70amp solar on a 200 amp main

[Joethemechanic]

Well, the black part, technically, could be the stabs on a plug-on bus.
But by and large your point is well taken, and is why Golddigger mentioned 200% in the first place.

So as long as any given breaker didn't exceed 200 amps we should be fine

 

[jaggedben]

So as long as any given breaker didn't exceed 200 amps we should be fine

In a side-by-side panel I could stick (2) 125A breakers on the same stabs, then I could theoretically have up to 250A on those stabs.

 

[tortuga]

Many panels have a per stab limit for breaker sizing. Its on the panel sticker.

 

[wwhitney]

You mean the informational note?

No, 2023 NEC 705.12(B) starts "For power source connections to distribution equipment with no specific listing and instructions for combining multiple sources," So once the 2023 NEC is adopted, equipment with the proper listing is exempt from 705.12(B).

Cheers, Wayne

 

[Joethemechanic]

In a side-by-side panel I could stick (2) 125A breakers on the same stabs, then I could theoretically have up to 250A on those stabs.

I just looked at a Square D homeline panel I have on a shelf out in the shop. The cross sectional area is pretty small where the stabs connect to the bus. I could see how they could possibly get overloaded

 

[ggunn]

You don't have to keep saying this. We can all tell the difference between a discussion of current code requirements, vs. a discussion of the reasoning behind them (or lack thereof). Some people here are involved in the code making process (or want to be). It's not off topic.

Feeling a bit cranky this morning?

 

[electrofelon]

Feeling a bit cranky this morning?

I do agree with Ben here, I mean I like to know the reasoning behind code requirements. Because Maybe there is something I am missing from a theory perspective, or more likely it will give me more fodder to rant against the NFPA with

 

[jaggedben]

Feeling a bit cranky this morning?

No, this is me being cranky.

 

[Joethemechanic]

So do these rules only apply to solar? How about other secondary energy sources? For example , what if I burned old tires to make steam to run a turbine that turns a generator? If I connected that with a back fed breaker on the opposite end of my bus, would I have to follow this 120% thingy

 

[electrofelon]

So do these rules only apply to solar? How about other secondary energy sources? For example , what if I burned old tires to make steam to run a turbine that turns a generator? If I connected that with a back fed breaker on the opposite end of my bus, would I have to follow this 120% thingy

Yes, it doesnt matter what the energy supply is

 

[mooreaaryan]

One option for sizing solar backfeed as a line side connection in a panel or subpanel is the "120% rule".
You find out the amp rating of the panel bus bars (which in your case will be at least 200A, but may be 225A if you are lucky. You need to find out for sure for your panel. )
You then multiply that by 120%. For you, that is a minimum of 240A.
The sum of the main breaker size and the nominal amperage of the solar installation must not exceed that number. Unless you have a 225A bus or reduce the size of the main breaker, that limits you to 40A. If you have a 200A bus, even a 175A main will not allow you to put in 70A of solar.

The motivation behind that rule is that by putting the main breaker and the solar backfeed breaker at opposite ends of the ;panel bus you can avoid overloading the bus if you have more than 200A of connected loads. The reason it is a 120% rule rather than a 200% rule is that the Code makers wanted to be totally conservative with a configuration that was not part of the panel testing.

Alright I confirmed with Square D engineering that the bussing is rated 225. Knowing that the bussing is 225 I can use the 120% percent rule which will allow me up to 270 amp minus 175 main breaker -95 amp of alternate source (solar). Am I interpreting these rules correctly?

 

[Joethemechanic]

Yes, it doesnt matter what the energy supply is

Damn government, they are such troublemaking busybodies

Can't do this, can't do that, can't burn scrap copper wire clean, can't operate a meth lab, can't dump tanker loads of waste industrial solvents down old mine ventilation shafts, It's all just to keep the little guy from getting ahead

 

[jaggedben]

Alright I confirmed with Square D engineering that the bussing is rated 225. Knowing that the bussing is 225 I can use the 120% percent rule which will allow me up to 270 amp minus 175 main breaker -95 amp of alternate source (solar). Am I interpreting these rules correctly?

Yes.

One thing in your posts isn't totally clear.
The 95A allowed is inverter continuous output times 125%.
So if the inverter continous output is 70A, then 70×1.25=87.5 so it works with downsizing the main to 175A. The solar breaker can be 90A.
If the inverter output times 125% is 70A or less (i.e. inverter 56A or less) then downsizing the main to 175A is not necessary.

 

[Joethemechanic]

inverter continuous output

What defines this? Like here in Hazleton I think we average like 3 solar hours per day. There is only going to be a brief period when the panels are putting out maximum

 

[bwat]

What defines this? Like here in Hazleton I think we average like 3 solar hours per day. There is only going to be a brief period when the panels are putting out maximum

Inverter datasheet

 

[electrofelon]

What defines this? Like here in Hazleton I think we average like 3 solar hours per day. There is only going to be a brief period when the panels are putting out maximum

because of this is why most PV systems are designed with DC wattage to inverter wattage (DC:AC ratio) of above 1. The result is peak inverter output (or close to it) for a larger chunk of the day.

The busbar rules make sense from a physics standpoint, but IMO are overly conservative.

 

[jaggedben]

What defines this? Like here in Hazleton I think we average like 3 solar hours per day. There is only going to be a brief period when the panels are putting out maximum

The inverter data sheet defines it. Available sunlight has nothing to do with it. Most solar inverters do end up outputting at the max for at least 3 hours on some days of the year. (In Hazleton you might average 3 solar hours but there will be days or even weeks when it's much higher.)

 

[mooreaaryan]

Yes.

One thing in your posts isn't totally clear.
The 95A allowed is inverter continuous output times 125%.
So if the inverter continous output is 70A, then 70×1.25=87.5 so it works with downsizing the main to 175A. The solar breaker can be 90A.
If the inverter output times 125% is 70A or less (i.e. inverter 56A or less) then downsizing the main to 175A is not necessary.

Perfect explanation. I am going to make a quick cheat sheet to keep these references nearby
I appreciate you taking you time to make sure I understood

 

[ggunn]

What defines this? Like here in Hazleton I think we average like 3 solar hours per day. There is only going to be a brief period when the panels are putting out maximum

DC makes no difference to AC wiring and OCPD requirements. The rated maximum AC current as shown on the inverter data sheet must be used to size everything.

 

[Joethemechanic]

DC makes no difference to AC wiring and OCPD requirements. The rated maximum AC current as shown on the inverter data sheet must be used to size everything.

I'm just not seeing how this could be considered a continuous (3 hour or more) ac current flow in a grid tie system without batteries. If each day we are only getting 3 solar hours in the course of say 8 to 10 hours of daylight that means the inverter is only going to operating at max current on the AC side for a brief period and the rest of the day at considerably less output.