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?
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.
JP John Paquay snipped-for-privacy@insightbb.com
"Building Your Own Kitchen Cabinets"
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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.
Most circuit breakers are not designed for high duty-cycle operation. A typical switch will handle 10's of thousands of on-off cycles, because that's what it's designed to do. Breakers aren't.
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.
What _size_ of breakers? The inspector here says the lo capacity onew, i.e. below 30A _are_ ok for switching, but the 30A and above are -not- designed to handle it.
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.
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.
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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