We are installing solar panels on the roof of an apartment building. Each of three inverters on the roof generates 34amp, 208v, 1-phase power which then feeds via three 50a 2-pole breakers into a combining panel with a 90 amp 3-phase main breaker. From this 90 amp breaker feeders descend 200' to a 100amp switch (fused at 90amp), mounted in an existing main distribution panel (MDP) in the basement electrical room.
The question arises: How to determine the size of the feeder from the basement to the roof? The max current generated by the PV panels is 68 amps in each of the three phases. Considering this alone, a # 4 THHN (95a rated) wire is appropriate (with 1.25 factor for continuos use), even considering voltage drop in the 200' run.
However, some have interpreted NEC 2005 290.64(B)(2) as applying to this feeder which is supplied with power via two over-current devices (90a on the roof and 90a in the bsmt) and that therefore the rating of the conductor must be at least 180 amps (90 + 90) (2/0 THHN). The suggestion is that someone could tap into this riser to feed loads which could cause the current in the riser (fed by BOTH the PV sources on the roof and the utility power in the basement) to exceed its 95 amp rating without activating either OCPD.
My interpretation of the code is that it applies only to busbars and conductors at the "point of connection", not to such a riser. In this view it makes sense to protect busbars & conductors at the building service in the basement where additional loads might be applied, not to protect against some fool indiscriminately tapping into risers dedicated to supply power to the building from a rooftop PV system.
What is the correct interpretation?
