120% Conductor Rule?

[shortcircuit2]

Here is a POCC connection scenario I would like to bounce off others here...

A dwelling has a 200amp service with a main breaker panel that has a feed thru connection to a downstream panel that is also rated at 200 amps. There are 4/0 alluminum SER conductors going to the downstream panel.

Now we connect the solar PV source to a 40amp breaker at the main panel per code. (200amp x 120% = 240)

Is there concern that the feeder conductors going to the downstream panel are now undersized?

 

[Dennis Alwon]

Since both of the 200 amp panels are under the 200 amp breaker I don't see that as a violation. Now you have a 200 amp and a 40 amp breaker which should be compliant.

Now depending on a few things the 4/0 may not be compliant

 

[don_resqcapt19]

How would connecting an additional power source to the first panel increase the load on the second panel? If the 4/0 aluminum feeder to the second panel was compliant prior to the solar installation, it will still be compliant after the solar installation.

 

[shortcircuit2]

How would connecting an additional power source to the first panel increase the load on the second panel? If the 4/0 aluminum feeder to the second panel was compliant prior to the solar installation, it will still be compliant after the solar installation.

I don't see that it will increase the load either. But will it increase the potential that the feeder conductors could to draw power up to 240amps?

 

[Smart $]

I don't see that it will increase the load either. But will it increase the potential that the feeder conductors could to draw power up to 240amps?

The only way I see it complying is if the downstream feeder and load-end OCP meet the requirements of a feeder tap [240.21(B)]. With what you've mentioned so far, and a couple assumptions, it fails to meet the criteria.

 

[Smart $]

The only way I see it complying is if the downstream feeder and load-end OCP meet the requirements of a feeder tap [240.21(B)]. With what you've mentioned so far, and a couple assumptions, it fails to meet the criteria.

PS: Providing assumptions are correct, and the main panel has the available spaces, relocating the feeder to a 2P-175A breaker would be the simplest solution.

 

[shortcircuit2]

PS: Providing assumptions are correct, and the main panel has the available spaces, relocating the feeder to a 2P-175A breaker would be the simplest solution.

Is there a code rule that would require this change to the 175amp overcurrent device for the feeder conductors?

 

[don_resqcapt19]

After additional thought, the 4/0 Al between the two panels is covered by 705.12(D)(2). Now the question becomes about 310.15(B)(7). First does is still apply in this application and second does this section actually change the ampacity of the conductor or does it just permit its use at a higher ampacity?

 

[shortcircuit2]

After additional thought, the 4/0 Al between the two panels is covered by 705.12(D)(2). Now the question becomes about 310.15(B)(7). First does is still apply in this application and second does this section actually change the ampacity of the conductor or does it just permit its use at a higher ampacity?

Thankyou Don...

In your 1st sentence...*After additional thought, the 4/0 Al between the two panels is covered by 705.12(D)(2).*

This is the answer I was looking for as I'm in the middle of a big discussion with others that the CONDUCTOR part of 705.12(D) (2) is a valid concern as applied to field installations of solar PV at its POCC. Other discussions have put emphasis that it should only apply to feeder tap conductor situations.

As I have pointed out in post #1 with the existing installation that I described...where a feeder is taken from the main service equipment to a panel down stream which depends on the service main for protection (which is allowed by 408.36), 705.12(D) (2) provides guidance to the installer that the existing feeder now must be addressed also.

The proposed change to 705.12(D)(2) in the current ROP, # 4-375a by Code-Making Panel 4, which was accepted appears to change the code language of 705.12(D)(2) and will remove the guidance for installers to address the protection of this CONDUCTOR.

shortcircuit

 

[Dennis Alwon]

So I am confused. Is the install compliant or not-- regardless of 310.15(B)(7) being an issue

 

[Smart $]

After additional thought, the 4/0 Al between the two panels is covered by 705.12(D)(2). Now the question becomes about 310.15(B)(7). First does is still apply in this application and second does this section actually change the ampacity of the conductor or does it just permit its use at a higher ampacity?

Is there a code rule that would require this change to the 175amp overcurrent device for the feeder conductors?

Need more details whether the feeder meets the tap rules.

Before debating issue raised by Don, let's first determine whether the subpanel feeder qualifies as the dwelling's main power feeder under 310.15(B)(7). Aside from this feeder, what is served by the main panel? IMO, if the main panel serves any load associated with the dwelling (not through the subpanel feeder), the feeder does not qualify as the main power feeder. 310.15(B)(7) says "all loads that are part [of] or associated with the dwelling unit." (text in red is a grammar correction, if anyone cares to "write it up")

 

[shortcircuit2]

Need more details whether the feeder meets the tap rules.

Before debating issue raised by Don, let's first determine whether the subpanel feeder qualifies as the dwelling's main power feeder under 310.15(B)(7). Aside from this feeder, what is served by the main panel? IMO, if the main panel serves any load associated with the dwelling (not through the subpanel feeder), the feeder does not qualify as the main power feeder. 310.15(B)(7) says "all loads that are part [of] or associated with the dwelling unit." (text in red is a grammar correction, if anyone cares to "write it up")

Thanks Smart$...

I know what the current code text says in 310.15(B) (7) and the installation I described would not be compliant in some cases since 2008...

BUT...under the 2005 NEC it was allowed under the text related to table 310.15(B) (6)...

So there are many installations existing in the real world that I have described that were code compliant at the time. Also there are parts of the country that are still under the 2005 NEC or earlier editions.

OK...lets look further...If the installation were according to 2008 NEC and later, a 4/0 aluminum conductor might still be used if the computed load at the time of installation were 180amps or less... you could use next size up for overcurrent protection.

If a 250 MCM aluminum conductor were used for the feeder...that still only comes in at 205amps...which is below the 240amps that would be required according to the 120% rule of 705(D)(2)

Do you follow me?

 

[Smart $]

Thanks Smart$...

I know what the current code text says in 310.15(B) (7) and the installation I described would not be compliant in some cases since 2008...

BUT...under the 2005 NEC it was allowed under the text related to table 310.15(B) (6)...

So there are many installations existing in the real world that I have described that were code compliant at the time. Also there are parts of the country that are still under the 2005 NEC or earlier editions.

OK...lets look further...If the installation were according to 2008 NEC and later, a 4/0 aluminum conductor might still be used if the computed load at the time of installation were 180amps or less... you could use next size up for overcurrent protection.

If a 250 MCM aluminum conductor were used for the feeder...that still only comes in at 205amps...which is below the 240amps that would be required according to the 120% rule of 705(D)(2)

Do you follow me?

Yes, I follow you.

First, clarify what NEC edition your installation is under.

IMO, the subpanel feeder compliance is bound to the edition you are currently under. It doesn't matter that it was compliant when installed... you are modifying the circuits supplying that conductor (in addition to the bus of the main panel).

250 kcmil aluminum would be in compliance. 205A at 100%, 246 at 120%.

 

[shortcircuit2]

My mistake...your right on the 250MCM...I'm under the 2011 MEC

But there are many installations in the field with the 4/0 aluminum and the current 705.12(D) (2) wording tells the electrician to address the feeder to the sub-panel.

You said...

IMO, the subpanel feeder compliance is bound to the edition you are currently under. It doesn't matter that it was compliant when installed... you are modifying the circuits supplying that conductor (in addition to the bus of the main panel).

Yes...so adding the PV to the main panel would require addressing the feeder to the sub-panel under the 2011 section 705.12(D)(2)

 

[Smart $]

...so adding the PV to the main panel would require addressing the feeder to the sub-panel under the 2011 section 705.12(D)(2)

IMO, yes.

If you supply the feeder through a breaker other than the main, you remove it from consideration.

 

[jaggedben]

[nevermind]

 

[jaggedben]

I think Don hit the nail on the head. If the 4/0 aluminum is rated for 200A, then it will still be compliant if only 40A of solar is added to the main panel. However it is not clear if 310.15(B)(7) establishes that the 4/0 aluminum is rated for 200A for the purposes of 705 or not.

On a side note, see 705.12(D7).

 

[Smart $]

I think Don hit the nail on the head. If the 4/0 aluminum is rated for 200A, then it will still be compliant if only 40A of solar is added to the main panel. However it is not clear if 310.15(B)(7) establishes that the 4/0 aluminum is rated for 200A for the purposes of 705 or not.

...

The subpanel feeder first has to qualify as the main power feeder for the dwelling in order for 310.15(B)(7) to apply. OP'er has replied to the post in which I brought this up and has mentioned it does qualify.

 

[don_resqcapt19]

I don't think that the feeder to the second panel qualifies under 310.15(B)(7) and even if it did, I don't think that section changes the ampacity of the conductor. The ampacity of the 4/0 AL is 180 so the maximum solar input breaker would be 15 amps for that 200 amp service.

 

[Smart $]

...I don't think that section changes the ampacity of the conductor. ...

I agree.