Sorry. Would you like a code reference?

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Old 09-06-2007, 08:16 PM   #3
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Hi MdShunk...yeah me again..lol

I have the code already(240.4)(D), Im just trying to figure out why they dont allow us to use 20A breakers if the wire is capable of handling that ampacity.
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Old 09-06-2007, 08:33 PM   #4
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Originally Posted by shocking View Post
Hi MdShunk...yeah me again..lol

I have the code already(240.4)(D), Im just trying to figure out why they dont allow us to use 20A breakers if the wire is capable of handling that ampacity.
Beats me. That's the nice thing about the code. We don't have to think too much if we don't want to. We can just follow what it says.
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Old 09-06-2007, 08:49 PM   #5
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Maybe then in the '08 NEC, they should just skip the "allowable ampacity" table. Dont tell me the cable can handle more, some of us might remember that instead of the correct overcurrent number. haha. Good nite...
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Old 09-07-2007, 12:19 PM   #6
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Quote:
why cant we have a 20A breaker on 14 AWG wiring if the allowable ampacity states it?
You can install a 20 amp or larger overcurrent protective device on a 14 AWG conductor under some of the specific conductor applications listed in 240.4(G).

For example in a branch circuit feeding an air conditioner you can use the minimum circuit ampacity listed on the nameplate to size the branch circuit conductors and install an overcurrent device sized according to the nameplate of the A/C to protect the branch circuit conductors.

Say I have an A/C with a minimum circuit ampacity of 18 amps and a maximum breaker size of 30 amps. I could use 14 AWG conductors for the branch circuit and install a 30 amp breaker to provide my short-circuit and groundfault protection for the branch circuit.

Chris
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Old 09-07-2007, 01:17 PM   #7
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Raider,

Using 14 AWG conductors with a 30 amp overcurrent protection would be a violation, since your going beyond a max of 15 amp overcurrent protection.

Once your conductors get near 30 amps, before tripping the breaker, wouldnt they be fried?
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Old 09-07-2007, 02:26 PM   #8
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Raider,

Using 14 AWG conductors with a 30 amp overcurrent protection would be a violation, since your going beyond a max of 15 amp overcurrent protection.

Once your conductors get near 30 amps, before tripping the breaker, wouldnt they be fried?
It is permissible to use a 30 amp breaker to protect 14 AWG branch circuits for A/C units.

240.4(D) says unless specifically permitted in 240.4(E) or 240.4(G) the overcurrent protection shall not exceed 15 amps for 14 AWG.

240.4(G) gives us specific conductor applications that permit larger overcurrent devices for small conductors. If you look at the list in 240.4(G) you will see Air-conditioning and refrigeration equipment circuit conductors Article 440, Parts III, IV.

Article 440 Part III is titled Branch-circuit short circuit and ground fault protection. We are permitted to follow the requirements of this section which allow the branch circuit short circuit and ground fault protection to be sized no larger than 175% of the motor rated-load current or branch-circuit selection current whichever is greater.

Take a look at Article 440 for sizing your circuits of an A/C unit and you will be suprized by what you can do.

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Old 09-07-2007, 02:45 PM   #9
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I see. It also says should not exceed 225% of motor load current.
So if we have 15A, we can go as far as 33.75 amps.
This also applies to pool pumps I would think, where they draw lots of current initialy.
But all this is not continous load.

Bottom line is stick with 15amp protection for 14 awg.
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Old 09-07-2007, 04:33 PM   #10
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Quote:
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Bottom line is stick with 15amp protection for 14 awg.
You'll change your tune, in time, when you start dealing with motor loads.
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Old 09-10-2007, 10:28 AM   #11
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You'll change your tune, in time, when you start dealing with motor loads.
Agreed,

There is no reason on a motor or A/C circuit, not to use the relaxed rules regarding short circuit and ground fault protection and install branch circuit conductors that are rated for the load and not the higher overcurrent device rating.

Chris
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Old 09-10-2007, 03:29 PM   #12
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additionally for derating
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Old 09-11-2007, 02:12 PM   #13
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Quite a way back the fellows who write the code took romex derating into consideration and required no more than a 15 amp breaker for #14 cable, no more than a 20 amp breaker for #12 and no more than a 30 amp breaker for #10 cables for most purposes. There are exceptions as other posters have noted. The notable part about all this is the fact that nm cables have de-rating factored into them even before you break the seal on the roll of cable. The current trend of increasing the rules about de-rating cables when "bundled" (code still has not properly defined just what that constitutes) for more than 24", when more than two cables are run up thru top plates where firestoping is used, ect, is in my opinion due to a lack of understanding on the part of current code panels on the previous history of the already present de-rating built into the cable to begin with. Soon just having two cables come into contact with each other if buried in thermal insulation will be enough to require further de-rating. Supreme overkill for a problem that did not exist in the first place.
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Old 09-13-2007, 11:49 PM   #14
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Quote:
Originally Posted by shocking View Post
Hi MdShunk...yeah me again..lol

I have the code already(240.4)(D), Im just trying to figure out why they dont allow us to use 20A breakers if the wire is capable of handling that ampacity.
Because they want to leave "head room" for the inevitable situation where a homeowner "fixes" a breaker tripping problem by replacing the breaker with a larger breaker. The fudge factor is there because of stupid homeowners.
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Old 09-14-2007, 03:25 AM   #15
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Quote:
Originally Posted by Reddy Kilowatt View Post
The fudge factor is there because of stupid homeowners.

There is a pretty high fudge factor when your helper pushes a fish tape into a live buss on a panel too! Ok, maybe that is a little different. It sure is quite a sound though.

~Matt
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Old 10-18-2007, 12:56 PM   #16
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also becareful, the most common rating of lugs a termination is 75 degree that changes everything
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Old 10-18-2007, 09:38 PM   #17
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Quote:
Originally Posted by Reddy Kilowatt View Post
Because they want to leave "head room" for the inevitable situation where a homeowner "fixes" a breaker tripping problem by replacing the breaker with a larger breaker. The fudge factor is there because of stupid homeowners.
That's it. It's a built in safety.
You think that wire is going to burn at 21A? Nope, just built in safety factors.
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Old 05-09-2013, 08:58 AM   #18
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You shouldn't use too small of a conductor anywhere, especially not on a motor like a air conditioning compressor. The reason being is when the compressor first starts the usual amperage is close to the LRA of the compressor for a very short period. If the conductor is undersized, even if there is an exception in the code for it, it's not a good idea.

Why? When that compressor firsts starts up a large current is going to flow through the wires, if they're undersized then they generate heat. The creation of heat has to come from somewhere, and it's coming from a voltage drop created. Compressors can be hard enough to start sometimes, but if you introduce a voltage drop at start-up you're going to find that the compressor has a premature life, cooks capacitors too quickly or has starting problems.

In general it's not good to get away with the bare minimum conductor because your trade off will always be heat and a drop in voltage. Your wire becomes a resistor if you will.
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Old 05-09-2013, 10:07 AM   #19
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It's always the n00bs
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Old 05-09-2013, 12:58 PM   #20
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It's always the n00bs
Eric, yes but still searchable on Google which is how I found it. It's a rather important subject so I joined to post that much needed reply. "Noob"? Maybe to the forum but I am speaking from a long established background in electrical engineering.

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