Sub Panel allowable ampacity

SolarPro

Senior Member
Location
Austin, TX
Yes, there are a lot of changes here, and generally for the better. There are now more options for a Code-compliant design. Here's the new language.

(D) Utility-Interactive Inverters. The output of a utility-interactive inverter shall be permitted to be connected to the load side of the service disconnecting means of the other source(s) at any distribution equipment on the premises.Where distribution equipment, including switchgear, switchboards, or panelboards, is fed simultaneously by a primary source(s) of electricity and one or more utility-interactive inverters, and where this distribution equipment is capable of supplying multiple branch circuits or feeders, or both, the interconnecting provisions for the utilityinteractive inverter(s) shall comply with 705.12(D)(1) through (D)(6).

(1) Dedicated Overcurrent and Disconnect. The source interconnection of one or more inverters installed in one system shall be made at a dedicated circuit breaker or fusible disconnecting means.

(2) Bus or Conductor Ampere Rating. One hundred twenty-five percent of the inverter output circuit current shall be used in ampacity calculations for the following:

(1) Feeders. Where the inverter output connection is made to a feeder at a location other than the opposite end of the feeder from the primary source overcurrent device, that portion of the feeder on the load side of the inverter output connection shall be protected by one of the following:

(a) The feeder ampacity shall be not less than the sum of the primary source overcurrent device and 125 percent of the inverter output circuit current.

(b) An overcurrent device on the load side of the inverter connection shall be rated not greater than the ampacity of the feeder.

(2) Taps. In systems where inverter output connections are made at feeders, any taps shall be sized based on the sum of 125 percent of the inverter(s) output circuit current and the rating of the overcurrent device protecting the feeder conductors as calculated in 240.21(B).

(3) Busbars. One of the methods that follows shall be used to determine the ratings of busbars in panelboards.

(a) The sum of 125 percent of the inverter(s) output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed the ampacity of the busbar.

Informational Note: This general rule assumes no limitation in the number of the loads or sources applied to busbars or their locations.

(b) Where two sources, one a utility and the other an inverter, are located at opposite ends of a busbar that contains loads, the sum of 125 percent of the inverter(s) output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed 120 percent of the ampacity of the busbar. The busbar shall be sized for the loads connected in accordance with Article 220. A permanent
warning label shall be applied to the distribution equipment adjacent to the back-fed breaker from the inverter that displays the following or equivalent wording:

WARNING:
INVERTER OUTPUT CONNECTION;
DO NOT RELOCATE THIS OVERCURRENT DEVICE.

The warning sign(s) or label (s) shall comply with 110.21(B).

(c) 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 that displays
the following or equivalent wording:

WARNING:
THIS EQUIPMENT FED BY MULTIPLE SOURCES.
TOTAL RATING OF ALL OVERCURRENT DEVICES,
EXCLUDING MAIN SUPPLY OVERCURRENT DEVICE,
SHALL NOT EXCEED AMPACITY OF BUSBAR.

The warning sign(s) or label (s) shall comply with 110.21(B).

(d) Connections shall be permitted on multiple-ampacity busbars or center-fed panelboards where designed under engineering supervision that includes fault studies and busbar load calculations.

(3) Marking. Equipment containing overcurrent devices in circuits supplying power to a busbar or conductor supplied from multiple sources shall be marked to indicate the presence of all sources.

(4) Suitable for Backfeed. Circuit breakers, if backfed, shall be suitable for such operation.

Informational Note: Fused disconnects, unless otherwise marked, are suitable for backfeeding.

(5) Fastening. Listed plug-in-type circuit breakers backfed from utility-interactive inverters that are listed and identified as interactive shall be permitted to omit the additional fastener normally required by 408.36(D) for such applications.

(6) Wire Harness and Exposed Cable Arc-Fault Protection. A utility-interactive inverter(s) that has a wire harness or cable output circuit rated 240 V, 30 amperes, or less, that is not installed within an enclosed raceway, shall be provided with listed ac AFCI protection.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Thank you for your input. Just for clarification, the 43A is the continuous output rated current of the inverters. My understanding (although I haven't confirmed it yet) is that the 2014 code, which has already been adopted here in Mass, utilizes this value now in calculations instead of the OCPD rating. I've still got some homework to do.
From Solapro's post:

"(b) Where two sources, one a utility and the other an inverter, are located at opposite ends of a busbar that contains loads, the sum of 125 percent of the inverter(s) output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed 120 percent of the ampacity of the busbar."
 

csoc64

Senior Member
Location
northeast
(2) Bus or Conductor Ampere Rating. One hundred twenty-five percent of the inverter output circuit current shall be used in ampacity calculations for the following:

(1) Feeders. Where the inverter output connection is made to a feeder at a location other than the opposite end of the feeder from the primary source overcurrent device, that portion of the feeder on the load side of the inverter output connection shall be protected by one of the following:

(a) The feeder ampacity shall be not less than the sum of the primary source overcurrent device and 125 percent of the inverter output circuit current.

(b) An overcurrent device on the load side of the inverter connection shall be rated not greater than the ampacity of the feeder.

So, If I am feeding my pv at the bottom end of my sub panel, is this location considered
"the opposite end of the feeder from the primary source overcurrent device"? If so, then (a) and (b) would not apply. Is this the correct interpretation?
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
(2) Bus or Conductor Ampere Rating. One hundred twenty-five percent of the inverter output circuit current shall be used in ampacity calculations for the following:

(1) Feeders. Where the inverter output connection is made to a feeder at a location other than the opposite end of the feeder from the primary source overcurrent device, that portion of the feeder on the load side of the inverter output connection shall be protected by one of the following:

(a) The feeder ampacity shall be not less than the sum of the primary source overcurrent device and 125 percent of the inverter output circuit current.

(b) An overcurrent device on the load side of the inverter connection shall be rated not greater than the ampacity of the feeder.

So, If I am feeding my pv at the bottom end of my sub panel, is this location considered
"the opposite end of the feeder from the primary source overcurrent device"? If so, then (a) and (b) would not apply. Is this the correct interpretation?
I believe so, yes.
 

jaggedben

Senior Member
So, If I am feeding my pv at the bottom end of my sub panel, is this location considered "the opposite end of the feeder from the primary source overcurrent device"? If so, then (a) and (b) would not apply. Is this the correct interpretation?
Assuming the feeder is connected to the top of the panel, yes.
 

jaggedben

Senior Member
Ok, so if that is the case, do I still need to oversize my feeders to a sub panel to account for the panel OCPD and the PV input current?
No, not under the 2014 code. It's not a situation "where the inverter output connection is made to a feeder at a location other than the opposite end of the feeder from the primary source overcurrent device".
 

Smart $

Esteemed Member
Location
Ohio
No, not under the 2014 code. It's not a situation "where the inverter output connection is made to a feeder at a location other than the opposite end of the feeder from the primary source overcurrent device".
I think he's referring to upstream on the utility side of a subpanel that's backfed by a PV System.
 

GoldDigger

Moderator
Staff member
It matters only if in the case of a main breaker panel the main breaker in the subpanel is smaller than the feeder breaker supplying it.
Otherwise the protection of bus and wires and the application of the 120% rule is identical.

Sent from my XT1080 using Tapatalk
 

csoc64

Senior Member
Location
northeast
It matters only if in the case of a main breaker panel the main breaker in the subpanel is smaller than the feeder breaker supplying it.
Otherwise the protection of bus and wires and the application of the 120% rule is identical.

Sent from my XT1080 using Tapatalk
Ok. So let me summarize to see if I've got this straight. This is my understanding. For my 200A bus main lug sub panel fed from a 100A breaker in my main panel, I can supply (200 x 1.2) - 100 = 140 amps (realistically, 100A max). My existing feeders to the sub panel, currently rated for 100A, do not have to be re-sized to accommodate the 43A of PV I am adding.
 

GoldDigger

Moderator
Staff member
To the first part of your question, yes. The subpanel bus is fine.
To the second part of your question, the answer depends on your code cycle and your AHJ.
By [2014] you would be fine. But under earlier versions you would need the feeder wire sized for 120A so that 120 x 120% = 144 would be greater than 100 + 43.
As long as there are no other load connections to the feeder wires it makes no sense to apply the 120% rule to them, but that is what the Code language arguably does.
Needless to say some inspectors will argue that :)


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jaggedben

Senior Member
Does it matter if the sub panel is a main breaker panel or a main lug panel with a 100A breaker in the main panel?
No.

Under the 2014 code, the feeder rating just needs to be appropriate for the breaker on the utility side.
(Under a strict reading of any previous code, it has to be sized for 5/6ths of the sum of the utility and inverter breakers.)
 

csoc64

Senior Member
Location
northeast
No.

Under the 2014 code, the feeder rating just needs to be appropriate for the breaker on the utility side.
(Under a strict reading of any previous code, it has to be sized for 5/6ths of the sum of the utility and inverter breakers.)
Thanks for your help. Do you know the specific code reference that applies. Just trying to get my ducks in a row for the inevitable challenge down the road.
 

Smart $

Esteemed Member
Location
Ohio
Thanks for your help. Do you know the specific code reference that applies. Just trying to get my ducks in a row for the inevitable challenge down the road.
From what I gather of 2014 Code, the feeder requirements revert back to Chapters 1-4,, so it would be 215.2, because 690 or 705 don't cover it specifically (i.e. where PV breaker is at opposite end of panel MCB).
 
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