Tman (tman9_at_comcast.netremoveunderscores) said...
I have a 8-place Square D subpanel with a 70A switch and a 30A breaker wired
with one of those mechanical interlocks that prevent both from being on at
the same time.
So I'll run the 30A breaker via 10/4 romex to an outside plug, which is
where I'll connect my approx 5500W portable generator.
Then I'll run the 70A switch (2 hots) and a neutral via some 6ga wire in a
conduit to a 60A 2-pole breaker in my main panel. Including a 8ga ground in
the same conduit, being sure that the ground and neutral are not tied
together in the sub-panel.
So far, so good. I have nearly the same setup, except the panel I have
came with two 30A breakers for the interlock. My connection to the generator
inlet is the same, except I used conduit with three 10 gauge wires for the
two lines and neutral and a 12 gauge grounding conductor (the ground can
be one size smaller, except once you get to 14 ga).
My connection to the main panel used #10 conductors and a #8 ground and
runs off a 40A breaker (sized larger in case I want to upgrade it later,
but the 30A breaker at the transfer panel is ultimately the overcurrent
protector here).
Then I'll load the remaining 4 spots in the subpanel up with the QOT tandem
breakers. One of these is for a 240 circuit (water pump), and i'll tie two
adjacent breakers together with a tie pin from Square D to make a 2-pole
breaker. This leaves 6 circuits for 120V service.
No, it doesn't. Unless Square D has come out with new mini-breakers that
provide a proper 2-pole option (I have seen some other manufacturers that
provide a 2-space mini breaker configuration, where the two outer breakers
are single pole and the two inner are tied together to give a 2-pole
breaker), you will have to use a full-size, two position breaker for your
240 volt circuit (a QO215 or QO220, for instance). This will leave you
with only two full positions on the panel, where you could install two
mini breakers (QOT1515, for instance) to give you four more circuits.
If I add all the breakers in the subpanel up, I'll have 70A (2x20, 2x15) per
leg, which exceeds the 60A subpanel feed breaker and certainly the 30A
generator feed breaker, and even more so, the 5500W generator capacity. But
I don't think the feed breaker will ever trip, and I'll just have to be
careful when on genny. Is there anything not kosher about this setup?
Not really. Have you ever added up all the breakers in your main panel (on
each leg)? It is not unusual for them to exceed the main breaker's rating.
A branch circuit protected by a 15 A or 20 A breaker typically does not
run constantly with that amount of current (forget, for the moment that
breakers are only supposed to be loaded to 80% of their capacity, since
80% of the sum of all breakers compared to 80% of the main breaker's
rating is still relatively the same comparison!).
Certain load assumptions are made when figuring out how many devices are
on a single circuit, and these assumptions are worst-case scanarios.
For instance, when planning a new circuit where general receptacles and
light fixtures are concerned (as oppposed to knowing exactly what will
be there), I tend to use 100 Watts for lights and 200 Watts for
duplex receptacles. Many receptacles in a home are there because code
requires you to be no more than six feet along a wall from an outlet, so
1500 Watts worth of receptacles on a 15 A circuit (1500 Watts is a close
estimate for 80% of 15 A at 120 volts!) may only end up with 200 Watts on
the circuit under normal use.
Some outlets require a dedicated circuit. So a 1200 Watt refrigerator
would use up one full circuit, but not be accounting for any more than
10 A when it was actually running.
I need to transfer a handful of circuits from the main panel to the sub
panel. Can I do this the easy way.. by leaving each circuit run in the main
panel, and splicing the hots ONLY with a wire nut to feed to the subpanel
via wire of the appropriate guage in a conduit? Anything to be aware of
here, especially if it is OK to leave the ground and neutrual for these
transferred circuits terminated in the main panel? I could run those to,
but not if I don't need to...
Hmmm. Something I didn't have to deal with, since mine was new construction
so all the transfer-switch circuits had their home runs to the transfer
switch. What you are essentially doing is running a "switch loop" for each
of these circuits. On the surface, this seems acceptable, but I would run
it past your inspector before getting started.
The cost of your permit actually buys you a bit of a consultant. Inspectors
are generally available for an hour in the morning to answer questions. Make
use of this service (just don't abuse it!).
--
Calvin Henry-Cotnam
"Never ascribe to malice what can equally be explained by incompetence."
- Napoleon
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