I just purchased a home built in 1905 and as I've gone through it I
found a number of surprises. One in particular is puzzling me. The home
has 100 amp service, and the wiring has been updated. There is a
detached garage with a subpanel. The subpanel in the garage is fed by a
2-2-2 al feeder cable that makes a run of about 90 feet, back to the
main panel in the house and connected with a 100 amp double pole
circuit breaker. It looks fairly new but it seems to be a long run for
a cable of that size. Theres not much to power in the garage typical
lights and outlets. Is this a typical and when I say typical I mean
In Canada, if it's got 90C insulation then #2 cable is good for 100A as
By my calculations pulling 100A on that cable would give a bit over 5V
drop, or about 2%.
However...it is not reasonable to pull that much off the main panel.
The exact amount that would be reasonable to supply to the subpanel
would depend on the details of the house load.
Are you sure the service isn't coming to the garage, and the house being
fed via that cable?
A 100a sub panel on 2ga al is technically a violation but the realiity
is, if the service is also 100a I can't see how you could ever get
100a in that feeder unless everything in the house was turned off.
I don't see this as likely to cause a problem since your not using anything
near 100 amps in the garage but if you are concerned just change the
breaker that feeds the subpanel to smaller one, 50 or 60 amps say. Plenty of
power for your garage and no chance ( however unlikely) of overloading the
cable that feeds the sub panel.
Is there a code reference that disallows you from connecting a 90 degree
conductor, and using it a full value? I checked with Square D, and while
they describe it the same as you, they don't indicate that you can't do it,
and only mention that you may have nuisance tripping as a result
110.14-C generally limits 100A or less terminations to 60C ampacity (75C
if rated for over #1 wire). This was a UL requirement, and thus
enforcable, before it appeared in the NEC a few code cycles ago. It
appeared in the UL Green book, which I'm sure everyone reads. One could
splice a larger wire with a 60C ampacity of 100A to the end before
landing it on the breaker.
Cable to a detached garage is not likely rated 90C so the cable itself
is probably not rated 100A.
On Mon, 20 Nov 2006 08:30:27 -0600, "Steve Barker LT"
The "residential" exception 310.15(B)(6) is only for the mian feeder
or service to the dwelling. It does not include feeders to sub panels.
As I said in my first response this is probably not important since
the whole house is running on a 100a service but technically it is a
It is important to me however, since I'm doing exactly that. Running a 100a
sub off a 200a main. My electrician has told me 3 fours and a bare 8 will
be fine. Could you cite the section pertaining to "it does not include
feeders to sub panels" please?
It's in 310.15(6). The glitch is more in the temperature rating of the
circuit breaker as #4 copper is good for 95 amps if it's 90c wire. I've
scoured Google about this and have found that some jurisdictions accept the
service feeder table for sub panel feeders as well, so your electrician may
be right in that regard
On Mon, 20 Nov 2006 14:10:06 -0600, "Steve Barker LT"
The #4 for 100a comes from the table referenced here
310.15(B)(6) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and
Feeders. For dwelling units, conductors, as listed in Table
310.15(B)(6), shall be permitted as 120/240-volt, 3-wire, single-phase
service-entrance conductors, service lateral conductors, and feeder
conductors that serve as the main power feeder to a dwelling unit and
are installed in raceway or cable with or without an equipment
grounding conductor. For application of this section, the main power
feeder shall be the feeder(s) between the main disconnect and the
lighting and appliance branch-circuit panelboard(s). The feeder
conductors to a dwelling unit shall not be required to be larger than
their service-entrance conductors. The grounded conductor shall be
permitted to be smaller than the ungrounded conductors, provided the
requirements of 215.2, 220.22, and 230.42 are met.
Notice it says "For application of this section, the main power feeder
shall be the feeder(s) between the main disconnect and the lighting
and appliance branch-circuit panelboard(s)."
In the case of the other poster, if his service was #4cu or #2al he
could use that size for his sub panel.
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