I have a 200A service in a garage. I would like to run cable from this to a 100A service in a house. I would also like to run from this to a separate 50A service in the house which will supply generator-backed-up power. Both runs will be single-phase 240V. The run lengths are no greater than 160'. The cables will run through buried PVC conduit.
How do I determine a suitable conductor size for these cables?
Considering ampacity alone, I would expect to be able to use #3 copper (SER or THHN) or #1 aluminum (SE) or #2 aluminum (THHN) for the 100A run.
By the same logic, for the 50A run, #6 copper (NM) or #8 copper (SE or THHN) or #6 aluminum (SE or THHN) or #8 aluminum (NM).
However, considering the voltage drop over 160', large conductors seem indicated.
To achieve a less than a 3% voltage drop for the 100A run, it seems like #2 copper (Vdrop 2.6%) or 1/0 aluminum (Vdrop 2.7%) will be required. For the 50A run, #6 NM copper (Vdrop 3%) or #3 SE aluminum (Vdrop 2.7%).
Another factor I know matters but that I don't know how to evaluate is the temperature at the breaker terminals. For example, will #2 copper in the 100A run remain below the required 60C or 75C when actual load approaches 100A?
I'm also not sure whether 3% is the right magic number to select for Vdrop. It seems allowed (encouraged?) to accept a 5% Vdrop in situations like this one. Yet allowing even 3% loss on a 100A line seems like a lot of wasted power. Have I understood this part of the decision properly?
And, not being a professional, I wonder if there are further questions I should be asking that I'm not even aware of.
So, what are the proper conductors for this scenario and why? I am interested in both code compliance as well as good performance of the resulting system (should those two not necessarily be the same).
Thanks.
Best Answer
Temperature at the terminals is handled for you
The NEC's ampacity charts handle temperature at the terminals for you, simply by way of you selecting the appropriate column from the chart for the temperature limit of your terminals.
3% for feeders is a good voltage drop rule of thumb
The reason 3% is used for feeder voltage drop is because we want overall voltage drop to be no more than 5% at max ampacity (some stuff can take more, but it's not particularly kind to certain loads, esp. motors), and the other 2% needs to be left for voltage drop across the branch circuit to the load in question. Varying this is possible, depending on the load, though -- a water heater might be OK with a bit of extra voltage drop, while your air conditioner would be better off with the full 240V at its terminals.
Of course, if you wish to analyze load diversity and come up with a more realistic design point than maximum load for the feeder to be at design voltage drop, that's your prerogative -- as long as the wire meets Code minima, it'll be able to handle the current safely.
Will it fit?
There are two final things you need to check:
Is your conduit big enough for your wires? An overstuffed conduit may overheat, in addition to making the pull far harder than it needs to be -- in fact, it's wise to oversize the conduit to keep pulling difficulty down and provide room for future expansion.
Will your wires fit in the breaker lugs? Most lugs accept a wide variety of wire, but it's good to double-check before you have everything pulled.
P.S. TORQUE MATTERS
Section 110.14(D) in the 2017 NEC requires that connections be torqued to manufacturer specifications; in practice, you'll need a torque screwdriver and/or torque wrench, both reading in inch-pounds, for this.