408.3(E)(2) DC Bus Arrangement.
Code Change Summary: A new subsection provides marking and identification requirements for direct current busses.
DC busses are quite common when dealing with PV systems or battery storage systems. In larger PV systems, combiner boxes are often used to combine multiple series strings pf PV modules in parallel. Multiple DC PV source circuits enter the enclosure and terminate to individual fuse holders which are tied into DC busbars where a single larger DC conductor terminates.
In the 2011 NEC®, the phase arrangement on 3-phase AC buses is A, B, C from front to back, top to bottom, or left to right, as viewed from the front of the switchboard or panelboard. The phase arrangement of DC busbars is not mentioned. In the 2014 NEC®, the code section has been rearranged and the phase arrangement of DC busbars is now addressed.
DC busbars are often comprised of DC positive and DC negative conductors. In a grounded DC two-wire system, one of the conductors is ungrounded and the other is grounded. The 2014 NEC® states that ungrounded DC buses may be in any order. The arrangement of DC buses must be field marked as to polarity, grounding system, and nominal voltage.
Below is a preview of the NEC®. See the actual NEC® text at NFPA.ORG for the complete code section. Once there, click on the “free access” tab and select the applicable year of NFPA 70 (National Electrical code).
2011 Code Language:
408.3(E) Phase Arrangement. The phase arrangement on 3-phase buses shall be A, B, C from front to back, top to bottom, or left to right, as viewed from the front of the switchboard or panelboard. The B phase shall be that phase having the higher voltage to ground on 3-phase, 4-wire, delta-connected systems. Other busbar arrangements shall be permitted for additions to existing installations and shall be marked.
2014 Code Language:
408.3(E) Bus Arrangement.
(1) AC Phase Arrangement. Alternating-current phase arrangement on 3-phase buses shall be A, B, C from front to back, top to bottom, or left to right, as viewed from the front of the switchboard, switchgear, or panelboard. The B phase shall be that phase having the higher voltage to ground on 3-phase, 4-wire, delta-connected systems. Other busbar arrangements shall be permitted for additions to existing installations and shall be marked.
Exception: Equipment within the same single section or multisection switchboard, switchgear, or panelboard as the meter on 3-phase, 4-wire, delta-connected systems shall be permitted to have the same phase configuration as the metering equipment.
Informational Note: See 110.15 for requirements on marking the busbar or phase conductor having the higher voltage to ground where supplied from a 4-wire, delta-connected system.
(2) DC Bus Arrangement. Direct-current ungrounded buses shall be permitted to be in any order. Arrangement of dc buses shall be field marked as to polarity, grounding system, and nominal voltage.