I have a 100A main breaker panel I want to use as a subpanel. I've
removed the tie strap between the ground and neutral bars. I bonded
the ground bar to the panel and made sure the neutral bar wasn't
bonded. I'm going to use a 60A double pole breaker in the main panel
to power the subpanel. My question is, can I connect the hot wires in
the subpanel to the 100A main breaker in the subpanel? That way I have
a way to open the circuit at the subpanel without having to go to the
main panel. Is it dangerous and/or a violation to have the 100A
breaker in the circuit which is protected by a 60A breaker?
My local inspector, who was *amazingly* reasonable about a bunch of other
things, would not allow the _same_ size breaker at both ends of a sub-panel
run. Had to be one size smaller at the sub-panel. I could -not- get a really
good explanation for 'why'. Closest approximation was that high-capacity
breakers are *NOT* intended to be used as manual switches. That, after you
manually throw them a small number of times, they will not trip reliably on
overload. The 'authoritative answer' was simply: "it's not allowed." *sigh*
"logically", there's no actual danger/risk. Except for the 'apparent' false
assurance, as regards much load you can have in the sub-panel. i.e., "if it's
got a 100A main breaker, I oughta be able to run 100A of downstream load."
This may not be a problem for -you-, since you know better; but what about
the *next* owner of the property?
On 2 Sep 2003 04:43:00 -0700, firstname.lastname@example.org (Bill) wrote:
If I'm understanding you correctly, you want to keep the subpanel's
main breaker intact so that you can use it as a disconnect.
You can keep a main breaker in the subpanel, but it can be no larger
than the breaker to which the subpanel feed is connected in the
upstream panel... so you can't do exactly as you've outlined. What
you've proposed is both dangerous and a code violation.
You can replace the subpanel main breaker with a 60A or smaller
breaker. IMO, a better approach would be to remove the subpanel main
breaker and install a main lug kit, then install an appropriately
rated NEMA disconnect next to, but upstream of the subpanel. Circuit
breakers really don't make very good switches, but they'll hold up to
infrequent switching for quite a while.
"Building Your Own Kitchen Cabinets"
With Glory and Passion No Longer in Fashion
The Hero Breaks His Blade. -- Kansas, The Pinnacle, 1975
And what part of it is dangerous. Seems no more dangerous
than having no main breaker in the subpanel, which is OK.
What is the problem with using circuit breakers as switches
to shut off circuits? Is any manual actuation of a circuit
breaker bad, or is it just manually switching it with an
electrical load on it that is bad.
I suspect that the saving factor here is that those breakers aren't
carrying anywhere near rated (trip) current so the switching is not a
particularly destructive event.
B a r r y B u r k e J r . wrote:
I believe this is incorrect. I built my house last year. 200A main
panel in the garage, 60A sub-panel in the basement. The sub-panel is
pulled off of a 60A-2P breaker in the 200A panel. The main breaker in
the sub-panel is 100A. Master Electrician told me to do it this way and
city inspector had no problems with it. At the time I did this
Minnesota had already adopted the 2002NEC.
In reviewing my response, as written it IS incorrect. Sorry for the
confusion. Hopefully a better explanation follows.
In talking with our code consultants a couple of years ago about this
exact same issue, we agreed that it *may not* violate the NEC, but
that it created enough *potential* for violations that it should never
be done. The 1990 NEC did not implicitly prohibit this practice, and I
don't believe the 2002 does, either, though I haven't studied it as
carefully yet. There are some articles that intimate that this might
be a problem, but none that come right out and say 'OK' or 'not OK'.
We work primarily with state code officials and offices, and our code
consultants advised that the state office would reject a permit on
this basis, while local officials will probably not even point it out
most of the time. The most applicable NEC 1990 articles were 240-3,
240-21, 230-90(a), 384-16(a).
I'm still 90% convinced that there is some article that addresses the
practice of having two breakers in series protecting the same load,
but I can't recall where it is. Could be I'm imagining that, and
anyway it could probably be argued that the 100A main breaker in the
subpanel is only protecting the subpanel bus.
The installation as described is not necessarily and inherently
dangerous, but it is very potentially dangerous, depending upon how
the subfeed wiring is done. The code and accepted practice only
require that the service feed to the subpanel, since it's protected by
a 60A breaker in the main panel, to be #6 THHN/THHW. It *could* be
sized large enough to handle the full possible 100A load, but we don't
know that it has been, and the sensible assumption is that it is not.
So my assumption is that it's #6.
Further, the idea that the subfeed is protected is only valid within
the five-minute time frame that it takes to change that 60A breaker to
a larger one.
Let's say a future not-so-sensible owner looks at the 100A subpanel
and decides he has plenty of capacity for for a ceramics shop with a
couple of 7kW kilns, a 5kW tankless water heater, maybe some electric
baseboards, and who knows what else, and assumes that since it's a
100A subservice, all he needs to do is to change the 60A breaker in
the main panel to get the capacity he needs -- which would be, in this
scenario, a clear code violation (NEC 110-10). Any good electrician
would verify the size of the subpanel conductors, but a typical DIY'er
may not - he'll just go the the Borg, buy a 100A breaker, change it in
the main panel, and think he's golden... until some frosty January
morning when the subpanel feed catches fire and burns the place down
from both ends while he's washing his hands and baking a couple of
ceramic gnomes. Somebody might look at this scenario and think, "how
often does this happen?", but our local fire inspectors have reported
that about 60% of house fires are electrical in origin, and about 50%
of those originate in service panels or branch wiring.
Because we are a very mobile society, we have to do things in a
forward-thinking way. We can't just figure out what works for us in
our situation -- we must also look at what could happen with the next
owner of our handiwork, and assume that he's a complete moron... and
he may well be. If a *potentially dangerous* installation is
performed, such as the one I've exampled, then the installation should
be permanently and appropriately marked at both ends.
As for using the main breaker as a switch, the code does implicitly
allow this for 120v and 277v *lighting* circuits provided that the
breaker is marked 'SWD'. But in general, breakers are not designed for
switch duty cycles, and I do not believe the code implicitly approves
switching at the breaker for any other use. IIRC, there are breakers
that *are* designed for higher duty cycles, but I've never looked for
or called for one. I'd bet my truck that the main breaker in the
majority of residential-grade main panelboards does not meet this duty
rating. It is, as somebody else pointed out, pretty common for
industrial installations to use breakers as light switches, because it
saves all the expense of installing switches and the associated
wiring. It's really not very good practice, but it is allowed.
When I decided to install a new subpanel for my garage and shop last
year, I had the option of installing a used 100A main breaker panel
which cost me nothing, or installing a 125A main lug only panel, which
cost me $30. I felt that the $30 was very well-spent, because it
eliminated ANY potential confusion. Since I fed the subpanel from a
70A branch breaker in the main panel and ran #6 THHN/THHW all in
conduit, I labelled the breaker in the main panel "70A Max - Do Not
Exceed", and put a label on the inside of the subpanel door indicating
that the feed was rated for only 70A. I didn't need to be able to
disconnect the subpanel at the subpanel, but if I had, another $30 for
a separate disconnect would have been well-spent, IMO.
Sorry for any confusion I may have created...
Not a problem as long as the conductors are being protected at thier ampacity.
In fact every load in your house is protected by at least 2 breakers/fuses in
series, the main and the branch circuit O/C device. In some cases there may be
a lot more than that. It is very common to have a sub panel with the up stream
O/C device providing protection for the feeder and another breaker at the sub
panel only serving as a disconnecting means. At that point it could be replaced
by a switch so a larger breaker is no problem.
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