120% Rule

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bwaryjasz

Member
Location
Pittsfield MA
Hello, first time writing in but I have been in the industry for 12+ years and I have a question revolving around NEC 705.12 of the 2014 code. If I am trying to interconnect a PV system into a sub-panel rated at 100A and my over current protection for my PV is sized at 50A I am reducing the breaker feeding that sub-panel from a 90A to a 70A to give me the allowable interconnection at that sub-panel. my question is do I still need to do these calculations back at the main panel that feed this sub panel? if so I will need to do a load side meter tap. System is 35 Enphase-S280 @ 1.13A each.
 

mwm1752

Senior Member
Location
Aspen, Colo
I had to take a double look for NEC 2014 -- 2011 was based on overcurrent device from both power scources. your instance doe seem to work the same way though -- NEC 2014 take the utility or sub panel OCPD in addition to the output current x 125%. Your output current happens to equal 49.43 Amps to techincally you could use a 49 Amp figure. 100 amp buss v 120% = 120 amps --- 120 amps - 49 amps = 71 amp breaker max for utility OCPD on buss. Let's say your output current was 32 amps x 125% = 40 amps. With the a conductor rated at 50 amps & an OCPD of 50 amps a 80 amp OCPD for the utility or sub panel could be used - the difference between 2011 & 2014 NEC code

I question - Can you use 240.6 to go to the next standard size OCPD of 80 amps?
 

bwaryjasz

Member
Location
Pittsfield MA
yes your calculations on my Max continuous output current of my Micro's is correct. which means I need to have OCPD on that at 50A. which puts me over the amperage on the 100A sub panel by 30A. The existing breaker feeding this sub-panel is 90A which allows me 10A more of interconnections but to allow my system to tie into this panel I need to free up 20A more of capacity which I will do by changing this breaker out to a 70A feeding the sub-panel. My question is NEC 705.12 does not talk or mention anything behind this panel only the bus that this breaker is on. So my question is if I'm sizing my PV output amperage at the 49.55 A which requires a 50A OCPD that would theoretically put me over the amperage of the main panel by 9 or so amps and I would need to do one of two things. Reduce the main breaker size of this panel which I do not want to do because of the loads in this panel, or change to a load side meter tap. Im just trying to see if I can reduce the amount of installation cost and ease the interconnect process or if I need to resort to the Tap.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Hello, first time writing in but I have been in the industry for 12+ years and I have a question revolving around NEC 705.12 of the 2014 code. If I am trying to interconnect a PV system into a sub-panel rated at 100A and my over current protection for my PV is sized at 50A I am reducing the breaker feeding that sub-panel from a 90A to a 70A to give me the allowable interconnection at that sub-panel. my question is do I still need to do these calculations back at the main panel that feed this sub panel? if so I will need to do a load side meter tap. System is 35 Enphase-S280 @ 1.13A each.
You must apply 125% of the inverter nameplate output current to the 120% rule in every panel that the backfed current passes through between the inverter(s) and the service.
 

mwm1752

Senior Member
Location
Aspen, Colo
The existing breaker feeding this sub-panel is 90A .

What is your service size? are you saying you have a 100 amp main panel with a 90 amp breaker to a 100 amp subpanel that has excessive load? does your main panel have other loads?
 
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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
What is your service size? are you saying you have a 100 amp main panel with a 90 amp breaker to a 100 amp subpanel that has excessive load? does your main panel have other loads?

The loads in the main panel are irrelevant to the issue unless he is trying to qualify the panel under 705.12(D)(2)(3)(c).

If he has a 100A panel with a 90A main, he has only (1.2)(100A) - 90A = 30A of headroom under 705.12(D)(2)(3)(b) - the 120% rule. His PV system would need a minimum of (35)(1.13A)(1.25) = 49.4A of panel headroom in every panel between the inverters and the service meter.

One quirk in the code is that to qualify under 705.12(D)(2)(3)(b) the contribution to the current on the bus is 125% of the combined inverter maximum output current, but under 705.12(D)(2)(3)(c) it's the rating of the breaker that injects the current onto the bus that is used in the calculation. Sometimes that makes a difference.
 
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bwaryjasz

Member
Location
Pittsfield MA
Thanks, I was thinking this was the case but wasn't sure because of the sub-panel being the location of the first interconnection. So touching on this as well if ever installing in a sub panel you need to make sure the breaker feeding that sub-panel is then moved to furthest spot from the main panel. correct? I am specifying a Tap interconnection for the system in question FYI.
 

mwm1752

Senior Member
Location
Aspen, Colo
The loads in the main panel are irrelevant to the issue unless he is trying to qualify the panel under 705.12(D)(2)(3)(c).

If he has a 100A panel with a 90A main, he has only (1.2)(100A) - 90A = 30A of headroom under 705.12(D)(2)(3)(b) - the 120% rule. His PV system would need a minimum of (35)(1.13A)(1.25) = 49.4A of panel headroom in every panel between the inverters and the service meter.

I'm just trying to get his explaination -- as now I know he has a 200A main with a total of 49.4 amp of pv power which is his direct question of does code make him reduce the 200 a main to 175 amps? & does he have to locate the now 70A OCPD to the opposite end of his 200 amp rated buss. yes
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
[So touching on this as well if ever installing in a sub panel you need to make sure the breaker feeding that sub-panel is then moved to furthest spot from the main panel. correct?[ /QUOTE]Farthest from the main breaker in the main panel, that is correct.

Another thing you could do if you had, for example, a 200A MDP with a 200A main breaker and you needed to interconnect more than 40A of PV is to change the MDP to a 225A panel and leave the 200A main breaker. That would give you (1.2)(225A) - 200A = 70A of headroom.
 
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bwaryjasz

Member
Location
Pittsfield MA
I'm just trying to get his explaination -- as now I know he has a 200A main with a total of 49.4 amp of pv power which is his direct question of does code make him reduce the 200 a main to 175 amps? & does he have to locate the now 70A OCPD to the opposite end of his 200 amp rated buss. yes

Yes my question was do I have to apply the 120% rule to the main panel as well and that has been answered. Thank you all. I would love to reduce the Main breaker size in the Main panel as well to ease the installation, but the amount of load on this panel is too much as there are other sub panels off of this main and many loads. not sure why the original electrician that did the service did not put in a 400A.
 

Smart $

Esteemed Member
Location
Ohio
...
One quirk in the code is that to qualify under 705.12(D)(2)(3)(b) the contribution to the current on the bus is 125% of the combined inverter maximum output current, but under 705.12(D)(2)(3)(c) it's the rating of the breaker that injects the current onto the bus that is used in the calculation. Sometimes that makes a difference.
I believe the case can be made that the lowest rated breaker in the supply chain can be used... which would be 50A in the OP's case, rather than 90A, or 70A if downsized as mentioned. Unfortunately I don't believe many will interpret Code to allow it as written. 2017 perhaps? If not, remember to submit for 2020. :D
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I believe the case can be made that the lowest rated breaker in the supply chain can be used... which would be 50A in the OP's case, rather than 90A, or 70A if downsized as mentioned. Unfortunately I don't believe many will interpret Code to allow it as written. 2017 perhaps? If not, remember to submit for 2020. :D

FWIW, my local AHJ has told us that they will not count the breaker connected to a surge protector in the calculation, so there is that.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
FWIW, my local AHJ has told us that they will not count the breaker connected to a surge protector in the calculation, so there is that.
Isn't that only relevant when trying to use the 100% rule?

Under normal circumstances the SDP is neither a load nor a source.
Or are you talking about the six handle rule for service disconnect with no main?
 

Carultch

Senior Member
Location
Massachusetts
Isn't that only relevant when trying to use the 100% rule?

Under normal circumstances the SDP is neither a load nor a source.
Or are you talking about the six handle rule for service disconnect with no main?

I agree.

Other examples of auxiliary circuits on a PV system dedicated panel would be meter voltage taps, as well as rapid shutdown/AFCI power. Any thoughts on whether these should be counted?
 

mwm1752

Senior Member
Location
Aspen, Colo
Yes my question was do I have to apply the 120% rule to the main panel as well and that has been answered. Thank you all. I would love to reduce the Main breaker size in the Main panel as well to ease the installation, but the amount of load on this panel is too much as there are other sub panels off of this main and many loads. not sure why the original electrician that did the service did not put in a 400A.

So what is the load calc for this service? & your statements do imply a line tap is require for this installation.
 
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