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Joined: Aug 2006
Posts: 7
Yeknom Offline OP
New Member
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?

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Joined: Apr 2002
Posts: 7,384
Likes: 7
Food for thought....90A from PV, 90 from ConEd, with a 'tap' along the way b'twn the two OCP's.

Scott35, this is your theoretical domain. My opinion now is 90 amps is all, unless you cut the riser.

Theoretically, you may be able to suck 180 A along the riser, but I fail to see where this could be a code issue, unless you installed the tap!

Yes, the rules you cited are for buss protection, etc., not to stop illegal modifications (taps in your scenario).

Joined: Jun 2004
Posts: 1,273
Unless you've got some trick synchronizing logic you have absolutely no reason to believe that three inverters are clocked right.

In my experience conversion of DC to 3-phase is only ever performed in a unitary inverter for this reason.

So, this set-up appears incorrectly engineered.


For the life of me I can't understand why any manufacturer would provide the extra logic needed to synch one-phase units. I can think of a ton of reasons why it wouldn't be done starting with warranty issues for field engineering by contractors.

Inverter power is non-harmonic. So I'd suspect that you're already undersized pretty much across the board.

That's one of the brutal realities WRT solar. You have to build for peak flows which will only ever happen during the hottest days. Average use will be a trivial fraction of that.


The whole solar energy economy is a scam. Without endless government subsidies it can never work anywhere at any time.

The obvious exception being low power, off the grid communications gear.

The desire to hold down carbon dioxide emissions is folly. Plants require it to grow -- it's fertilizer to them. NASA proved that 45 years ago when testing for closed cycle food provisioning for astronauts.

All that happens with higher concentrations is that plants grow even faster and suck it out of the atmosphere. That a scientific fact. One of the reasons for increased crop yields across the planet is this effect.

There is active research attempting to directly produce bio-diesel from modified algae. Left unsaid is that every scheme uses super concentrations of carbon dioxide to kick things along!

So, instead of a runaway heat effect we are left with simply more plant life everywhere.


The other one that kills me is the shtick about oceanic carbon dioxide hurting shell fish. What a riot. Carbon dioxide is THE essential component required to deposit their shell. Like plants, shell fish suck down carbon dioxide like we breath air. If denied to them, they stop growing / die.

In an ancient age, carbon dioxide was very much more present. Shell fish grew like crazy. That's why we have the white cliffs of Dover. Imagine how many tons of carbon dioxide are trapped there!


But if that isn't enough, a SINGLE volcano is able to emit more carbon dioxide than our entire nation. And that has been going on since the Earth was the Earth.

73,000 years ago Taba, a super volcano, let loose enough carbon dioxide to beat all. We're still here. So much for melting the ice caps, etc.


One thing that is going to crimp home owned solar: roof repairs and roof falls.

You can be certain as the years go by many a man will fall of the roof attending to the solar array.

But a real kicker is the need to have the array removed before the roof can be repaired -- and then put back.

To maintain the warranty, the original contractor is going to want to handle his panels.

Last edited by Tesla; 11/04/10 03:12 PM.


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