AC combiner box circuit breaker sizing question

[solar-dan]

Hi, I asked this question on the solar-electric forum, and wanted to get the opinion of folks here too:

Given an Enphase-based grid-tie system with multiple circuits, producing no more than a total of 32A, combined in a combiner box, does the total size of the circuit breakers in that box need to follow the 40A rule for a 200A service? For example, could I have 3 x 20A breakers in the box for 3 circuits, and rely on the 40A breaker at the panel to protect the panel? Or do you think the breakers in the box must also add up to 40A or less? thanks

[peakbagger]

I did notice in Home Power this month that the next code cycle appears to allow the actual maximum rated inverter output to be used instead of the breaker size. I got tagged with that as I have three grid tied inverters with a maximum output rating of around 35 Amps but the breakers added up to over 50 amps (on a 200 Amp panel). My solution was to run the three into a subpanel and then run it to a 40 Amp breaker on the main panel. My understanding of the next code cycle is that I wont need the subpanel.

I will let the code folks confirm or deny as I dont claim to be an electrician (Steams my thing)

[solar-dan]

I did notice in Home Power this month...

Is that the April/May 2014 issue? thanks

...just downloaded that... code change is described on p. 86 that allows using 125% of inverters output rather than OCPD (breaker) size... problem solved!

[Mike90250]

I asked this question on the solar-electric forum, and wanted to get the opinion of folks here too:

If Mom won't give you a cookie, ask Grandma !

It's best you ask the inspector how he wants to see it done, sometimes, they have no basis in reality, but are the only ones who can sign it off.

[peakbagger]

Beat me to it !.

I also like the clarification on protecting the DC wiring, No need for a fence around a pole mount.

[mapmaker]

problem solved!

Not necessarily. It's not solved until the inspector agrees. --mapmaker

[solar-dan]

Not necessarily. It's not solved until the inspector agrees. --mapmaker

The current installation does not have a problem. I was trying to anticipate adding a 3rd circuit in 2 or more years, so hopefully the rest of world will catch up to this change by then (i.e., no sense complicating current installation based on outdated [and electrically nonsensical] rules regarding circuit breakers).

[inetdog]

I did notice in Home Power this month that the next code cycle appears to allow the actual maximum rated inverter output to be used instead of the breaker size. I got tagged with that as I have three grid tied inverters with a maximum output rating of around 35 Amps but the breakers added up to over 50 amps (on a 200 Amp panel). My solution was to run the three into a subpanel and then run it to a 40 Amp breaker on the main panel. My understanding of the next code cycle is that I wont need the subpanel.

I will let the code folks confirm or deny as I dont claim to be an electrician (Steams my thing)

Well, the first question will be when your state adopts the 2014 version of the National Electrical Code (NEC). Some inspectors may be willing to make a judgement call on an installation looking ahead to the upcoming changes to their locally adopted code, but that would be rare.

The 2014 NEC has quite extensive changes in the area of PV, and not all of them make life simpler.

[solar-dan]

Here's the relevant quote from Home Power (Apr/May 2014, p. 86) for those w/o access:

"Section 705.12(D)
The most significant changes in Article 705 are in 705.12(D),
which further clarifies the ways a PV system can be connected
on the load side of the AC service disconnecting means,
particularly 705.12(D)(2), “Bus or Conductor Ampere Rating.”
Under earlier editions of the Code, busbar calculations were
performed using the rating of the overcurrent protection
devices (OCPDs) that were supplying power to the busbar
(the main, or supply breaker, plus any back-fed breakers
connected to grid-tied inverters).
One significant change is that 125% of the inverter’s rated
AC output current, rather than the actual back-fed breaker
size, is used for busbar calculations. The advantage is that
the installed OCPD size is usually larger than the calculated
minimum OCPD size (since, if the calculation doesn’t result
in a standard size, it has to be rounded up), but 125% of the
inverters’ rated output current (typically a smaller value than
the installed OCPD size) can be used in busbar calculations,
potentially allowing larger PV systems to be interconnected
on the load side of a service."

[carultch]

Here's the relevant quote from Home Power (Apr/May 2014, p. 86) for those w/o access:

"Section 705.12(D)
The most significant changes in Article 705 are in 705.12(D),
which further clarifies the ways a PV system can be connected
on the load side of the AC service disconnecting means,
particularly 705.12(D)(2), “Bus or Conductor Ampere Rating.”
Under earlier editions of the Code, busbar calculations were
performed using the rating of the overcurrent protection
devices (OCPDs) that were supplying power to the busbar
(the main, or supply breaker, plus any back-fed breakers
connected to grid-tied inverters).
One significant change is that 125% of the inverter’s rated
AC output current, rather than the actual back-fed breaker
size, is used for busbar calculations. The advantage is that
the installed OCPD size is usually larger than the calculated
minimum OCPD size (since, if the calculation doesn’t result
in a standard size, it has to be rounded up), but 125% of the
inverters’ rated output current (typically a smaller value than
the installed OCPD size) can be used in busbar calculations,
potentially allowing larger PV systems to be interconnected
on the load side of a service."

The way I interpret this, is "don't let rounding errors be a show stopper".

For over 90% of all your situations, it will not matter. Because it still is 125% of your inverter output current rating that gets used in the calculation. The only difference, is now you don't have to round up to the breaker size, when adding it with the main supply breaker, to verify that it is < 120% of the busbar rating.

I have yet to be put in a situation the numbers are so borderline, that the new rule is needed.

A few common examples:
200A bus w/ 200A MCB: maximum value of 1.25*current = 40A. This is a standard size, so the rule change doesn't matter.
400A bus w/ 300A MCB: maximum value of 1.25*current = 180A. The new rule helps, because the maximum breaker would otherwise be 175A.
400A bus w/ 400A MCB: maximum value of 1.25*current = 80A. This is a standard size, so the rule change doesn't matter.
600A bus w/ 500A MCB: maximum value of 1.25*current = 200A. This is a standard size, so the rule change doesn't matter.
600A bus w/ 600A MCB: maximum value of 1.25*current = 120A. The new rule helps, because the maximum breaker would otherwise be 110A.

[carultch]

If you combined your breakers at a dedicated AC combining panel upstream of your interconnection point, and you interconnect at a 40A breaker, it will not matter. This would comply, no matter which of the past three versions of the NEC applies. As long as the total circuit is no more than 32A, to be protected by a 40A breaker on the load side of the 200A panel w/ 200A MCB.

By "dedicated AC Combining panel", I mean a panelboaord that is dedicated to only breakers associated with the PV system. No loads. In fact, you should mark it with a label to indicate not to add loads. Once you add loads, then you are subject to the NEC705 rules that may apply.

If you eliminate the AC combiner, and connect at 3/ 20A breakers at your point of interconnection, then you are might want to check with your AHJ to see if you can follow the new NEC2014 rule for 705.12(D), even if your state isn't on the NEC2014. Your inspector may make you have AFCI breakers, as required by NEC2014 for microinverter systems, if you are trying to take advantage of NEC2014.

It is common for AHJ's to allow existing buildings (on which you do new work) to remain compliant with the codes they were once compliant with, and new work to follow the current standard. Most likely, they would also allow new buildings to become compliant with future codes not yet adopted by the AHJ, as long as all new work is completely compliant with the new code.

In otherwords, don't expect to cherry-pick new codes that don't apply yet, picking only the new rules that help you, and ignoring the rules that you don't want to follow. If you are going to follow NEC2014, you should plan on following all of it.

Hi, I asked this question on the solar-electric forum, and wanted to get the opinion of folks here too:

Given an Enphase-based grid-tie system with multiple circuits, producing no more than a total of 32A, combined in a combiner box, does the total size of the circuit breakers in that box need to follow the 40A rule for a 200A service? For example, could I have 3 x 20A breakers in the box for 3 circuits, and rely on the 40A breaker at the panel to protect the panel? Or do you think the breakers in the box must also add up to 40A or less? thanks