705.11 [2023 NEC] & MLO service panel

[tortuga]

An existing Main lug Only service panelboard (MLO) has 4 main breakers installed (service disconnects), and space for two more in the middle.
The existing breakers, installed under 230.90(A) Ex. 3, are 125A, 60A, 60A and a 40A at the bottom.
Presumably the calculated load is 200A.
Looking at the 2023 NEC 705.11, what is the maximum amount of PV that can be connected to this panel?
And does it matter where the breakers are located in the panel?

 

[wwhitney]

An existing Main lug Only service panelboard (MLO) has 4 main breakers installed (service disconnects), and space for two more in the middle.
The existing breakers, installed under 230.90(A) Ex. 3, are 125A, 60A, 60A and a 40A at the bottom.
Presumably the calculated load is 200A.
Looking at the 2023 NEC 705.11, what is the maximum amount of PV that can be connected to this panel?

160A of continuous inverter current, on up to two breakers.

I take it you read 2023 230.71(B) Exception as allowing additional breakers to be added to this existing service equipment?

And does it matter where the breakers are located in the panel?

No.

Cheers, Wayne

 

[ggunn]

An existing Main lug Only service panelboard (MLO) has 4 main breakers installed (service disconnects), and space for two more in the middle.
The existing breakers, installed under 230.90(A) Ex. 3, are 125A, 60A, 60A and a 40A at the bottom.
Presumably the calculated load is 200A.
Looking at the 2023 NEC 705.11, what is the maximum amount of PV that can be connected to this panel?
And does it matter where the breakers are located in the panel?View attachment 2569790

Under the latest NEC, panels like this are no longer allowed. If the AHJ has adopted the latest code you might run into "you touch it, you fix it". That could get expensive.

 

[tortuga]

Yeah 160A thats what I was figuring also (38.4kva), thanks for the feedback.

I take it you read 2023 230.71(B) Exception as allowing additional breakers to be added to this existing service equipment?

Yes I did, and it kinda creates a disincentive to replace these old panels.

 

[jaggedben]

Yes I did, and it kinda creates a disincentive to replace these old panels.

Forcing people to do hot work to replace the old panels just because the old panels have some slightly more exposed parts than the newer ones really defeats the purported purpose of the new rule, which is to protect people from working up against exposed live parts. (Whether electrician or utility lineman, someone is likely to have to do live work to do the disconnect/reconnect.) I for one very much support the exception.

 

[don_resqcapt19]

A breaker in a MLO service panel is a line side tap and the amount of connected solar can equal the rating of the service. Based on the information provided, the service rating is 200 amps.

 

[ggunn]

A breaker in a MLO service panel is a line side tap and the amount of connected solar can equal the rating of the service. Based on the information provided, the service rating is 200 amps.

And as wwhitney said, that's 160A of inverter continuous output current.

 

[PWDickerson]

It is very likely that the bus stabs are rated less then the bus itself, and then this will determine the max backfeed current of the system. The max current per stab should be on the panel sticker somewhere. If you utilize both of the available 2-pole spaces, you should be able to go up to 160A continuous.

 

[retirede]

It is very likely that the bus stabs are rated less then the bus itself, and then this will determine the max backfeed current of the system. The max current per stab should be on the panel sticker somewhere. If you utilize both of the available 2-pole spaces, you should be able to go up to 160A continuous.

Wouldn’t the max current per stab have to at least be as large as the maximum size breaker listed for the panel?

 

[Carultch]

An existing Main lug Only service panelboard (MLO) has 4 main breakers installed (service disconnects), and space for two more in the middle.
The existing breakers, installed under 230.90(A) Ex. 3, are 125A, 60A, 60A and a 40A at the bottom.
Presumably the calculated load is 200A.
Looking at the 2023 NEC 705.11, what is the maximum amount of PV that can be connected to this panel?
And does it matter where the breakers are located in the panel?

Hot-bus MLO panelboard interconnections have been uncharted territory since inception. I haven't seen a 705 section that addresses specifics of this situation, that protects the busbar from overload.

A hot bus MLO panelboard no longer meets the current standard, and the NEC didn't provide guidance on what it means for adding circuits to existing MLO panelboards. Service disconnects are now required to be in separate enclosures, or separate switchboard sections. The closest rule I can think of that would justify anything close to this, is the sum of breakers rule, but it's very likely that any existing service panel already has more breakers than the busbar rating.

Ideally, if you were on an NEC edition where you could make use of this panelboard (by omission), you'd put your breaker at the opposite end of the main lugs, so that current is subtractive instead of additive.

 

[ggunn]

Ideally, if you were on an NEC edition where you could make use of this panelboard (by omission), you'd put your breaker at the opposite end of the main lugs, so that current is subtractive instead of additive.

Why would this make any difference? The current available to the bus in an MLO service panel is already much higher than the rating of the bus.

 

[tortuga]

Ideally, if you were on an NEC edition where you could make use of this panelboard (by omission),

Along with 2023 230.71(B) Exception, I would think all jurisdictions that adopt the NEC have some language that exempts existing installations from having to be brought up to current code, like your Rule 3 in Mass:

Additions or modifications to an existing installation shall be made in accordance with this Code
without bringing the remaining part of the installation into compliance with the requirements of
this Code. The installation shall not create a violation of this Code, nor shall it increase the
magnitude of an existing violation.

That allows one to add a circuit to a MLO panel that complied with a prior edition of the code, as long as the new breaker meets the current code.
And meeting current code could have a number of gotchys;

• The installer has do a load calc to justify the 230.90(A) Ex No.3,
• The new main breaker(s) cannot exceed the stab rating (as PWdickerson mentioned)
• and the new breaker has to have the appropriate fault current rating for the service (possibly more than 10kaic).
  
  I dont find an enforceable NEC section that dictates the position of the breakers(s) on the bus for this case.

 

[ggunn]

I dont find an enforceable NEC section that dictates the position of the breakers(s) on the bus for this case.

I agree. The provisions of 705.12(B) for bus qualification do not apply for line side connections in a MLO MDP, although if the connection to the bus comes from an inverter combiner panel, 705.12(B) does apply to the bus in the combiner panel.

 

[jaggedben]

Hot-bus MLO panelboard interconnections have been uncharted territory since inception. I haven't seen a 705 section that addresses specifics of this situation, that protects the busbar from overload.

It's just a supply side connection. Since at least the 2014 NEC, iirc, 705.11 (nee 705.12(A)) has restricted the output of interconnected sources to not overload the service conductors back to the utility.
But otherwise, the busbar is still protected by the load breakers. Or by Article 220 if using 230.90 Exception 3. As it has always been. There is no need for article 705 to further address specifics.

A hot bus MLO panelboard no longer meets the current standard,

For reasons that have nothing at all whatsoever to do with overload or amp ratings. Guarding personell from live parts was the only issue there and 705 didn't need to weigh in on that whatsoever.

and the NEC didn't provide guidance on what it means for adding circuits to existing MLO panelboards.

In my opinion it actually did, by referencing article 230 in 705 as always should have been done. In doing so it incorporated any article 230 rule changes to disconnects to apply to solar disconnects. There was no need for any additional guidance with respect to overload or ampacities.

...

 

[Carultch]

It's just a supply side connection. Since at least the 2014 NEC, iirc, 705.11 (nee 705.12(A)) has restricted the output of interconnected sources to not overload the service conductors back to the utility.
But otherwise, the busbar is still protected by the load breakers. Or by Article 220 if using 230.90 Exception 3. As it has always been. There is no need for article 705 to further address specifics.

I don't agree that it is protected by the load breakers, unless your existing situation is that the original load breakers add up to less than the busbar rating. A hot-bus panel that is filled with more breaker amps than its rating, is only "protected" by the fact that it's statistically unlikely to overload. A user could deliberately turn everything on, overload the busbar, and no OCPD would prevent this from happening.

Why would this make any difference? The current available to the bus in an MLO service panel is already much higher than the rating of the bus.

It makes a difference, because placing it at the opposite end avoids adding current to the problem. It isn't reflected in the NEC, but it is still best practice to distribute sources to avoid additive currents, and take advantage of current cancelling where you can.

 

[jaggedben]

I don't agree that it is protected by the load breakers, unless your existing situation is that the original load breakers add up to less than the busbar rating. A hot-bus panel that is filled with more breaker amps than its rating, is only "protected" by the fact that it's statistically unlikely to overload. A user could deliberately turn everything on, overload the busbar, and no OCPD would prevent this from happening.
...

But all that is true regardless of whether an additional power source is added to a panel. The source isn't going to change what the loads consume.

 

[ggunn]

It makes a difference, because placing it at the opposite end avoids adding current to the problem. It isn't reflected in the NEC, but it is still best practice to distribute sources to avoid additive currents, and take advantage of current cancelling where you can.

But if the PV breaker is in the middle of the bus the sources below the PV supply draw just as much current as they would have if the PV breaker was not there, and the bus in the panel already has much more current available to it than the bus rating. Granted, the current density is reduced above the PV breaker, but I don't see how the current density below the PV breaker is increased by the addition of the PV.

 

[ggunn]

But if the PV breaker is in the middle of the bus the sources below the PV supply draw just as much current as they would have if the PV breaker was not there, and the bus in the panel already has much more current available to it than the bus rating. Granted, the current density is reduced above the PV breaker, but I don't see how the current density below the PV breaker is increased by the addition of the PV.

Of course, I meant loads, not sources. Typing before coffee....