PV back feed breaker location, Why?

[Fred B]

Why is it required to install the back feed breaker at the opposite end of bus from the primary source breaker? (One option to get the solar interconnection.) I know the code says so, but what makes it necessary?

 

[ggunn]

Why is it required to install the back feed breaker at the opposite end of bus from the primary source breaker? (One option to get the solar interconnection.) I know the code says so, but what makes it necessary?

The idea is to reduce the risk of hot spots on the bus where the current is higher than the bus rating. The 120% number is a compromise between code writers and has no firm basis in electrical theory, but the code is the code.

 

[Dennis Alwon]

I agree it gives space so that the extra heat can dissipate thru the bus

 

[wwhitney]

Why is it required to install the back feed breaker at the opposite end of bus from the primary source breaker?

Say I connect a 100A source and a 20A source at one end of a 100A bus, and a 120A load at the other end of the bus. The entire bus carries 120A, which overloads it.

Versus connecting a 100A source at the top end of the bus, a 20A source at the bottom end of the bus, and a 120A load in the middle of the bus. The bus above the 120A load carries 100A, and the bus below the 100A load carries 20A. No part of the bus carries above its rating.

Cheers, Wayne

 

[electrofelon]

Why is it required to install the back feed breaker at the opposite end of bus from the primary source breaker? (One option to get the solar interconnection.) I know the code says so, but what makes it necessary?

It is not always required, in some cases the PV breaker can go anywhere. But Basically with two sources, you can theoretically exceed the bus rating of the panel board (IMO it's ridiculous to worry about for 200 A residential panels and they should be exempt, but I'm not the code decoder).

 

[ggunn]

It is not always required, in some cases the PV breaker can go anywhere.

One case is where the sum of the rating of the OCPD on the utility side plus 125% of the maximum inverter current is less than the rating of the bus, but of course you know that.

Some 150A - 200A panels have a 225A rated bus (Eaton CH, for example); that comes in handy sometimes.

 

[jaggedben]

Note that there are similar rules for feeder taps to PV, for the same reason.

 

[ggunn]

Note that there are similar rules for feeder taps to PV, for the same reason.

And of course I know that, too.

 

[jaggedben]

And of course I know that, too.

My comment was to the OP, and everyone.

 

[ggunn]

My comment was to the OP, and everyone.

Also of course; didn't you see the " "?

 

[brycenesbitt]

Some 150A - 200A panels have a 225A rated bus (Eaton CH, for example); that comes in handy sometimes.

Is that what's meant by "Solar ready" that seems to be appearing on packaging for panels?

 

[jaggedben]

Is that what's meant by "Solar ready" that seems to be appearing on packaging for panels?

In some cases, but see here:

Solar ready service

I understand that a solar ready panel comes with a kit that gets installed on the line side of the service disconnect. if you take the same panel that is solar ready vs one that is not solar ready, is there any difference in the rating of the buss? I assume by doing the line side connection...

forums.mikeholt.com

 

[Fred B]

One case is where the sum of the rating of the OCPD on the utility side plus 125% of the maximum inverter current is less than the rating of the bus, but of course you know that.

Some 150A - 200A panels have a 225A rated bus (Eaton CH, for example); that comes in handy sometimes.

I get the code is the code, and requirement for not more than 125% of bus. But with usual intent for back feed net metering, doesn't such limitation really only benefit the utility comp, by counting the PV as an additional load?

Was hoping for a more technical rational reason to give the guy other than "the code says so". Prevent heating hot spots ok maybe, but in most cases I would see a larger "true load" from a range or subpanel than contributed by typical back feed PV.

 

[wwhitney]

Was hoping for a more technical rational reason to give the guy other than "the code says so".

See post #4.

Cheers, Wayne

 

[jaggedben]

I get the code is the code, and requirement for not more than 125% of bus. But with usual intent for back feed net metering, doesn't such limitation really only benefit the utility comp, by counting the PV as an additional load?

Was hoping for a more technical rational reason to give the guy other than "the code says so". Prevent heating hot spots ok maybe, but in most cases I would see a larger "true load" from a range or subpanel than contributed by typical back feed PV.

How do you figure that it benefits the utility company if the customer has to upgrade their service panel? That's just labor for them (disconnect/reconnect) without any additional revenue (actually reduced kwh revenue after solar).

These rules are the result of code writers imagining worst case scenarios, not typical scenarios.

 

[ggunn]

I get the code is the code, and requirement for not more than 125% of bus. But with usual intent for back feed net metering, doesn't such limitation really only benefit the utility comp, by counting the PV as an additional load?

Was hoping for a more technical rational reason to give the guy other than "the code says so". Prevent heating hot spots ok maybe, but in most cases I would see a larger "true load" from a range or subpanel than contributed by typical back feed PV.

PV isn't an additional load, it's an additional power source on the bus. If a panel is rated at 200A and the bus is also rated at 200A, the rule is designed to split the power sources between the two ends of the bus so that no point on the bus gets more current than it can handle and to limit the backfeed to 40A (125% of 32A) for a total of 120% of the bus rating. The 120% number is a bit arbitrary, but the backed breaker location isn't. Apply Kirchoff's Current Law.

 

[Fred B]

benefits the utility company if the customer has to upgrade their service panel?

No mater the size of upgrade and you are limited by the existing loads and bus size POCO will never be at a deficit to the customer owing the customer money. Only way is to put in a 400A to provide for a 200A load to get 200A PV, and then hope no one adds to the big panel with all the empty space. or go to supply side tap, but even then it is limited.

Only option to get bigger really is supply side taps.

 

[wwhitney]

Only way is to put in a 400A to provide for a 200A load to get 200A PV

No. You can put 200A of PV (160A of inverter output current) and 200A of load on a 200A service (once you get the POCO's OK, the service transformer might be too small for 160A of inverter output current). You just need to use the feeder interconnection rules (2020 NEC 705.12(B)(1)) rather than the busbar interconnection rules (2020 NEC 705.12(B)(3)).

Cheers, Wayne

 

[Fred B]

PV isn't an additional load, it's an additional power source on the bus. If a panel is rated at 200A and the bus is also rated at 200A, the rule is designed to split the power sources between the two ends of the bus so that no point on the bus gets more current than it can handle and to limit the backfeed to 40A (125% of 32A) for a total of 120% of the bus rating. The 120% number is a bit arbitrary, but the backed breaker location isn't. Apply Kirchoff's Current Law.

But what you are doing is adding the PV as if it is a load, limiting it by combining it with the other loads not exceeding the bus capacity. Not even looking at actual loads but each breaker capacity.
705.12(B)(3)
(3) The sum of the ampere ratings of all overcurrent devices on panelboards, both load and supply devices, excluding the rating of the overcurrent device protecting the busbar, shall not exceed the ampacity of the busbar. The rating of the overcurrent device protecting the busbar shall not exceed the rating of the busbar. Permanent warning labels shall be applied to distribution equipment displaying the following or equivalent wording

use the feeder interconnection rules (2020 NEC 705.12(B)(1)) rather than the busbar interconnection rules (2020 NEC 705.12(B)(3)).

Even using this method you will not get a net metering in excess of usage for POCO to owe customer.

 

[jaggedben]

But what you are doing is adding the PV as if it is a load, limiting it by combining it with the other loads not exceeding the bus capacity. Not even looking at actual loads but each breaker capacity.
...


Even using this method you will not get a net metering in excess of usage for POCO to owe customer.

No no no no no. Just no.

Not sure quite where to start to address your apparently fundamental misunderstanding of how all this works. But amperage ratings and the rules in 705 just have nothing to do with actual consumption or net metering tabulations. I think you need start with the difference between an kW and a kWh and go from there. The accumulation and offsetting of consumption and production energy over time has essentially nothing to do with overcurrent protection. I can have a 200A residential service and if I almost never use more than 50A then the 200A rating says zilch about my energy consumption.