Wire gauge and current capacity

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  #1  
Old 09-27-04, 11:50 AM
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Wire gauge and current capacity

I'm surprised a search of the forums didn't turn this up, but...

Is there a chart somewhere that shows the amount of current than each AWG wire can handle? For example, I know you need at least 14g on 15A circuits and 12g on 20A circuits. Is there a chart somewhere here or on the net that shows this for more AWG sizes?

Thanks!
 
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Old 09-27-04, 12:21 PM
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There are several reasons why the standard table is not so easy to apply, due to derating and other code requirements. Here an example of the table
http://www.contactelectronics.com/14...escription.htm
 
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Old 09-27-04, 12:38 PM
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However, be warned that applications of this table frequently results in the wrong answer. For example, even though you may have THHN wire does not mean that you can use the column of the table under THHN.

The rule of #14 for 15 amps, #12 for 12 amps, and #10 for 30 amps is easy to apply, is all that most DIYers need, and is usually (but not always) correct. If you need more amps than that, then you have to apply other factors not covered by this table.
 
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Old 09-29-04, 02:55 PM
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Thanks, John, I think that info will be good enough.

I am rewiring a large chunk of the garage and my studio, as most of the old stuff is 2-wire ungrounded. I want to run 2 pairs of 10ga (two separate phases) for a total of 60A capacity out to the garage/studio from the house. That will be sent to a subpanel with 4 15A breakers. 3 for the studio and one for the garage.

Anyone know if 4 10 gauge wires will fit in 3/4" conduit?

BTW, what is THHN wire?

Thanks!
 
  #5  
Old 09-29-04, 03:38 PM
rlrct
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"I am rewiring a large chunk of the garage and my studio, as most of the old stuff is 2-wire ungrounded. I want to run 2 pairs of 10ga (two separate phases) for a total of 60A capacity out to the garage/studio from the house. That will be sent to a subpanel with 4 15A breakers."

You can't run (2) sets of #10 to feed a 60 amp subpanel. While paralled conductors are allowed, it's not until you get to much larger conductors and ampacity levels (like larger than your house service). So, for a 60 amp subpanel, you would pull #6 THHN/THWN.

"Anyone know if 4 10 gauge wires will fit in 3/4" conduit?"

Given that you need to run #6, you can just fit it in 3/4" - but that's going to be a full conduit. Your pull will be a lot easier if you run 1" conduit.

"BTW, what is THHN wire?"

THHN is individual conductor wire with a thermoplastic layer of insulation and nylon outer jacket. The W in THWN means it's wet-location rated.

---------

Based on your questions and comments, it sounds as if you should get a book at the local Home Depot or Lowes and do some reading before you try to run a subpanel. Look for the book Wiring Basics. I say this because you wanted to run (4) #10 conductors to feed the 2 legs of a subpanel. You do need a total of (4) conductors for the subpanel, but need to understand how to size the conductors, what the difference is between the grounded (neutral) and grounding conductors and how they terminate in your subpanel. Is your garage/studio a detached building? If it is, there are other things you need to learn about/do as part of running a subpanel out to the garage/studio.

Rob
 

Last edited by rlrct; 09-29-04 at 03:39 PM. Reason: spelling typo
  #6  
Old 09-29-04, 04:00 PM
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Geoff, your use of terminology is a bit unusual, so it's tough to tell if you're doing it correctly or not. Especially troubling is your use of the phrase "two pair". You do need to run four wires, but these are not two pairs.

When you say four #10 wires, you must mean two hots, one neutral, and one grounding wire. And the two hots will be connected to a 30-amp double-pole breaker in the main panel.

Although we know what you mean when you say "a total of 60A", it's a bit worrisome. You won't have more than 30 amps flowing in any wire.
 
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Old 09-29-04, 04:16 PM
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Sorry for being unclear. I'll try to explain a little better.

Four wires = two hots, two neutrals. Each hot/neutral pair will carry 30A. The garage/studio will have its own local ground, since it is a separate building from the house. So what I want to run in the conduit are two separate 30A circuits. They'll be opposite phases, so I can run the noisy a/c, garage tools, etc. on one phase and hopefully keep the other phase relatively quiet for the studio equipment.

Once in the building, the two 30 pairs will go to a sub-panel with four 15A breakers, three for the studio and one for the garage.

Does that make sense?



And Thanks rob, I will look for that book next time I'm at HD (a frequent occurance...) Unfortunately, the conduit's already run (underground, and under concrete for that matter) so I either have to live with less current capacity or try and cram four 6 gauge wires inside.
 
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Old 09-29-04, 04:40 PM
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Geoff,

What you say makes sense. It also violates the code.

A 240v subpanel needs 4 conductors: (2) hots, (1) "neutral" and (1) grounding. Because this is a separate building, you will need to drive ground rods and bond the ground rods to the panel and the grounding conductor. The "neutral" and grounding conductors are isolated in your subpanel; they are not bonded together as they are at your service equipment.

You are not running (2) pairs of circuits.

For the same amount of work, you could pull larger conductors and run a 40, 50 or even 60 amp, 240v subpanel. That would give you the power you need now plus extra capacity when you decide to buy other machinery for the garage/workshop.

Rob
 
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Old 09-29-04, 04:55 PM
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Hmm. I suppose I ought to do a bit of reading before I start anything else, then.

I figured this oughta be easy. I'm an electrical engineer by day and design aircraft radar, but I don't know squat about residential wiring and electrical codes...
 
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Old 09-29-04, 07:35 PM
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Your plan has nothing going for it at all:
  • Two neutrals is a code violation.
  • Two neutrals increases the cost of materials.
  • Two neutrals provides no more power than one.
  • Two neutrals increases the voltage drop.
  • There is only one place to connect the neutral feeder in the subpanel, so you won't even know what to do with the two neutrals if you had them.
Since you are a EE, you can surely understand that two out-of-phase currents added together cancel each other out. That's what makes one neutral safer, and one neutral less voltage drop.
 
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Old 09-30-04, 09:36 AM
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Hm. I always thought that the two phases provided to your home were two out of the three that are on normal insustrial power type stuff. Each of the three phases is 120 out with respect to the other two, so when you sum all three up, you get zero current in the neutral. When you've only got two phases 120 out, (which is what they provide to residences, right?) they shouldn't sum to zero, right? If they are 180 out of phase, of course they would sum to zero, but I always thought that they were ony two out of the three, so you should have some residual neutral current.

Now that I think about it, though, I suppose that the worst case neutral current would be with one phase pulling 30A and nothing on the other phase. In that case, you'd have 30A on the neutral wire. I'd think that it'd only get better as you added loads on the other phase, although it may not toatally cancel out.

Soooo.... if I want a 240V 60A subpanel, I need to use 6 gauge wire? I thought that 10 gauge wire was rated for 30A. Or is the code different for a sub-panel feeder wire? Also, Do I need to run the ground to the main panel if my sub-panel is grounded at the garage/studio building?

You guys are great and very informative for a knucklehead like myself. Thanks!
 
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Old 09-30-04, 09:47 AM
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It is incorrect to refer to the two hot wires entering your home as two phases. They are not two phases. The power entering your house is single phase. The two wires are opposite each other, so that there is no neutral current if they both have the same current through them.

When you have 240 volts at 60 amps, you have 60 amps available through each hot conductor at 120 volts, meaning that the wire needs to be able to handle 60 amps. You actually end up with 120 amps at 120 volts.

You should run the ground wire between the two panels.
 
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Old 09-30-04, 10:07 AM
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Ahh. It is starting to make sense now. I feel like I've had an epiphany! We only get one 240V phase. Then of course the currents cancel nicely on a 240V appliance.

So when I use the term "60A subpanel," does that mean 60A at 240V? (120A @120V) or does it mean 60A @ 120V. I think I will only need 60A at 120V, and even that is overkill.
 
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Old 09-30-04, 10:10 AM
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I always thought that the two phases provided to your home were two out of the three that are on normal insustrial power type stuff
Well, there you go. You now know that you've always thought wrong.

if I want a 240V 60A subpanel, I need to use 6 gauge wire? I thought that 10 gauge wire was rated for 30A
We have a terminology disconnect. Since you're an engineer, you know how important precise terminology is. The term "240-volt 60A subpanel" refers to a subpanel served by a feeder providing 60 amps at 240 volts, i.e., 14.4KW. Each of the two hot feeders must be capable of carrying 60 amps. I can't quite tell whether you want a 240-volt 30-amp subpanel (7.2KW) on 10-gauge copper wire, or a 240-volt 60-amp subpanel (14.4KW) on 6-gauge copper wire. If all you need is four 15-amp 120-volt breakers, then the 30-amp subpanel is sufficient, even if all four 15-amp circuits were running at maximum capacity simultaneously (which you will never even approach in real life) due to load diversity. Remember, these two 30-amp hot wires are 180 degrees out of phase, so it is never proper to add 30 plus 30 and get 60. That just isn't good electrical math.
 
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Old 09-30-04, 10:25 AM
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Yup, you were a bit confused on the naming issues.

When a panel is described as an N amp panel, that means N amps on each of the supply phase conductors. Doesn't matter if it is a 120V 'balanced ground' system only capable of supplying N*120 VA or a 480V three phase system capable of supplying N*480/1.732 VA. If your description of your loads is correct, then you only need a '30A' 240V feed to the garage.

In theory, you could do this with 10/3 + ground, protected by a double pole _30_ amp breaker.

It probably pays to install the 60A feeder using the 6/3 cable, simply because the costs of the subpanel, the supply breaker, and your installation time will swamp the difference in cost between the 10/3+g and the 6/3+g...the 6/3 is much stiffer and harder to work with, but in some circumstances requires less protection, and thus might be easier to install. For example, 10/3 run across joists needs a running board or bored holes, but the 6/3 can simply lay across the joists.

As a side note: the '2 phases of a 3 phase system' that you describe is not that uncommon in large multifamily dwellings such as apartment buildings. But for most homes, the standard is to tap one of the three distribution feeders to supply the primary of a transformer. The transformer has a 240V center tapped secondary, and the feed into the home has two 'hot' conductors and a neutral conductor. So most homes have single phase center tap service. Just to add a little confusion, however, it is entirely proper to call the 'hot' conductors the 'phase' conductors

Finally, if you were to have the '2 phases of a 3 phase system', then in general the current flow in the 'neutral' would _not_ balance out. As you note, in a balanced 3 phase system, if all _3_ phases carry the same current, then the neutral balances out. Thus it should be clear that if only _2_ of the phases are carrying a current of X, that the neutral must also carry a current of X, since X is what the third phase would have had to carry to balance things out.

-Jon
 
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Old 09-30-04, 10:39 AM
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Originally Posted by John Nelson
Well, there you go. You now know that you've always thought wrong.
I hate it when that happens....



Actually I'm glad that I've got all that mostly straight now.

Originally Posted by John Nelson
If all you need is four 15-amp 120-volt breakers, then the 30-amp subpanel is sufficient, even if all four 15-amp circuits were running at maximum capacity simultaneously (which you will never even approach in real life) due to load diversity. Remember, these two 30-amp hot wires are 180 degrees out of phase, so it is never proper to add 30 plus 30 and get 60. That just isn't good electrical math.
I think all I need right now would be a 240V 30A subpanel, but I'll go ahead and run as beefy wire as I can fit in 3/4" conduit in case I want to upgrade to greater load capacity in the future. Rob mentioned that four 6 gauge wires in 3/4" conduit will be pretty tight, though.

Thanks!
 
  #17  
Old 09-30-04, 01:15 PM
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Originally Posted by gdgross
I think all I need right now would be a 240V 30A subpanel, but I'll go ahead and run as beefy wire as I can fit in 3/4" conduit in case I want to upgrade to greater load capacity in the future. Rob mentioned that four 6 gauge wires in 3/4" conduit will be pretty tight, though.
(4) #6 wires is the max you can run in 3/4" conduit - that's what I meant by a "full" conduit. You couldn't run, for example, (4) #4 wires in 3/4" conduit. You need to check on specifically what type of conduit was run - the different types have varying fill capacities. If it's metal or schedule 40 PVC, you can run (4) #6. If it's schedule 80 PVC, you can only run (4) #8.

Even if you choose to run #8 conductors, that would let you run a 50 amp subpanel. Myself? I'd pull the #6 if I could and just put in the 60-amp sub. Once you're running the higher ampacity conductor (and using a suitably rated panelboard, which isn't a big deal) - the only other item to upgrade is the 2-pole feeding breaker and that's not an expensive upgrade. The vast amount of time and effort will be spent on pulling the wire, mounting your subpanel, driving ground rods, etc.

One other thing. Do it right - pull a permit and have it inspected.
 
  #18  
Old 09-30-04, 01:21 PM
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4 #6 THHN conductors is pretty much the maximum permitted in a 3/4" conduit. But the conduit fill rules are made so that at the maximum things won't be too tight. If there are few bends in this conduit, you will have no problem with the #6 conductors.

See http://www.westernextralite.com/resources/fill.htm

Note, however, that the equipment ground conductor for this does not need to be full size. If you plan for 60A, then you will need 3 #6 conductors plus 1 #10 EGC conductor.

If this garage/studio is _detached_, then there are a whole mess of rules that you have to deal with. I strongly suggest that you do the reading suggested above prior to buying any materials, and that you post your complete plan here for review.

Finally, you will not find a 30A sub panel. The smallest that you will find is 60A, and it may be cheaper to get a 100A; simply because of economies of scale. There isn't much price difference between a 30A and a 60A breaker, so I'd suggest that you will get your best 'bang for your buck' by using 3#6 + 1#10 and a 60A breaker. If you want to save money now but not have the expansion capability, go with 4#10 and use the 30A breaker.

-Jon
 
  #19  
Old 09-30-04, 05:23 PM
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Thanks, everyone. How do I tell if it's schedule 40 or schedule 80 PVC?

And I do plan on pulling a permit, but I won't be starting this project until for a month or so.

<edit> yes, the garage/studio is detached
 

Last edited by gdgross; 09-30-04 at 05:42 PM.
 

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