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.
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.
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
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
That would make this all easier.
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.
On Dec 11, 4:32 pm, email@example.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?
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
Thanks to *everyone* who has been so generous with their time and
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
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
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.
On Dec 10, 10:51 pm, firstname.lastname@example.org 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.
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
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.
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