MLO Panel questions

[jumper]

A new member has a few questions regarding an old post. I created this thread for him.

Regarding this thread:

http://forums.mikeholt.com/showthread.php?t=150617&p=1457020#post1457020

He asks the following:

I have a few questions about this thread:

1. Isn't the sub panel ALREADY protected to 80A by the 80 breaker in the main panel?

2. Is the logic of installing a 80A Main breaker in the 125a sub panel that it is permanent and cannot be as easily removed for a larger breaker as the 80a breaker in the main?

3. In the event there are no loads on the sub, and the PV is fully cranking close to 60A, does that not mean that the 60a will be backfed through the 80a in the main 200a panel which, in and of itself, would have a backfeed limit of 40a according to the 120% rule?

Any light on this would be most helpful. thx

 

[Smart $]

1. Yes.

2. I have no idea what the logic is. It does not change anything. However, there are those that believe the 120% "rule" only applies to the <sub>panel and not the upstream equipment.

3. Yes.</sub>

 

[Smart $]

3. Addendum: The panel would be limited to +40 if the bus rating is 200A. The bus could be rated higher than the panel rating, which would allow the extra 20A. For example, the bus could be rated 225A, allowing for 70A of solar backfeed (200A + 70A = 225A × 125%).

Don't forget that the feeder is also subject to the 125% rule.

 

[Carultch]

3. Addendum: The panel would be limited to +40 if the bus rating is 200A. The bus could be rated higher than the panel rating, which would allow the extra 20A. For example, the bus could be rated 225A, allowing for 70A of solar backfeed (200A + 70A = 225A × 125%).

Correction: it is a 120% rule, not a 125% rule.
200A + 70A = 225A × 120%

Don't forget that the feeder is also subject to the 125% rule.

Please discuss an example. It is my understanding that a feeder is only subject to requiring enough ampacity for either the source or load, and to be protected by all possible OCPD's.

 

[Smart $]

Correction: it is a 120% rule, not a 125% rule.
200A + 70A = 225A × 120%

Yep... brain fart. :ashamed1:

Please discuss an example. It is my understanding that a feeder is only subject to requiring enough ampacity for either the source or load, and to be protected by all possible OCPD's.

No need to discuss. It is in the black and white of 705.12(D)(2) of the 2008 NEC, which IIRC is the edition the original thread and install was under.

(2) Bus or Conductor Rating. 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.

 

[Carultch]

Yep... brain fart. :ashamed1:

No need to discuss. It is in the black and white of 705.12(D)(2) of the 2008 NEC, which IIRC is the edition the original thread and install was under.

Oh, I see. NEC2008 hasn't applied to my location since before I started working. So unfortunately, I probably overlook some of the rules in it that no longer apply.

NEC2014 no longer contains the word "conductor" in that rule.

705.12(D)(2)(3)(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% of the inverter(s) output current and the rating of the overcurrent device protecting the busbar shall not exceed 120% of the ampacity of the busbar. The busbar shall be sized for the loads connected per Article 220. A permanent warning label shall be applied to the distribution equipment adjacent to the backfed breaker from the inverter, displaying the following or equivalent warning."