backfeed

[jerrywilson]

i am installing a siemens solar backfeed panel.the max amp rating for the backfeed is 60a.now i have to install the backfeed breaker on the main buss if over 60a.my inspector failed this,saying that it exceeds the buss rating on a 200a panel.is he right?how do i install a 70a solar backfeed now?

 

[gar]

120727-1041 EDT

Suppose you mount the backfeed breaker at the same end of the bus as the main breaker. Further assume all breakers have a precise trip point of exactly the breaker rating.

The bus could be loaded up to the point of the sum of the two breakers. Thus, 200 + 60 exceeds 200 and therefore exceeds the bus rating. The solar supply can be viewed as an approximately constant current source with voltage largely defined by the power company. This does not mean that there aren't possibly large variations in the bus voltage as the solar energy available and loads change.

Suppose instead the backfeed breaker is at the opposite end of the bus from the main breaker. Then depending upon how the load was distributed across the bus the rating might not be exceeded. I would tend to agree with the inspector.

Can you backfeed ahead of the main breaker?

It is really all about the amount of heat generated in the bus. If you have multiple sources feeding a panel, then the sum of all those sources is the current that could flow into the bus.

.

 

[Little Bill]

i am installing a siemens solar backfeed panel.the max amp rating for the backfeed is 60a.now i have to install the backfeed breaker on the main buss if over 60a.my inspector failed this,saying that it exceeds the buss rating on a 200a panel.is he right?how do i install a 70a solar backfeed now?

The panel should have the buss rating on the label. If not, check with the mfg. Most 200A panels that I've seen will take a 100A breaker.
Unless the inspector read the rating on the panel, I would have to disagree with him and ask him to show you how he came up with that rating.
Better yet, find out yourself and if it's rated more than 70A, show him and he will have to accept the install.

 

[iwire]

The panel should have the buss rating on the label. If not, check with the mfg. Most 200A panels that I've seen will take a 100A breaker. U
Unless the inspector read the rating on the panel, I would have to disagree with him and ask him to show you how he came up with that rating.
Better yet, find out yourself and if it's rated more than 70A, show him and he will have to accept the install.

This is a PV back feed so the 200 amp main from the utility and the 60 amp back feed from the PV total 260 amps possibly supplying the panel which exceeds the bus bar rating.

 

[fmtjfw]

2011 NEC

2011 NEC

705.12 Point of Connection.
(D) Utility-Interactive Inverters.

(2) Bus or Conductor Rating. The sum of the ampere ratings
of overcurrent devices in circuits supplying power to a
busbar or conductor shall not exceed 120 percent of the rating
of the busbar or conductor.
Exception: Where the photovoltaic system has an energy
storage device to allow stand-alone operation of loads, the
value used in the calculation of bus or conductor loading
shall be 125 percent of the rated utility-interactive current
from the inverter instead of the rating of the overcurrent
device between the inverter and the bus or conductor.

(7) Inverter Output Connection. Unless the panelboard is
rated not less than the sum of the ampere ratings of all overcurrent
devices supplying it, a connection in a panelboard
shall be positioned at the opposite (load) end from the input
feeder location or main circuit location. The bus or conductor
rating shall be sized for the loads connected in accordance
with Article 220. In systems with panelboards connected in
series, the rating of the first overcurrent device directly connected
to the output of a utility-interactive inverter(s) shall be
used in the calculations for all busbars and conductors. A
permanent warning label shall be applied to the distribution
equipment with the following or equivalent wording:

WARNING
INVERTER OUTPUT CONNECTION
DO NOT RELOCATE THIS
OVERCURRENT DEVICE

 

[NorthwestPV]

Do a line side tap on the wire before the main breaker.

 

[jerrywilson]

after a little investigating,the insp. is right.the amp rating of the buss is only 200 and 120%is 240a....therefore the largest backfeed is 40a on the buss.I think my only option right now is to possibly derate the main to 150a..(.which means i need to do a ;load calc. on the house to see if thats suffecient)THE LOCAL POWER CO. HAS A VERY SHORT LIST OF APPROVED PANELS.CUTLER HAMMER MAKES A 225 RATED BUSS WHICH WOULD ALSO WORK IF TEP WILL APPROVE...THANKS TO EVERYBODY WHO SENT ME POSSIBLE FIXES...

 

[Little Bill]

This is a PV back feed so the 200 amp main from the utility and the 60 amp back feed from the PV total 260 amps possibly supplying the panel which exceeds the bus bar rating.

I ain't used to going backwards!:slaphead:

 

[iwire]

I ain't used to going backwards!:slaphead:

I understand, it goes against all we are used to.

Wait till you have to connect a standard disconnect switch to a circuit that is live on both sides ....... which side is line???????? (The utility side, but you have to think about it.)

 

[fmtjfw]

Utility interactive inverters drop voltage very quickly

Utility interactive inverters drop voltage very quickly

If you install a disconnect between the commercial power and the output of the inverters, the inverters must be "utility interactive" which means that absent commercial power the inverters are going to cease output very quickly. So quickly, in fact, that the backfed breaker they come in through need NOT have a retaining clip/screw because the voltage goes away so quickly it is not a hazard.

Thus the disconnect will only have voltage on the line (commercial) side.

 

[ggunn]

I understand, it goes against all we are used to.

Wait till you have to connect a standard disconnect switch to a circuit that is live on both sides ....... which side is line???????? (The utility side, but you have to think about it.)

You don't have to think about it very hard, though. If your inverter is feeding a service through a fused disco and you have the utility connected to the load side, how are you going to change the fuses?

 

[iwire]

If you install a disconnect between the commercial power and the output of the inverters, the inverters must be "utility interactive" which means that absent commercial power the inverters are going to cease output very quickly. So quickly, in fact, that the backfed breaker they come in through need NOT have a retaining clip/screw because the voltage goes away so quickly it is not a hazard.

Thus the disconnect will only have voltage on the line (commercial) side.

You don't have to think about it very hard, though. If your inverter is feeding a service through a fused disco and you have the utility connected to the load side, how are you going to change the fuses?

Guys, I am not an idiot

My point was simply this, when you start doing PV work that it is unnatural after doing things one way for 20 or 30 years to turn things around and feed a panel that is already fed.

 

[ggunn]

Guys, I am not an idiot

My point was simply this, when you start doing PV work that it is unnatural after doing things one way for 20 or 30 years to turn things around and feed a panel that is already fed.

Sorry, no offense intended. Connecting AC discos backwards is, according to inspectors I have dealt with, the single most common error they encounter in installations. There was another thread in here recently that spoke of an engineer who insisted that the utility be connected to the load side of a disco and would brook no argument. My point in response was that if that engineer would THINK about what he was saying rather than just spouting his first impression, he would immediately see how wrong he was. He wouldn't have to think that hard; with the utility on the load side of a fused disco, how do you change the fuses without electrocuting yourself?

Anyway, I didn't mean to cast aspersions in your direction; when I said "you", I meant it in the general sense.

 

[Little Bill]

Guys, I am not an idiot

My point was simply this, when you start doing PV work that it is unnatural after doing things one way for 20 or 30 years to turn things around and feed a panel that is already fed.

I agree with everything except the first line, the jury is still out on that!:lol:

 

[bradleymattlin]

backfeed breaker

backfeed breaker

The biggest breaker for a 200 amp panel is 40 amp for back feeding solar. So what we do a alot is do a line side tap. But you need a disconnect within ten feet. After the tap to the disconnect the wires have to be in a metal conduit or flex. We use a device called couple taps and they have teeth that bit into the wires and are insulated and ul listed.