Main Breaker Less than total of Breakers
xosder11
| Posted in Energy, Heating & Insulation on
Just a question here to satisfy my curiosity. Why have I seen on numerous occasions in distribution panels where the sum of all the breakers is greater that the main breaker. For example, in an apartment they have a 100a main and the total of all the circuit breakers is higher than 100. Would anyone care to explain to me why this is allowed, what the formulas or rules of thumb are pertaining to this. Does it have a lot to do with the individual set up and what types of equipment is on each circuit, or is it dictated by a more general code requirement. It never has made sense to me, and it’s just one of those things I have no answer for. It’s really bugging me
Thanks,
Andrew
Edited 8/3/2006 11:25 am ET by xosder11
Replies
circuits don't normally pull their full load, steady state.
bobl Volo, non valeo
Baloney detecter WFR
Right, I got that. If they did the main would trip. It's just the whole what if they did, thinking like an engineer designing a bridge that could support 18 wheeleers end to end in every lane containing the maximum allowable load and then adding a factor of safety to that and designing a bridge to support that load. Why, is the same thinking not applied here? I'm sure once it get's explained better I'll get it.
Trying to think this through...(and doing a bad job at it) if you had a panel with a hundred amp main, and you had 9 15a circuits totaling 135a, and each of those circuits was pulling 12 amps which would be their safe operating capacity, that is still like 109 amps.
Consider the bridge is a toll bridge. Each lane can support 10 fully loaded semis at a time in a crunch. There are a total of 30 lanes. The total bridge is limited to 200 trucks at a time, despite the fact that more could go on because at most times there isn't enough traffic to fully fill the bridge.
to continue the analogy - if you overload the "bridge" with too many "trucks", it just stops traffic - it doesn't collapse and kill everyone.
http://jhausch.blogspot.comAdventures in Home BuildingAn online journal covering the preparation and construction of our new home.
Suppose you were pulling 12A on all 9 circuits totalling 108A. This is safe because then the main would trip before the wiring overheated. The protection is for the circuits themselves not from preventing breakers from tripping.
As was stated before, you're never gonna run full load on all circuits unless you specifically try to do it.
Crank your water heater to 150, plug in a few space heaters, turn on the air conditioning, all stove burners & oven, etc.
The main breaker protects the incoming wires from overload, regardless whether the overload is caused by a short in the panel or an overload caused by too many breakers operating below their capacity. The code rules are designed to minimize main breaker trips, but not to eliminate them. To design the supply for all breakers operating at their trip capacity would merely require us all to waste an enormous amount of copper. The main breaker would STILL be required.
I would propose you think of it like your plumbing system:
You have a limited size water supply. You also have many, many water taps that draw on that supply. Unless you are realy trying though, you are unlikely to have so many taps running at the same time to cause and real problem.
BUT - If you, your partner, your kids, the neighbor kids, your mom the gardener, and your brother running the car wash charity fundraiser in the driveway are all operating taps at the same time you WILL have a problem, as your system was not designed to handle this enormous demand. Upgrading your "service entrance" would take care of this. Not that even after upgrading the service entrace of the water, you could STILL pull more water than it could supply because you can add an unlimited number of hose bibs, sinks, showers, etc... but are never likely to use them all at once.
Rebuilding my home in Cypress, CA
Also a CRX fanatic!
What is the way to determine how much total amperage in terms of individual breakers could be placed on say a 200a main breaker?
However many slots there are in the panel?
The NEC has calculations to give the minimum amperage for your main breaker and minimum circuit size for certain rooms (kitchens, baths)...
I think you're looking at this backwards. The individual breakers protect the wiring for whatever circuit they are on - they are not there to prevent the main breaker from tripping. The main breaker is there to protect the wiring coming into the house.
Theoretically, you could load up the panel with 20A breakers, (provided they are all wired with #12 cu) & you'd still be safe. In the event you run them all at near full capacity, you'd trip the main breaker & your incoming power lines would be protected.
Ok, so the only purpose of the Main breaker is to protect all the wires on the meter side, and to allow for a disconnect switch to deenergize the bus bars in the panel for any work to be done without disconecting the meter. And the nec tells you what the minimum size of the main breaker is so that you won't keep tripping it all the time, or hopefully ever for that matter. Thanks for playing along. I had to learn something today.
Here is how the service is sized.http://www.taunton.com/finehomebuilding/pages/bh0019.asp
What is the way to determine how much total amperage in terms of individual breakers could be placed on say a 200a main breaker?
Article 220 of the NEC.
The general idea that not every breaker will be running at full load at the same time is true. The Code recognizes this. It has demand factors. In section 220, you get to pick the larger of heating or cooling load. The Code requires a 20 amp circuit for the bathrooms because of hair dryers. But most of the time, it is just charging an electric razor.
An important point, that few seem to realize, is that the 200 amp breaker protecting the panel is actually two 200 amp breakers connected together. Each side will allow up to 200 amps to pass through it without tripping. Thus you could theoretically load a 200 amp pamel with up to 400 amps. But, it is best to limit continual loads to 80% of rated capacity.
[The bridge analogy is somewhat shaky. There is a dynamic load involved. A 20 ton truck traveling at 55 mph will have more dynamic impact on a bridge than a static truck siting still on the bridge waiting in line for the toll booth.]
~Peter
The size of the main breaker is based upon the expected (calculated) needs of the home. On the other hand, individual circuits are usually sized for a "worst case."
Since it's rare for everything to be running all the time, the main breaker need not be anywhere near the sum of the individual breakers.
Think of your house wires as a road network... just because your car can do 90 mph, the streets are not sized for every car to be out, doing 90, at the same time.