Understanding 705.12B

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Pitt123

Senior Member
Reviewing a design for a solar PV system that is tying into a panel. This panel is located in a parking garage and is fed from another distribution panel in the main building which is fed from the utility.

The inverter from the PV system is a 250kW inverter and therefore requires a 400A breaker at the interface panel. The interface panel has an 800A rated bus with an 800A breaker.

Looking at this it appears to be a violation of 705.12(D)(2) which says that the OCPD's from the PV inverter and the panel utility supply feed (in this case fed from an upstream panel) cannon exceed 120% of the bus rating. 120% of the 800A bus rating is 960A which is less than the 1200A total of the two breakers. Am I right in saying that there is a problem here even though this panel isn't fed directly from the utility but is fed from the utility source via an upstream distribution panel?

The way I see it we have (2) options which would be to either 1) verify that the interface panels buss is capable of 1000A continuous rating, or 2) if possible reduce the main breaker to a 500A breaker if the bus is only rated for 800A continuous duty.

Since the inverter is rated at 250kW then it needs to have a 400A breaker at the interface per the NEC.

Do you guys agree with the two options above?

What if the main breaker is an 800A frame but is set for a 500A trip setting. I assume its the trip setting we're concerned with, not necessarily the frame size?
 

BillK-AZ

Senior Member
Location
Mesa Arizona
Note that both of the panels and the cables need to be evaluated for the 120% factor. The cable will have to have any temperature deratings applied.

Only the 400A PV breaker is considered when evaluating the distribution panel in the main building.

All breakers need to be evaluated for back feed. Some interchangeable trip breakers are marked 'Line' and 'Load'. The breaker with the reduced trip should be permanently labeled for the required reduced setting or that the trip unit must not be increased above the 500A.

Difficult to "verify that the interface panels buss is capable of 1000A " as this will require re-listing. UL and others offer a field evaluation service for this and supply side connections when the OEM did not provide for such.
 

Pitt123

Senior Member
Note that both of the panels and the cables need to be evaluated for the 120% factor. The cable will have to have any temperature deratings applied.

Only the 400A PV breaker is considered when evaluating the distribution panel in the main building.

Thanks for the help

Can you please clarify your two statements.

What do you mean when you say that both of the panels and the cables need to be evaluated for 120%? Are you saying that the 120% rule must be applied to the panel that the inverter is directly connected to as well as the upstream panel in the main building that serves this panel in the garage? So when an inverter panel is fed from a series of multiple panels upstream must the combination of the 400A pv breaker with each panel upstreams main be evaluated for the 120% rule? Does this rule apply to the cable as well?

It sounds like with your second statement above you are clarifying what I suspect above in that you must use the 400A pv breaker along with the main breaker in each panel upstream to ensure that the (2) breaker combinations satisfys the 120% rule? I guess for all upstream panels as well the feeder breaker feeding the downstream equipment leading to the pv inverter must always be completely at the other end of the panel from the main breaker.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
The 120% rule only applies to the panel that the inverter output is feeding. It does not apply to the feeder conductors nor any upstream panels.

What, other than the inverter output, is connected to the 800 amp panel?
 

BillK-AZ

Senior Member
Location
Mesa Arizona
Can you please clarify your two statements.

What do you mean when you say that both of the panels and the cables need to be evaluated for 120%? Are you saying that the 120% rule must be applied to the panel that the inverter is directly connected to as well as the upstream panel in the main building that serves this panel in the garage?


YES

So when an inverter panel is fed from a series of multiple panels upstream must the combination of the 400A pv breaker with each panel upstreams main be evaluated for the 120% rule? Does this rule apply to the cable as well?

Yes, the situation is now clearer with the 2011 NEC. See 705.12(D)(2) "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".

It sounds like with your second statement above you are clarifying what I suspect above in that you must use the 400A pv breaker along with the main breaker in each panel upstream to ensure that the (2) breaker combinations satisfys the 120% rule?

Yes.
I guess for all upstream panels as well the feeder breaker feeding the downstream equipment leading to the pv inverter must always be completely at the other end of the panel from the main breaker.

Only if you want to use the 120% rule. For instance, If you have a 200A OCPD feeding a 225A panel, then a PV breaker of up to 25A can be placed anywhere in the panel.
 

Smart $

Esteemed Member
Location
Ohio
The 120% rule only applies to the panel that the inverter output is feeding. It does not apply to the feeder conductors nor any upstream panels.

...
I agree. The output of the inverter is the max that can be backfed upstream on the utility supplied conductors.
 

Pitt123

Senior Member
The 120% rule only applies to the panel that the inverter output is feeding. It does not apply to the feeder conductors nor any upstream panels.

So you are saying that I would not have to apply this rule to any panels that may be upstream of the inverter panel?

What, other than the inverter output, is connected to the 800 amp panel?

The panel also serves other loads and panels and transformers via its feeders.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Can you please clarify your two statements.

What do you mean when you say that both of the panels and the cables need to be evaluated for 120%? Are you saying that the 120% rule must be applied to the panel that the inverter is directly connected to as well as the upstream panel in the main building that serves this panel in the garage?

YES

So when an inverter panel is fed from a series of multiple panels upstream must the combination of the 400A pv breaker with each panel upstreams main be evaluated for the 120% rule? Does this rule apply to the cable as well?

Yes, the situation is now clearer with the 2011 NEC. See 705.12(D)(2) "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".
If the panel with the inverter output breaker in it is a MLO panel then the 120% rule would apply to the panel feeder, but it does not apply beyond the OCPD that protects the panel that has the inveter connection. If the panel that has the inverter connection has a main breaker the 120% does not apply to the panel feeder conductors. None of the upsteam panels or conductors can see excessive current as these devices are protected by their OCPDs and downsteam power source (inverter) does not change this.
 
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