The minimum size would be 18" wide by 7.5" high.
You might get lucky and find a stocked box 18" x 12" but I doubt it.
A standard size will be 18" x 18"
The depth can be as shallow as want and still get the 3" raceways in.
If this really has to look good you may want to have some made 18"W x 10"H x 6"D
You may already know this but I would not cut the 4/0s I would tap them with some sort of connector.
If you can find a 4/0 to 10 awg hypress H tap you would really have a trouble free connection.
Also if this is a 30 or 20 amp circuit and you are under the 2002 code the grounding conductor will need to be 4/0 when run with the 4/0s and 10 awg when run with the 10 AWGs
If the circuit is higher than 30 the EGC will still need to be increased in proportion to the ungrounded conductors.
Yes the grounding conductor for a 20 or 30 amp circuit may be required to be larger than the grounding conductor for a 40 amp circuit.
Read 250.122(B) carefully for this info.
[This message has been edited by iwire (edited 03-11-2004).]
