feeder wire for a subpanel

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On Wed, 12 Dec 2007 18:48:45 -0800 (PST), Big_Jake

That really doesn't mean anything since you won't find a termination listed for 90c. The 90c column is really just there for derating purposes. An example is if you had 8 THHN conductors in a pipe you have to derate them down to 70% of their ampacity 310.15(B)(2) but since THHN is a 90c conductor you can start derating from the 90c ampacity. So for a #12 you would start derating from 30a, get 21a but you are still limited by 240.4(D) that makes it 20.
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On Dec 13, 7:17 am, snipped-for-privacy@aol.com wrote:

So the termination at the breaker of the main panel and the connection to the buss at the other end aren't rated for 90c?
JK
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Almost certainly not -- but they're marked. Check 'em.
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Doug Miller (alphageek at milmac dot com)
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see what big jake said.
s

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On Mon, 10 Dec 2007 20:51:16 -0800 (PST), snipped-for-privacy@pookmail.com wrote:

If you are really using that much up there I doubt you really need any space heaters. I bet you would do fine with 60a. That is a lot. That is 14,400 watts. How big are these griddles?
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Good point. I won't need one of the space heaters whenever I'm using the griddles. The griddles draw 12A each, and do release a lot of their heat into the room. The other heater is needed, though, as it's in a different room.
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You are calculating your demand incorrectly. You're max draw is 70 amp @ 120 volt. The feeder you intend to run is 100 amp @ 240 volt. That's 200 amps @ 120 Volt. What gfretwell suggested #6 copper, is 60 amp @ 240 volt or 120 amp @ 120 volt. More than enough for your load

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You're right, I think my max is around 70A at 120 volt. I guess I was presuming you feed a subpanel with 240V so that it provides a balanced load to both bus bars. So you're saying I could feed the panel with a 60A 240V breaker using 6/3 romex, and then pull more than 60A at 120V (e.g. install 7 20A 120V circuits with a max simultaneous draw around 70A)?
That would make this all easier.
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That's one reason. The other reason is to supply 240V loads.

Absolutely -- as long as you don't pull more than 60A on any one leg at any time. 30 on one leg, 40 on the other, will be just fine.

Yep.
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Doug Miller (alphageek at milmac dot com)
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wrote:

From what you say there 40a with #8/3 would be more than enough. You will balance 80a across the 2 hot legs but going 60a is just a few bucks more for 6/3. The breakers and panel will probably be the same.
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On Dec 11, 4:32 pm, snipped-for-privacy@aol.com wrote:

So does that mean I would only need a 60A double-pole breaker in the main to feed the subpanel? This breaker will feed two hots (240v) to the subpanel bus bars, and then I can pull up to 96A (80% of 120) of 120v from that subpanel?
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snipped-for-privacy@pookmail.com wrote:

Yes.
Yes.
The 80% rule applies to continuous loads -- defined as maximum current for more than three hours.
You only identified about 70A of load anyway...
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Doug Miller (alphageek at milmac dot com)
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It means you can pull 60 amps on each leg for a total of 60 amps @ 240 volts or 120 amps @ 120 volts

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Should that 60A double-pole breaker be a 120/240v or just a 240v? I would presume the latter, but I see both at the store so thought I'd ask.
Thanks to *everyone* who has been so generous with their time and expertise!
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On Thu, 13 Dec 2007 22:06:03 -0800 (PST), snipped-for-privacy@pookmail.com wrote:

You just need a regular 240v (2 pole) breaker. One labelled 120/240 is probably a GFCI. ($50 or more?)
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You want a 240V breaker that has two breakers tied together with a common bar. When you turn the breaker off, you want to turn off BOTH legs of the 240V supply.
You DO NOT want the double breakers with two individual switches, as that could still leave one half of the supply powered if you didn't flip both switches.
Just to clarify, if you run 6/3 wire with a 60A breaker, that's 60 amps on EACH side of neutral (60 on the black wire, 60 on the red wire).
If you balance half of your 120V loads on one side, and the other half of your 120V loads on the other, it should support close to 120Amps total. I'm not sure what the derating requirements are, but it should be more than enough to support the 70A loads you are planning.
Anthony
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On Dec 10, 10:51 pm, snipped-for-privacy@pookmail.com wrote:

Can you use one inch rigid exterior conduit and go up the outside of the house then back into the house on the top floor? Running it along a downspout maybe so it's not conspicuous. Then it will be easy to just pull whatever wire you want.
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If you want to install a 100A breaker for your subpanel, you'll need to run four #2 copper wires (or 1/0 aluminum). I've never seen sheathed cable this large, even at my local electrical supply stores, so you'll probably have to run individual conductors in conduit.

You'll need at least 1-1/4" PVC conduit for the four #2 wires, though I'd go with 1-1/2" as it's a lot more common and would be easier to pull wires through.
The largest flexible conduit I've seen is 1", so you'll probably need to use rigid conduit.

You could run rigid conduit along the underside of the joists if appearance isn't an issue. Otherwise, if the conduit has to run perpendicular to the joists, you'll need to use lots of short sections with couplings. A lot more work, but doable. Plumbers do it all the time.
Keep in mind, the holes you drill for the conduit can't be any larger than 1/3 the depth of the joist, and must be at least 2" from the top and bottom edge of the joist. A 2" hole for the conduit would be too big to fit in a 2x6 joist, but you should be fine if you drill your holes in the center of 2x8 or larger joists. I'd still choose surface mounting if that's an option.
You'll need pulling elbows where you change directions (basically an angle with a removable cover).
Installing conduit would be a lot of work, but it would offer better protection and make changes easier in the future.
However, if you think you can get by with a 60Amp panel, you could use 6/3 romex cable which is commonly available at most home centers (it's frequently used for installing kitchen ranges).
If you really think you'll need 100 amps or larger, maybe you could install two 60A subpanels, fed with two separate 6/3 cables? That would be a lot easier to install, and give you 120A total, assuming you have space for two 60A breaker pairs in your main panel.

I recommend you pick up a copy of "Code Check Electrical". It condenses all the various code requirements for cable sizes, conduits, derating requirements, etc. Be warned though, it's not a "how-to" book, it's more reference material. You'll probably have to read through the important sections multiple times to catch everything.
Anthony
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