Supply Side (line side) Interconnections and the 7th disco

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2011 NEC Section 705 allows for supply side interconnections. However is does not specify how large these connections can be. I read an article in 2012 SolarPro by Mike Holt which states, by the way, that the size of the PV system OCP must be below the rating of the service entrance conductors. This article then goes on to discuss that PV systems are not services and that one may install the 7th disconnect on a given service. This article also has a drawing that shows an OH service to a CT/Term can and then a gutter servicing the six service disconnects. But what about service gear?
My question relates to this type of installation on a service gear without a main switch.
We are looking to put 200A of PV into an 800A rated gear. The gear has a collection of SIX existing disconnects (30A, 100A, three 200A and a 400A). We are hoping to add a 7th disconnect to the gear but have been called on it by an PE electrical engineer and the AHJ plan reviewer stating that we need to follow the 120% rule. This rule clearly only applies to Load side interconnections and would limit us to 160A of PV but only if there was a main at 800A to complete the calculation, which there is not.
If we are installing as described is this not Supply Side (line side)?
How do we size the PV interconnection for Supply side?
Does the 120% rule apply here?
 
2011 NEC Section 705 allows for supply side interconnections. However is does not specify how large these connections can be. I read an article in 2012 SolarPro by Mike Holt which states, by the way, that the size of the PV system OCP must be below the rating of the service entrance conductors. This article then goes on to discuss that PV systems are not services and that one may install the 7th disconnect on a given service. This article also has a drawing that shows an OH service to a CT/Term can and then a gutter servicing the six service disconnects. But what about service gear?
My question relates to this type of installation on a service gear without a main switch.
We are looking to put 200A of PV into an 800A rated gear. The gear has a collection of SIX existing disconnects (30A, 100A, three 200A and a 400A). We are hoping to add a 7th disconnect to the gear but have been called on it by an PE electrical engineer and the AHJ plan reviewer stating that we need to follow the 120% rule. This rule clearly only applies to Load side interconnections and would limit us to 160A of PV but only if there was a main at 800A to complete the calculation, which there is not.
If we are installing as described is this not Supply Side (line side)?
How do we size the PV interconnection for Supply side?
Does the 120% rule apply here?

I agree that the 120% rule does not apply to your situation and that you may have a 7th service disconnecting means, however mounting it in the same panelboard may be a little sticky. Look at 230.40 and 230.71. You are allowed up to six disconnects for the additional set of service entrance conductors allowed by 230.40 ex 5. If you are adding an additional breaker to a mlo service panelboard, I'm not really sure you have another set of service entrance conductors. It seems silly but tapping the service buss in an approved way to a new enclosure seems to match the wording much better IMO.
 

Carultch

Senior Member
Location
Massachusetts
2011 NEC Section 705 allows for supply side interconnections. However is does not specify how large these connections can be. I read an article in 2012 SolarPro by Mike Holt which states, by the way, that the size of the PV system OCP must be below the rating of the service entrance conductors. This article then goes on to discuss that PV systems are not services and that one may install the 7th disconnect on a given service. This article also has a drawing that shows an OH service to a CT/Term can and then a gutter servicing the six service disconnects. But what about service gear?
My question relates to this type of installation on a service gear without a main switch.
We are looking to put 200A of PV into an 800A rated gear. The gear has a collection of SIX existing disconnects (30A, 100A, three 200A and a 400A). We are hoping to add a 7th disconnect to the gear but have been called on it by an PE electrical engineer and the AHJ plan reviewer stating that we need to follow the 120% rule. This rule clearly only applies to Load side interconnections and would limit us to 160A of PV but only if there was a main at 800A to complete the calculation, which there is not.
If we are installing as described is this not Supply Side (line side)?
How do we size the PV interconnection for Supply side?
Does the 120% rule apply here?

For a supply side interconnection, the OCPD cannot exceed the rating of the service. If there are multiple of them, the sum cannot exceed the rating of the service. You do not add up all the load service disconnects with it, you just use the PV system disconnects.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I agree that the 120% rule does not apply to your situation and that you may have a 7th service disconnecting means, however mounting it in the same panelboard may be a little sticky. Look at 230.40 and 230.71. You are allowed up to six disconnects for the additional set of service entrance conductors allowed by 230.40 ex 5. If you are adding an additional breaker to a mlo service panelboard, I'm not really sure you have another set of service entrance conductors. It seems silly but tapping the service buss in an approved way to a new enclosure seems to match the wording much better IMO.

This nails it, in my opinion. Kind of strange, but it is what the code actually says.

Good luck working with the AHJ on this. They are already wrong about the 120% rule so don't expect them to have thought about whether a PV disconnect can be a service disconnect. Regarding the 120% rule itself, I've had a couple experiences where it worked to point out that there is no breaker 'supplying' the busbar from the utilty side, thus the sum of the utility and solar is ... just the solar. Give that a try,.

BTW, the rule limiting the size of the PV output to the size of the service is in the 2011 code. It was not in previous codes. It also passes the common sense test.
 

Carultch

Senior Member
Location
Massachusetts
BTW, the rule limiting the size of the PV output to the size of the service is in the 2011 code. It was not in previous codes. It also passes the common sense test.

Absolutely.

One thing I notice about the wording of the code, is that they are talking about the sum of the breakers/OCPDs for interconnection. I think this could use a rewording, in the case that rounding errors make a difference.

Certainly if the breakers aren't behind a master PV system breaker, it makes sense to add them up.

But if you have a lot of breakers in an AC combiner that has a main breaker, it doesn't make sense to add up all the branch breakers including their rounding errors. It makes more sense to only take the master breaker.
For instance, 13 x 15 kW inverters, each with a 25A breaker at 480Vac. The main needs to be 300A, even if the sum of branch breakers is 325A.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Absolutely.

One thing I notice about the wording of the code, is that they are talking about the sum of the breakers/OCPDs for interconnection. I think this could use a rewording, in the case that rounding errors make a difference.

Certainly if the breakers aren't behind a master PV system breaker, it makes sense to add them up.

But if you have a lot of breakers in an AC combiner that has a main breaker, it doesn't make sense to add up all the branch breakers including their rounding errors. It makes more sense to only take the master breaker.
For instance, 13 x 15 kW inverters, each with a 25A breaker at 480Vac. The main needs to be 300A, even if the sum of branch breakers is 325A.
This is fixed in the 2014 code. 125% of the inverter's maximum output current is carried all the way back to the service for the 120% rule calculations. The size of the OCPD is no longer used.
 

shortcircuit2

Senior Member
Location
South of Bawstin
The 2014 Code still uses the breaker value in 705.12(A)...

How about...

The interconnected inverter(s) maximum output current rating shall not exceed the current carrying capacity of the service entrance conductors they are connected to.

Need the 125% language in there somehow?
 

shortcircuit2

Senior Member
Location
South of Bawstin
Maybe this...

705.12(A) (2nd sentence)

The sum of 125 percent of interconnected inverter(s) maximum output current rating shall not exceed the current carrying capacity of the service entrance conductors they are connected to.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Here's how I might do it:

[New Section]
705.11 Service Rating

The rating of the service shall not be less than 125 percent of the total maximum output current ratings of all utility-interactive inverters connected to that service according to 705.12(A) and (D).

[Remove second sentence of 705.12(A).]
 

SolarPro

Senior Member
Location
Austin, TX
What am I missing?

We have to have OCP at the POC for a supply-side connection. 690.8 says we have to oversize the OCP for inverter output circuits at 125% of the source. It seems like we already have to do what your proposed proposals tell us to do.
 

shortcircuit2

Senior Member
Location
South of Bawstin
I see your point Solarpro. It is covered (125%) in 690.8

But we were working on the language in 705.12(A) and the reference to "sum of the ratings of all overcurrent devices" and were thinking to replace with..."sum of 125 percent of interconnected inverter(s) maximum output current rating"

Then I wanted to change "shall not exceed the rating of the service" to ..."shall not exceed the current carrying capacity of the service entrance conductors they are connected to"

Now my reason for the change to the rating of SERVICE ENTRANCE CONDUCTORS from the RATING OF THE SERVICE...is that many services have undersized SE conductors such as in residential and the 83% rule, so if I were to connect PV power at the rating of the service switch, the SE conductors may be overloaded in some cases.

Although I have never seen a service back-fed with PV at the rating of the service in any case yet. Has anyone?
 

Carultch

Senior Member
Location
Massachusetts
But we were working on the language in 705.12(A) and the reference to "sum of the ratings of all overcurrent devices" and were thinking to replace with..."sum of 125 percent of interconnected inverter(s) maximum output current rating"

I'd clarify it to "sum of the ratings of PV System master overcurrent device(s)".

A master overcurrent device is connected from its line side directly to the service conductors, without any intermediate overcurrent device. A PV system master overcurrent device shall be permitted to be supplied by any number or sum of inverter overcurrent devices on its load side, as long as its rating is at least 125 percent of its load side inverter current ratings.

Then I wanted to change "shall not exceed the rating of the service" to ..."shall not exceed the current carrying capacity of the service entrance conductors they are connected to"

I agree. To me, it doesn't make sense to make a 200A "line side tap" onto anything smaller than 3/0 Copper or 250 AL. Even if the service conductors are suitable for a 200A service.
 

SolarPro

Senior Member
Location
Austin, TX
Your suggested language would be a big improvement, clear and concise. I'm pretty sure that's what they want us to do. So why not say it directly. :slaphead:
 

shortcircuit2

Senior Member
Location
South of Bawstin
Could it be considered a Code violation by connecting a PV System through a Line Side Connection to service entrance conductors installed with the dwelling 83% rule, 310.15(B)(7)...?

I mean the Service Conductors would no longer be used to only supply the ENTIRE LOAD to that dwelling unit...the Service Conductors are now also being used to deliver the PV System current to the utility.

If so, replacement of the (83%) Service Conductors would be required.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
The 2014 Code still uses the breaker value in 705.12(A)...
Then I must have completely misinterpreted a dialog we had in this subforum a few months ago about this issue. That was before I got my 2014 NEC, though, and I hadn't thought about it since then.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Could it be considered a Code violation by connecting a PV System through a Line Side Connection to service entrance conductors installed with the dwelling 83% rule, 310.15(B)(7)...?

I mean the Service Conductors would no longer be used to only supply the ENTIRE LOAD to that dwelling unit...the Service Conductors are now also being used to deliver the PV System current to the utility.

If so, replacement of the (83%) Service Conductors would be required.
I raised the same question from a different angle, namely that the service wires are not supplying the entire load because the PV is supplying some of that load some of the time.
Nobody agreed with my argument.
As for your analysis, the grid tie inverter (GTI) is not a load, IMO.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Could it be considered a Code violation by connecting a PV System through a Line Side Connection to service entrance conductors installed with the dwelling 83% rule, 310.15(B)(7)...?

I mean the Service Conductors would no longer be used to only supply the ENTIRE LOAD to that dwelling unit...the Service Conductors are now also being used to deliver the PV System current to the utility.

If so, replacement of the (83%) Service Conductors would be required.
I raised the same question from a different angle, namely that the service wires are not supplying the entire load because the PV is supplying some of that load some of the time.
Nobody agreed with my argument.
As for your analysis, the grid tie inverter (GTI) is not a load, IMO.

I think you have to look at the justification for the 83% allowance on service conductors and feeders, which itself could be a source of argument.

If the justification is essentially a load factor, i.e. an assumption that the entire load won't be used at once, then it does not make sense to obviate the allowance because PV might be supplying some of the load some of the time. But it does make sense to obviate it if the rated PV output could exceed 83% of the conductor rating, because that would be a continuous full output for several hours on sunny days.

Put it another way: power can travel only one direction on the conductor at a time. If it's serving the load, 83% is allowed. If it's serving as an inverter output to the service, 83% is not allowed.

If the justification is something else, then maybe it doesn't matter.

Then I must have completely misinterpreted a dialog we had in this subforum a few months ago about this issue. That was before I got my 2014 NEC, though, and I hadn't thought about it since then.

All of the language regarding load side connections (705.12(D)) was changed from overcurrent device to 125% of the inverter output. But it wasn't changed for supply side connections (705.12(A)). Probably it was just not thought about because it was in a different section.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
All of the language regarding load side connections (705.12(D)) was changed from overcurrent device to 125% of the inverter output. But it wasn't changed for supply side connections (705.12(A)). Probably it was just not thought about because it was in a different section.

Ah. That 'splains it. Thanks.
 
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