I have wiring and circuits that can exceed 100 amps in both locations.
So, in theory, I could load up both locations with loads that the internal wiring could safely handle, say 120 amps on both and either the 100 amp breakers would trip in the circuit boxes or the mains at the meter would go.
Is it common for this to be the case in new construction or are the number of circuits or the total amperage of the breakers held under the total of the main breakers?
I’m not likely to ever have 200 amps pulled at once but I could and I wonder if it is within code.
The purpose of the main breaker is primarily to protect the wiring coming into the building. Yes, it’s possible to almost fully load each branch circuit breaker without tripping any of them yet overload the main breaker, causing it to trip. There is nothing wrong with this, per the NEC (provided that no branch circuit is continuously loaded to near capacity–I think the max continuous load is set out to be 60% of the nominal rating, IIRC), however, if this is actually happening frequently, you’ll want to contact your electric utility and discuss with them about installing a higher-current service.
It’s called “load diversity” and quantified by “diversity factor”.
The idea is that you will not pull the maximum possible load on all the circuits at the same time. Should you approach that condition you will trip the main, which is there to prevent the bus bars in your breaker box from overheating.
For a house, if you add up the 3-4 biggest loads (dryer, electric water heater, electric range for example) and still have 25-30% of the service capacity as reserve you will probably never have a problem. Note: Use the actual draw of the loads, not the breaker rating, which is at least 30% higher.
Sizing the panel, service, transformer, etc. for “worst case” would add a lot of expense that is just not needed.
FWIW, I have seen lots of tripped branch circuit breakers, but only four cases of main breakers tripping. One was due to a shorted 5 HP motor that had it’s own time delay (for starting) fuse that also tripped. In one case the main was actually defective. Another case had loose connections (causing heat, which tripped the breaker). The fourth case was something of a mystery: A thunderstorm knocked the power out, when power was restored, the main on one panel was found to be tripped. It may have been associated with the lightning that took the power out, or a surge when power was restored.
I’ve only seen two tripped mains-one was a defective 150A main which was tripping ~ 70A according to the inductive ammeter, and the other was my fault-a fumble fingered moment while feeding #4 solid Cu into an energized panel to land the driven ground, it slipped from my fingers, and as this was a service replacement, acres of bus were open. #4 conducted 200 amperes, for a very short time.
So, how much of that wire or the bus was vaporized? And did you save the safety glasses to show to people the importance of eye protection when doing halfway foolish things like working hot? :eek:
As for the OP, add me to the list of folks that have yet to see a main breaker tripped out from “normal” overload.
I did once trip a main by putting a hard short across the two legs of the 240-volt feed. (forgot to break out the stupid tab on a duplex switch that I was replacing…) Put the power back on, and the building main must have been running just close enough to max that the current rush while the branch circuit breaker was thinking about tripping was enough to trip three breakers - the branch, the subpanel and the building main. Ker-POW! (Actually, it was a deep buzz and some clanking as wires were flailing around inside the conduit.)
OK, who’s got the next Stupid Electrician Trick story?
Well, here’s one related to the OP question – max capacity of main wiring.
Years ago, my grandmothers old house was 3 floors, 8 bedrooms, very old wiring (some knob-and-tube, some wires fished thru pipes that supplied the original gaslights). The main service was only 60A, 4 circuits, 15-A screw-in fuses for each, plus a pair of big blade fuses for the electric stove.
Her children gave her an electric dryer for Christmas, and I had to wire an outlet for it. The only way to do so without re-wiring the whole house was to run it in parallel with the electric stove circuit. Which I did, then warned Grandma that if she used the electric oven when the dryer was on, she would likely blow these fuses. She told me that would be no problem, because "laundry is done on Monday, and baking on Wednesday’, so they wouldn’t be used at the same time. (I guess after 75 some years, your schedule is pretty set!)
But that is what is meant by ‘load diversity’. People seldom have all the electrical circuits in the house going at full capacity at the same time.
Thanks guys… That ‘splained’ it just fine. I was just worried about adding circuits in the shop for added plugins. Too easy to overload a circuit in the shop and so I have a lot of plugins and want more. I am a fan of having three drills all with different things for repetitive tasks. 2 different drill sizes and a screw driver bit or some such. Only using one at a time but if something cuts in automatically on the same circuit about the time I hit the circular saw and the wife loads up the ½" drill and the little 120 V compressor cuts in and I have the 500 W floods on and … You get the picture, not altogether a big load but I need to spread it around even if we are working in the same area. I am not a fan of extension cords all over the place.
