Section 690.64. Again.

[tallgirl]

The '08 NEC seems to contradict itself (see (B)(7)), as well as the '08 ROP on the size of the largest backfeed breaker.

My understanding was that if I have a 200A service the size of the largest backfeed breakers was reduced by the sum of the breakers serving loads. A proof, I offer a piece of wire and two OCPDs connected in series by said piece of wire.

If the piece of wire is just a piece of wire, rated for 200A continuous, I can backfeed all 200A from an inverter into the supply. But if I call the piece of wire a "buss bar", at most I can back feed 40A.

What's wrong -- the understand of the section, or the section itself?

 

[don_resqcapt19]

Julie,
The proposal that was accepted at the ROP stage would have permitted a 200 amp main and a 200 amp backfed breaker on the same bus as long as the breakers were placed on the opposite ends of the bus. This was changed in the comment stage to only permit feeds into a panel that do not exceed 120% of the panel rating. The following is part of comment 13-77 which was submitted by CMP 9 who has jurisdiction over Article 408.

...CMP-9 agrees that it is possible to assure that the busbars of panelboards
receiving supply current from two sources can be arranged so the busbars will
not exceed their ampacity (the “opposite end” scenario accepted under this
proposal), however, CMP-9 points out that such an arrangement allows for up
to double the amount of load to be taken from the panel for indefinite periods
of time. Current product standards do not anticipate the effect of I2R heating
losses under these conditions, which could severely impact the performance of
essential components within the distribution equipment. Before the NEC
recognizes this type of connection, it is essential that careful testing be carried
out to determine the acceptable parameters that should be applied in these
cases. ...

 

[tallgirl]

Julie,
The proposal that was accepted at the ROP stage would have permitted a 200 amp main and a 200 amp backfed breaker on the same bus as long as the breakers were placed on the opposite ends of the bus. This was changed in the comment stage to only permit feeds into a panel that do not exceed 120% of the panel rating. The following is part of comment 13-77 which was submitted by CMP 9 who has jurisdiction over Article 408.

Yup, that's what I remembered. But I somehow also remembered that the backfeed had to be reduced by all load-serving disconnects.

So, if I had a 200A service, with 150A of loads connected, I could backfeed 200A * 120% - 150A = 90A.

I guess I have to admit CMP 9 or whomever made the best decision. Too many variables to insure the installation is maintainable since adding another 50A OCPD to that panel would mean the bars could be overloaded.