jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
Okay I'll try...
Busses on the supply side of the service disconnect in an MLO panel (or switchboard) can only see as much current in normal operation as the load breakers allow. If a load breaker draws too much current, the breaker will trip.
Adding a PV source doesn't change that. Any load breaker that draws too much still trips, so the total current the busbar can carry remains the same.
On a load side busbar, say 200A protected by a 200A main breaker, there may be 300A of branch breakers. If you then add a 60A source breaker, the loads can now draw more current than the main breaker allows by itself. So you've defeated the overcurrent protection scheme required elsewhere in the code and 705 places limits on how much you can do that.
There are only a few places in the code where overload protection isn't strictly sized to conductors. 230.90 Exception 3 is one, and the one which makes the least sense in my opinion, at least in its current unlimited form. (Do they really mean you can have 400A of breakers on a 100A service just because of a load calc?) 705 is another. Motors and fixture wires are a couple others I can think of. I don't really see any major philosophical discrepancy here. If anything, I think 230.90 ex3 should be more limited (but I won't be the one asking them to do that).
Busses on the supply side of the service disconnect in an MLO panel (or switchboard) can only see as much current in normal operation as the load breakers allow. If a load breaker draws too much current, the breaker will trip.
Adding a PV source doesn't change that. Any load breaker that draws too much still trips, so the total current the busbar can carry remains the same.
On a load side busbar, say 200A protected by a 200A main breaker, there may be 300A of branch breakers. If you then add a 60A source breaker, the loads can now draw more current than the main breaker allows by itself. So you've defeated the overcurrent protection scheme required elsewhere in the code and 705 places limits on how much you can do that.
There are only a few places in the code where overload protection isn't strictly sized to conductors. 230.90 Exception 3 is one, and the one which makes the least sense in my opinion, at least in its current unlimited form. (Do they really mean you can have 400A of breakers on a 100A service just because of a load calc?) 705 is another. Motors and fixture wires are a couple others I can think of. I don't really see any major philosophical discrepancy here. If anything, I think 230.90 ex3 should be more limited (but I won't be the one asking them to do that).
