I would like to run a subpanel to my garage. I am thinking about a
100amp. What I need to know is what gauge wire? Copper or aluminum?The
distance will be about 75ft by the time the wire snakes through the
house. Anybody have any tips.....
<< I am thinking about a 100amp. What I need to know is what gauge wire? Copper
or aluminum?The distance will be about 75ft by the time the wire snakes through
the house. Anybody have any tips.. >>
Your cart is way ahead of the horse here. If your intent is to do it yourself,
get down to the city hall and pull a copy of the building codes for your
project. Find out about permits, too, so you won't get tripped up by an easily
avoided mistake. The codes themselves are the easy way to figure out what works
and what doesn't, regardless of the casual advice from Uncle Willie or the
clueless clerks at the box store. Your library or book store will have copies
of guides to the NEC code that are very enlightening. Good research and common
sense planning is the hallmark of all successful projects. Good luck.
You want to use a 100 or 125A panel, but you don't have to feed it with
that much juice. I have a 100A panel in my garage, on a 60A feeder from
the main panel. Mine is wired with a mixture of #6 copper wire and #4
aluminum (the aluminum is outdoor overhead cable.)
You need do a load analysis to see how much power you really need and
work from that. If you have a welder that you use at full power, a big
air compressor, and a chop saw, *and they all run at the same time* you
might need 100A. But I doubt it.
Fifty and 60 amp breakers are cheap. 100 amp breakers are expensive.
Seventy amp breakers are hard to find, but don't cost much more than
60's. The smaller breakers will allow you to run smaller (cheaper and
easier to handle) feeder wires.
You will need to run 4 wires, or 4-conductor cable because the garage is
attached to the house.
I like to use aluminum for long runs at high-amperage, and copper for
Welders don't care about voltage drop from long undersized supply wires,
but most other things do.
Thanks for the info. Currently I have a 200amp main breaker panel with
a 60amp feed going to a 100amp fuse panel(in the garage) fed with #4
copper (3 conductors, 2 hot and 1 neutral) the only thing the fuse
panel feeds is my AC. What I want to do is upgrade fuse panel (to a
breaker) so that it is grounded properly and add a couple of 20amp
circuits in the garage for basic power tools and a small compressor.
To do it right, you need another wire. :-(
Is that neutral wire insulated or bare? Is the wire in conduit, or are
you using multi-conductor cable ("romex" or service entrance cable) and
Your existing fuse box probably has 2 hot wires and a ground wire, and
no neutral, because that's what the AC wants.
If you are really lucky, that existing ground wire is insulated and you
can safely make a neutral out of it with a little white tape, and run a
separate #6 copper ground wire. Or the wires are in conduit and you
stuff in one more wire.
Except for the problem of needing 4 wires and you only have three, a
100A "main lug" panel with 6 or 8 spaces would just drop right in where
your fuse box is.
The wire is a service entrance type(3 wire), no conduit. I was planning
on replaceing the wire because it is a "cloth" cover and it looks
rather old. The way I see it is I need a #4 copper with 4 wires (2hot,
#6 copper is OK for a 60A feeder, and that's more than enough based on
what you've said. You will still use a 100A panel because they are so
common and inexpensive.
#4 is better than #6 if the expected load is high enough cause voltage
drop problems. I'm not an electrician, and I don't have my books handy,
but I think all you need is a #6 neutral and a #6 ground even if the
hots wires are #4.
You'll need to install a ground kit in the new subpanel, and probably
remove a preinstalled "bonding screw" or strap to isolate the neutral bar.
I am sure that if you wanted to hire an electrician you would have done so.
I am sure that if you have to ask the guage of the wire it is because you
are doing your homework to ensure that the project is done right, not
because you are an unqualified dolt just waiting to electricute a small
Please ignore the hecklers with no intent on actually answering your
Your question is multi-faceted. I am going to attempt to address each issue
A: You might be able to get away with less than 100A for a garage and it
might save you a small amount on materials. However, the labor for 100A is
going to be about the same as for 60A. If cost is a major concern calculate
to ensure 60A is sufficient (it probably is) and go with a 60A box. If cost
is not a major concern skip the calculation step and make yourself happy
with the 100A breaker box-- not because it is necessary, because you want
Q: Copper or aluminum?
A: Copper conducts electricity better than aluminum. An aluminum wire sized
to carry a given number of ampers of electricity will be larger than the
corresponding copper wire and therefore harder to work with / larger bend
radius / etc. Aluminum wires also need a special conductive coating at
their termination to prevent oxidation. Oxidation can lead to poor
conductivity which can lead to heat and ultimately fires. So why does
anyone use aluminum? People use aluminum because it is cheaper.
Q: What gauge wire?
A: There are two questions you must ask yourself when determining the wire
i) at what gauge will the wire become so hot that it poses a fire risk
a) 4 AWG copper is acceptable for 100A service entrance / feeder
b) 2 AWG aluminum or copper-clad aluminum is also acceptable for 100A
service entrance / feeder
ii) at what gauge will the voltage drop become unacceptably large
VD @ 75 degrees F = 2 * L * R * I / 1000 where
L = one way length of circuit (ft)
R = conductor resistance in ohms per 1000 ft
0.308 <== 4 awg cu
0.318 <== (0.194/0.61) <== 2 awg al
I = load current (amps)
VD = 2 * 75 * 0.318 * 100 / 1000 = 4.77 V
You will have 4.77 V less at the subpanel in the garage than the main
breaker. You will lose more volts when you run wires around inside your
garage. How much you can lose is partly based on how many volts there is to
begin with at your main breaker and partly a judgement call. If you believe
you are losing too much upgrade to 2 awg cu and your voltage drop will be
Q: Additional tips?
A: Go down to the library and ask for a copy of the NEC 2002 handbook. ISBN
#0-87765-462-x You might have to ask for it by inter-library-loan. The
2005 NEC is out but your library probably won't have it.
Hope this helps,
i am sure this guy means well. im am sure he feels he can help you, but
he's not there. he doesnt have to live with what is done. please ignore
the guys who think a couple paragraphs in a newsgroup post will give you
enough information to do this job.
Check out www.homewiringandmore.com
This site is run by a master electrician, who also is an electrical
inspector for the state of Indiana.
Free advice, very competent. He's mostly concerned that DIY'ers know the
correct way to do things.
Consider it a learners tool, much like a reference manual.
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