Does 125% Rule apply to back fed solar breaker - Inverter Output Circuit?

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Tradesmanx

Member
Location
Sammamish, WA
According to NEC 690.9 (B): Overcurrent Devices in pv system DC circuits...shall be rated with one of the following: "not less than 125% of the maximum currents calculated in 690.8(A)..."

However, this relates to DC circuits. The Inverter output is AC since the DC circuit has already been converted to AC electricity. I can't find anything to suggest the circuit breaker needs to be 125% of the inverter output rating. In this case, the Solar Edge 10000 single phase residential inverter has a continuous output rating of 42 amps. If the backfed breaker was required to be 125%, we would put in a 60amp breaker. We used 6 AWG THHN wire rated at 65 or more amps, so no problem there. But, why would you protect 42 amps with a 60 amp breaker? This makes no sense. Yet, there does seem to be an industry standard to size the breaker at 125%. But, I can't find that in the 2017 code. In fact, the code says no overcurrent protection is required at all...as I read it. For instance, a line side tap has no overcurrent protection. No breaker. I would like to use a 50 amp breaker and call it good.

What I read in the code says, "690.8(A)(3) The maximum current shall be the inverter continuous output current rating." Am I missing something? If so, what? Where does the code direct inverter output overcurrent circuit breakers to be 125% of the inverter continuous current rating?
 

Carultch

Senior Member
Location
Massachusetts
According to NEC 690.9 (B): Overcurrent Devices in pv system DC circuits...shall be rated with one of the following: "not less than 125% of the maximum currents calculated in 690.8(A)..."

However, this relates to DC circuits. The Inverter output is AC since the DC circuit has already been converted to AC electricity. I can't find anything to suggest the circuit breaker needs to be 125% of the inverter output rating. In this case, the Solar Edge 10000 single phase residential inverter has a continuous output rating of 42 amps. If the backfed breaker was required to be 125%, we would put in a 60amp breaker. We used 6 AWG THHN wire rated at 65 or more amps, so no problem there. But, why would you protect 42 amps with a 60 amp breaker? This makes no sense. Yet, there does seem to be an industry standard to size the breaker at 125%. But, I can't find that in the 2017 code. In fact, the code says no overcurrent protection is required at all...as I read it. For instance, a line side tap has no overcurrent protection. No breaker. I would like to use a 50 amp breaker and call it good.

What I read in the code says, "690.8(A)(3) The maximum current shall be the inverter continuous output current rating." Am I missing something? If so, what? Where does the code direct inverter output overcurrent circuit breakers to be 125% of the inverter continuous current rating?

It has to do with continuous loads in general, which when using a standard duty OCPD, you are required to size the OCPD at 125% of the continuous operational amperes. Solar is considered a continuous load. The intent is such that heat build up within the fuse or breaker, doesn't cause it to nuisance trip when the circuit is operating at the current it is supposed to carry. The OCPD is not there to protect the inverter itself (it has its own means of doing that), but rather the wire and terminations between the inverter and breaker.

If you use a continuous duty breaker in a situation where you can use it, then the 125% rule wouldn't apply.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
According to NEC 690.9 (B): Overcurrent Devices in pv system DC circuits...shall be rated with one of the following: "not less than 125% of the maximum currents calculated in 690.8(A)..."

However, this relates to DC circuits. The Inverter output is AC since the DC circuit has already been converted to AC electricity. I can't find anything to suggest the circuit breaker needs to be 125% of the inverter output rating. In this case, the Solar Edge 10000 single phase residential inverter has a continuous output rating of 42 amps. If the backfed breaker was required to be 125%, we would put in a 60amp breaker. We used 6 AWG THHN wire rated at 65 or more amps, so no problem there. But, why would you protect 42 amps with a 60 amp breaker? This makes no sense. Yet, there does seem to be an industry standard to size the breaker at 125%. But, I can't find that in the 2017 code. In fact, the code says no overcurrent protection is required at all...as I read it. For instance, a line side tap has no overcurrent protection. No breaker. I would like to use a 50 amp breaker and call it good.

What I read in the code says, "690.8(A)(3) The maximum current shall be the inverter continuous output current rating." Am I missing something? If so, what? Where does the code direct inverter output overcurrent circuit breakers to be 125% of the inverter continuous current rating?

Looking at 690.9(B), which refers to 690.8(A) (specifically 690.8(A)(3)), I don't see that. My 2014 code book doesn't say 690.9(B) only applies to DC. 690.9(A) does but (B) is not dependent on (A). I haven't seen the 2017 code book yet, so I can't comment on what it says.
 

Tradesmanx

Member
Location
Sammamish, WA
690.9(B)

690.9(B)

I think I'm getting this now. The first sentence of 690.9(B) refers to DC circuits, but the second sentence (2017) code book apparently doesn't. So, where would I get a continuous current breaker and what are the conditions where I can use one of those?
 

Smart $

Esteemed Member
Location
Ohio
I think I'm getting this now. The first sentence of 690.9(B) refers to DC circuits, but the second sentence (2017) code book apparently doesn't. So, where would I get a continuous current breaker and what are the conditions where I can use one of those?
They are called 100%-rated circuit breakers (Google it).

You have to use them in conjunction with a panelboard listed for 100% operation and breakers therein must be 100%-rated. All major manufacturers offer such rated equipment.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I think I'm getting this now. The first sentence of 690.9(B) refers to DC circuits, but the second sentence (2017) code book apparently doesn't. So, where would I get a continuous current breaker and what are the conditions where I can use one of those?
Why? I would just design the OCPD to 125% of inverter current and be done with it. In fact, I do that multiple times every day.

I would hazard a guess that the added expense of the 100% rated breaker would be greater than what you would save by going with the lower rating. Also, using a 100% rated breaker does not remove your obligation to consider 125% of the inverter Imax in calculating the bus loading in whatever panel you interconnect with if you are working under the 120% rule.
 

Tradesmanx

Member
Location
Sammamish, WA
120 percent rule (ceiling)

120 percent rule (ceiling)

Most of the PV systems I install are at the 10kw size. Thus, I have to use larger than a 40 amp breaker to backfeed into. Unfortunately, most panelboards will only accept a 40 amp breaker max because it's rare to find larger than a 200 amp panel busbar rating and the 120 percent rule imposes a restriction. This means I have to do a line side tap (which doesn't work with battery backup) or pull the main 200 amp breaker and replace with a 175 amp. But, usually the main breaker is very expensive (like $180.00).
 

Carultch

Senior Member
Location
Massachusetts
Most of the PV systems I install are at the 10kw size. Thus, I have to use larger than a 40 amp breaker to backfeed into. Unfortunately, most panelboards will only accept a 40 amp breaker max because it's rare to find larger than a 200 amp panel busbar rating and the 120 percent rule imposes a restriction. This means I have to do a line side tap (which doesn't work with battery backup) or pull the main 200 amp breaker and replace with a 175 amp. But, usually the main breaker is very expensive (like $180.00).


In NEC2014 and later, it isn't the actual rating of the interconnection OCPD that matters in the 120% rule, but rather the 125% of the total nameplate AC current. In otherwords, it is the value that you use for calculating a standard interconnection OCPD, before you round it up to the OCPD you are actually using. The intent of this rule was to A) avoid rounding errors being a show stopper, and B) to allow interconnection at subpanels, without the subpanel's feeder breaker in the main panel mattering. One reason you might do B, is if you are designing an off-grid system with a critical loads panel.

To meet that rule in NEC2014+, you'd have to use 125% of the inverter current rating in the 120% rule calculation, regardless of whether you use a standard breaker or a continuous duty breaker.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
In NEC2014 and later, it isn't the actual rating of the interconnection OCPD that matters in the 120% rule, but rather the 125% of the total nameplate AC current. In otherwords, it is the value that you use for calculating a standard interconnection OCPD, before you round it up to the OCPD you are actually using. The intent of this rule was to A) avoid rounding errors being a show stopper, and B) to allow interconnection at subpanels, without the subpanel's feeder breaker in the main panel mattering. One reason you might do B, is if you are designing an off-grid system with a critical loads panel.

To meet that rule in NEC2014+, you'd have to use 125% of the inverter current rating in the 120% rule calculation, regardless of whether you use a standard breaker or a continuous duty breaker.

A minor point, possibly, but if you are qualifying a bus under 705.12(D)(2)(3)(c), the pertinent numbers are the OCPD ratings themselves, not 125% of max inverter current. In hindsight, there was a system we built where we had to go from a 400A panel to a 600A AC combiner panel because the sum of the breaker ratings exceeded 400A by a small amount. In that case, if we had used 100% rated breakers we might have been able to get into the 400A panel. I answered my own question, it appears.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Most of the PV systems I install are at the 10kw size. Thus, I have to use larger than a 40 amp breaker to backfeed into. Unfortunately, most panelboards will only accept a 40 amp breaker max because it's rare to find larger than a 200 amp panel busbar rating and the 120 percent rule imposes a restriction. This means I have to do a line side tap (which doesn't work with battery backup) or pull the main 200 amp breaker and replace with a 175 amp. But, usually the main breaker is very expensive (like $180.00).


In most cases, a 200 amp service does not have a load anywhere near 200 amps. One trick is to replace the 200 amp main breaker with a 175 amp breaker and then you can have up to 65 amps from the inverter.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
In most cases, a 200 amp service does not have a load anywhere near 200 amps. One trick is to replace the 200 amp main breaker with a 175 amp breaker and then you can have up to 65 amps from the inverter.

Another is to keep the 200A breaker and replace the 200A bus with a 225A bus. That boosts the headroom from 40A to 70A.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Yes but if you have an existing service panel that costs 20-30 times as much. :happyno:
I think you exaggerate, but at any rate some jurisdictions require that you justify with a load analysis the downsizing of a MDP main breaker. If the load breakers total up to less than the rating of the reduced main, I guess it's a no-brainer, but if not, it might be a pain.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I think you exaggerate, but at any rate some jurisdictions require that you justify with a load analysis the downsizing of a MDP main breaker. If the load breakers total up to less than the rating of the reduced main, I guess it's a no-brainer, but if not, it might be a pain.

I don't think I exaggerate. $85-150 for a breaker vs $3000 minimum for a 200A service panel replacement around here. And 1 man hour in the field vs 16 or more. And yes, some AHJs make me show them a load calc but that's just another half hour in the office. I have yet to do a load calc for a house with a 200A service that actually required more than 175A.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I don't think I exaggerate. $85-150 for a breaker vs $3000 minimum for a 200A service panel replacement around here. And 1 man hour in the field vs 16 or more. And yes, some AHJs make me show them a load calc but that's just another half hour in the office. I have yet to do a load calc for a house with a 200A service that actually required more than 175A.

I don't deal with procurement, but we replace panels fairly often and I don't think they cost anywhere near that much.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I don't deal with procurement, but we replace panels fairly often and I don't think they cost anywhere near that much.

Maybe you don't work in northern California. :p And the cost isn't mostly material on service panel upgrade, unlike a main breaker downsize.

I grant that we don't do our own service panel replacements, so it costs us more. But even if we did our own work it would still be easily 10 times the cost of a main breaker downsize. Not to mention the customer has to live without power for 4-8 hours instead of 10-20 minutes.
 
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