Why is it mandatory for a subpanel to have two ungrounded entrance cables? Also, what is the reasoning for a subpanels need to have two buses?
Is it to accommodate today’s (and future) demand?
Starting with the 2008 National Electric Code, the only acceptable way to wire a subpanel is with a four-wire feed. Two hots, one ground, and one neutral wire.
Distribution panels, or load-side panels, are downstream from the panel containing the main service disconnect(s). In these panels, the neutral and grounds should be separate, and the neutral bus should be isolated from the panel enclosure.
The only exception to this is in existing detached structures where no metallic path exists between the structures. In this exception, a connection between the grounded conductor and the metal case via a bonding jumper is permitted. According to the 2008 NEC, this is not allowed in new construction, so, in all cases, a 4-wire feed to the detached structure is required in order to isolate the grounded conductors from the equipment grounding conductors.
There are two methods of providing ground continuity back to the service panel:
1. four conductor feeders with:
two hot or ungrounded conductors;
one neutral or grounded conductor; and
one grounding conductor.
2. three conductor feeders with:
two hot or ungrounded conductors;
one neutral or grounded conductor; and
equipment grounding through conduit/tubing, electrically linking the two panels (allowed by section 250.118 of the NEC).
If I’m understanding this right, all subpanels are required to have two hot conductors (not just new construction). Am I interpreting this correctly?
Now I see what this is referring to. In the 2008 NEC they removed the permission to use a 3-wire feeder where the neutral was bonded to provide equipment grounding that’s why you may need a 4-wire feeder when the voltage is 120/240. The statement:
“According to the 2008 NEC, this is not allowed in new construction, so, in all cases, a 4-wire feed to the detached structure is required in order to isolate the grounded conductors from the equipment grounding conductors”.
is incorrect because a feeder can still be only 3 wires (ungrounded, neutral and EGC) if there are only 120 volt loads.