Breaker on #6 copper

[Doug Miller]

If I offended you, I'm sorry - it was unintentional.

[zxcvbob]

If you have a 30A breaker already, use it. If it ever trips (or just before you sell the house), replace it with a 60A.

Bob

[Steve Barker]

Well good greif! the damn wire will handle about 3 times what they say they will. thats why.

s

[terry]

A couple of the posts here seem to make the assumption that a wire with a 'rated safe' ampacity of 50 amps (i.e. that it can carry 50 amps under normal conditions of voltage drop and length of run etc.) will immediately heat up and burn off if it is made to carry 55 amps for intermittent periods of time. Not so. A 50 amp rated wire (say #6AWG) can probably carry twice that even if getting a bit warm and with a certain amount of voltage drop. And anyone who has ever dealt with an emergency situation where one presses into service whatever reasonable size conductors are available and has a good feel for the basics of electrical resistance, current flow and voltage drop will understand that. In this case we are discussing 'normal' domestic workshop conditions to meet safety and code requirements. With only one or two people working a maximum home workshop load is unlikely to be close to the rated capability. The statement "is only capable of carrying 55 amps" is probably only true in regard to maximum current that could be carried continuously. The reference 'is only capable of carrying 55 amps "at 120V", is superfluous. We are considering current here, not voltage. For example: Today we had two people working on a friends personal vehicle in our domestic garage; the maximum load at any one time was, intermittently;

1) A small wire welder maximum about 20 amps (@230 volt) 2) A sander/grinder or hand drill, maximum 5 amps (@120 volt). 3) A halogen lamp. 2.5 amps (@ 120 volt) Total (intermittently) 27.5 amps, plus maybe another light that was on. Maximum 30 amps say all fed from a panel wired with #6AWG and a 50 amp (Square D) breaker from the main house panel about 40 cable feet distant. Nothing blinked or went dim etc. Caught fire or popped a breaker.

[mm]

I don't understand any of the 3 degree things.

[mm]

Absolutely. The heat, if any, generated by the current flowing through the wire is totally dependant on how much current is flowing, and unrelated to the voltage applied to the whole circuit.

I think this will help a few people and won't help others at all.

E=IR (Remember that. :) ) In other words: EMF=Current times resistance. In other words, in this case: Voltage = 55 amps x R (resistance)

Power = Voltage x Amps. (Remember this.)In other words, in this case: Watts = voltage x 55 amps.

But the voltage in the formula above is not the 110 or 220 that comes from the wall. It's the voltage drop from one end of the wire to the other, which is small, because the resistance of the wire is small. The total voltage drop of an appliance plugged in to a wall is 110 or

220 or whatever, but almost all of that voltage drop is in the appliance, and not in the cord going to it. It's the saw or washer or water heater or mixmaster that is designed to do the work, and the wire to it is designed to have low resistance and not impeded the electrficity getting to the appliance. But, in this case we're talking about running 6 gauge wire with a 60 amp breaker, even though the wire is rated at only 55 amps. So how much heat (power or watts, iirc) will be generated in that wire if the current goes up to just shy of 60 amps, even though it is rated at 55 amps.

Look at the formulas at the top, and then continue here:

Since voltage = 55 amps x R, then: Watts = 55 amps x R x 55 amps. In other words: Watts = (55 amps) squared x R.

Where R is the resistance of the wire feedinn the appliance.

This is not the number of watts given off by a lightbulb, if there is a lightbulb in the appliance, or used by the motor if there is a motor in it. It's the number of watts given off by the wire feeding the appliance. And it is totally unrelated to the voltage applied to the appliance**

**Except insofar as if higher voltage were applied, more current woudl go through the same resistance wire and appliance.

[Doug Miller]

Temperature rating of the insulation (some insulating materials can withstand higher temperatures than others).

[Tony]

That is not quiet right! "voltage drop" in the line times current equals watts which generate heat in or on the line so voltage does have some effect.

EMF= Electro Motive Force

In other words, in this case: IxR=E

In other words:can you make simple (E=Volts) x (I=Curent) =(W=Watts)

And for any General use #6 wire three in conduit is rated for 65 Amps.

[dpb]

...

Well, yes and no...depends on whether it is a constant current or constant voltage source (or another way to consider it is in regards to the chicken and egg :) )...

For a power circuit fed from the grid, one can consider it a constant voltage source.

There is current flow through the circuit only because there is a voltage drop from that source to the neutral.

How much voltage drop is in the wiring depends on the resistance of the wiring, that is correct. But how much current flows through the wire depends on the _total_ resistance from the source back to the neutral (assuming a series circuit, the current through each component is the same), not just the wire. So, given that there is a fixed voltage supply and for a particular piece of equipment on the circuit in the shop, the current will also be (nearly) constant and the voltage drop across the wiring will be determined by that current. Consequently, of the wiring loss, the situation looks more like a current source rather than a voltage source.

But then again, one can analyze it as if there were a fixed voltage impressed over the cable and arrive at the same numerical result...

V = IR

P = VI = (IR)I = I^2 R

Which is more fundamental; the VI or I-squared R form? All depends on point of view... :)

...

[Doug Miller]

That is incorrect -- the ampacity depends on the insulation. Type TW or UF

6AWG conductors, for example, have an ampacity of 55A.

[Eigenvector]

Don't worry about, I have thin skin lately.

But back to my question, which was more out of interest than contesting a ruling. The 5 extra amps is more than just the wire, what about the device at the other end of that wire - it'll be taking on an additional 5 amps too. Maybe not important, if it has its own fuses, they'll blow too. I was just curious is all.

Now this next part is just my mind asking questions, not making personal attacks.

A lot of people are firing back claiming the wire can handle 65 A, 60 A,

2000A, whatever, those current carrying capacities aren't advertised on the wire bundle, so how would an electrician know that? I'm presuming an electrician isn't schooled at the same level as an Electrical Engineer. So looking at a wire and being able to tell the ampacity of it seems liberal to me. When they allow higher breaker sizes it also tells me that the NEC conventions are largely anecdotal or arbitrary as opposed to calculated or theoretical values - which is even more worrisome to me. I would expect them to state restrictions and rules more along the lines of "This is the theoretical limit of this particular wire, plus a safety margin of 1.5 - you may not use something higher than this value" Rather than, "Just use the next highest one, they don't make the correct one for it." If they were to state something like that, I would also expect them to qualify it by stating the reason why they make that allowance. Like I said, just me asking questions.

[Speedy Jim]

Indeed, electricians are required to know all those fine details.

To get some feel for the breadth of Code issues, look at Mike Holt's pages which include a very busy forum on Code issues:

Jim

[dpb]

Eigenvector wrote: ...

The electrician doesn't have to know what the theoretical current-carrying capacity of a conductor is -- all he has to do is learn the basic rules of NEC (or whatever particular code variant he is working under).

The NEC is a product of the NFPA which is a nonprofit organization formed initially by a bunch of insurance underwriters for the purpose of trying to bring some order into common practice and to reduce the prevelance/frequency of fires owing to poor practice (and, given the time in which they started, not in small part, to define what good practice entailed.)

The code is pragmatic and not intended as a technical treatise or engineering specification. That saic, there are bases for each rule and reasons for the rule and the exceptions to the rules. As others have said, the tendency is to make the rules conservative with respect to actual practices that would be an imminent and immediate danger.

Code is written by committee of member representatives and is, for the most part, a volunteer activity. For an overview of the Code development process, see the following link...

Having served on another Standards committee subcommittee in the past with similar rules, it is a protracted process to say the least... :)

--

[mm]

Wow. Somehow I thought all the degree numbers were fortions of a angle! (Maybe because degree didn't end in s. :) ) Thanks.

[HeyBub]

Like most "regulations," there is no requirement that a "reason" be given. Often the regulation is arbitrary and based on emotional satisfaction rather than science (silicone breast implants, arsenic levels in municipal water systems).

Other times, like having a licensed electrician (or union member), be responsible for changing a light bulb, the underlying reason is political or economic.

Sometimes, like the regulations prohibiting PEX in California, the regulation is merely a pride issue.

[Tony]

Doug: no disrespect to you are any one ales in this forum, I have no degree to speak of so I am not going to contradict you But from my understanding insulation do not have anything to do with wire Ampacity or in the other words capacity to carry amount of current. Wire insulation rating is for soly purpose where that wire can be used to be able to carry it's rated current. example # 10 wire insulated with rubber is good up to maximum

140 Deg. F regardless if is carrying 30 amps. or 10 amps. because at that temp. rubber will start to melt. I use lot of glass cover wire and Teflon but # 10 wire is good only for up 30 amps. higher amperage requires larger conductor 8 or 6 whatever I may need so I don't see insulation been factor here I am sorry that is the way I see it Tony

[RBM]

In wire sizes 10-12-14, the NEC limits the ampacity to 30-20-15 regardless of insulation type, but in larger sizes the NEC allows different amperages depending upon insulation type, as well as other factors

[Doug Miller]

No, actually, they won't. A device draws whatever it draws. A 1200W heater, for example, pulls 10A at 120V whether the circuit it's on has a 15A breaker or a 100A breaker.

Nope; see above.

You don't tell the ampacity just from looking at the wire. Well, you might if you have NEC Table 310.16 memorized, but it's the table that tells you, not the wire.

It should be worrisome if your conclusion were correct; fortunately, it's not.

It's important to remember that the Code allows going up *one*, and only one, breaker size, and then only if the listed ampacity does not correspond to a standard breaker size. For instance, a wire with a listed ampacity of 50A may

*not* be breakered at 60A, because 50A is a standard breaker.

There isn't really any difference between the two situations, and the latter is much more convenient to implement in practice.

Perhaps the most important thing that you're missing here is the part of

240.4(B) that says "provided all of the following conditions are met" -- conditions which I only summarized in my earlier posts in this thread. It seems to me that it's time to quote the first one in full, and comment on it:

"(1) The conductors being protected are NOT part of a multioutlet branch circuit supplying receptacles for cord-and-plug-connected portable loads." [emphasis added]

Let's examine the possibilities under which the bump is permitted by this condition:

1) Circuit has no receptacles at all. That means it's feeding either a subpanel, or direct-wired stationary equipment. In the former case, the load will depend on which circuits in the subpanel are in use, and is unlikely to ever be at maximum. In the latter case, the load is precisely known, and the safety (or lack thereof) is readily determined. 2) Circuit has a single receptacle, or multiple receptacles, for cord-and-plug-connected NON-portable equipment. Again, the actual load can be readily determined: you know what's going to be used on the circuit, because it's sitting right next to the outlets, and it isn't going anywhere. A circuit supplying outlets for a table saw and a 5HP air compressor would be a good example of this category. 3) Circuit has a single receptacle for cord-and-plug-connected portable equipment. It is unlikely that any Code-compliant circuits can exist in this category: the *lowest* listed ampacity that would be permitted to be bumped is a 55A conductor breakered at 60A; it is a Code violation to install a receptacle with a rating lower than that of the overcurrent device on any circuit over 20A; and any load which requires a 60A (or higher) rated receptacle is highly unlikely to meet anyone's definition of "portable".

In short, this means that the bump up to the next higher breaker size is limited to circumstances in which the load is either limited, or more or less fixed, and readily predictable.

[Doug Miller]

Well, I'm sorry, but your understanding is incorrect. Ampacity depends on multiple factors, and insulation is one of them. I'll try to explain.

Exactly. And the rating depends on the type of insulation (among other things) -- this is laid out clearly in Table 310.16 of the National Electrical Code.

Which is exactly why wires with rubber insulation are limited to carrying currents low enough to *not* heat the wire up that much. Wires insulated with materials capable of withstanding higher temperatures are allowed to carry more current because they can get hotter without damaging the insulation and creating a hazard.

The insulation *isn't* a factor for AWG10 and smaller wires, because the NEC specifically limits the allowable breaker rating for AWG10, 12, and 14 conductors to 30A, 20A, and 15A respectively, regardless of what insulating material is used.

For wires AWG8 and larger, the ampacity *does* depend on the insulation. For example, AWG8 wires with Type TW insulation are limited to 40A -- but with Type THHN insulation, the allowable ampacity is 55A.

[dpb]

HeyBub wrote: ...

Some of those _may_ have some merit, but I'm doubtful of most...

W/ the NEC, there is a very long and involved process of building the Code and while it isn't required (nor even very meaningful) to publish every nuance of "why" underlying the actual code provisions, there are defensible reasons for them and there has to be sufficient basis to win the approval of a sizable number of participants. At last time I looked there were some 80,000 members of the NFPA and the NEC had something otoh 5,000(!) peer reviewers. With that many folks involved, anything _too_ arbitrary or one-side simply isn't going to fly.

It may appear arbitrary, but like most things, the details are far more complex than they may appear from the outside.