Breaker on #6 copper

[Toller]

I am putting in a subpanel for my workshop. I need another 240v line, so I am ripping out the two 120v lines and the 240v line I have already put in and replacing them all with a subpanel. Wish I had done it right the first time, but that boat has sailed.

I am running #6 copper. As I read it, it is good for 55a, and since there is no 55a breaker, I can use a 60a. Is that correct?

On the other hand; the most I will ever ever use at one time is a bandsaw and DC, which draw 27a together (which is why I need another 240v). I happen to have a 30a breaker. Any reason not to just use that? It is over the 80% rule, but I presume that applies to the wire rather than to the breaker. If it trips, I can always replace it.

The #6 was only a few dollars more than the #8; so what the heck...

[RBM]

60 is fine

[Tony]

100 foot run in dry condition good up to 65 amp. (ITE ret.) 3 wire conduit size 1"

[M Q]

No. Use a 50A breaker (or less).

The 80% applies to the breaker, so with a 30A breaker you can only run 24 A continuous load. And maybe you might also want to run some lights, a vacuum, a radio, a charger for some handheld tools, etc.?

[David Martel]

RBM,

Is the OP wrong that the #6 line is rated for only 55 amps or are you saying that a 60 amp breaker is safe with wire that will fail at 55 amps. Something is wrong here.

Dave M.

[M Q]

...

I should clarify my comments here. The ampacity of the #6 wire depends upon the type of wire. OP didn't say what type. If it is a type that has a 55 A ampacity, then use 50 A breaker. Other types of #6, may have ampacities of

65, 75, or other.

[Doug Miller]

Nothing is wrong at all. The NEC specifically permits going up to the next higher standard breaker size when the rated ampacity of the wire (55A in this case) does not correspond to any standard breaker size.

[Doug Miller]

Wrong. Please familiarize yourself with the NEC before attempting to answer electrical questions.

A 60A breaker is perfectly fine:

"Devices Rated 800 Amperes or Less. The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all of the following conditions are met ..." [2005 NEC, Article 240.4(B)]

The conditions can be summarized as not a multiple-receptacle circuit, no matching standard breaker, and next size up not > 800A.

[Doug Miller]

Correct; this is specifically permitted under Article 240.4(B).

Remember that the "80% rule" applies only to circuits serving a continuous load, which is defined as "a load where the maximum current is expected to continue for three hours or more." [2005 NEC, Article 100]

This usage probably doesn't meet that definition.

Actually, it applies to both (see Article 210.19(A)(1) [wire] and 210.20(A) [breaker]), but, as noted above, this installation doesn't appear to meet the definition of a continuous load anyway, so it doesn't matter.

[Doug Miller]

Clarifed or not, your comments are still wrong. It's perfectly OK to use a 60A breaker on a conductor with a listed ampacity of 55A, under Article 240.4(B).

Likewise, if the listed ampacity is 65A, a 70A breaker may be used, and if the listed ampacity is 75A, an 80A breaker may be used -- all because standard breaker sizes do not include 55, 65, or 75 amps.

[Ignoramus12500]

Yes, it is correct.

I did the same thing, ran a 60a circuit to my garage, and I regret to this day that I did not make it a 100A circuit.

Spend a few more bucks and get a 100A circuit. You can then buy some large equipment (welding, plasma cutters) and run them inexpensively.

i

[gfretwell]

If this is THHN/THWN wire in conduit you can use the 75c column of

310.16 (65a). If you are using a non-metalic cable NM-b or UF-b you are specifically limited to the 60c column (55a) but you can still "round up" so 60a breaker is OK either way. You can't use the 90c column (75a) because the breaker terminations are only rated 75c. The 90c column is generally only there for derating purposes.

[Wade Lippman]

No, I can't see ever running the bandsaw for 3 hours. Thanks

[Doug Miller]

And even if you were, it's very unlikely that it would be pulling maximum current the entire time.

[Eigenvector]

That doesn't make sense to me at all.

Why allow the breaker to trip at a higher rating than the wire? Conceivably the wire would fail without the breaker ever tripping. That presumes the wire rating of 55 A means it is only capable of carrying 55A at 120V

[M Q]

wrote: ...

Mea Culpa. I was wrong on two points. Thanks for correcting me.

I will now slink off with my tail between my legs.

[Doug Miller]

Whether it makes sense to you or not, it *is* the Code. Article 240.4(B), to be exact.

What do you mean by "fail"? Melt? Start a fire? Neither will happen as a result of applying a 60A breaker on a 6AWG copper conductor.

The vast majority of the time, a typical circuit with 60A overcurrent protection isn't going to be carrying anywhere near 60A. In any event, the difference between 60A and 55A on a #6 wire is not going to mean the difference between the wire igniting vs. not igniting. I imagine that the NFPA (authors of the NEC) decided that five amps on a wire that size wasn't enough to worry about. You could always ask them.

[Wade Lippman]

My cottage had a 23a water heater on #12 for 35 years before I found it and replaced it with #10. That is about the worst possible thing, since it comes on at full draw, and stays on for a long period of time; yet nothing happened. Code is rather conservative, which prevents many fires.

And don't forget, the wire is 90degree and the panel and breaker are

75degree; yet the 55a limit is as though they were 60degree. So the extra 5 amps is not a big deal.

[RBM]

You're also presuming that the heat generated by pulling 60 amps through a #6 copper wire would cause it to reach it's flash point. I'm pretty confident that the NEC has figured in considerable leeway

[Eigenvector]

You got better treatment than I did from him.