Another 120% rule question

P

photonboy

Member
Location
Berkeley, CA, USA
Occupation
Ex roof monkey, current desk jockey
Apologies if this is basic, for some reason I can't find a clear answer.

Scenario:

100A Main bus w 100A Main breaker, so 20A allowed to backfeed
100A load breaker in Main feeding 125A rated sub-panel
30A PV backfeed into this 125A sub

My thinking is the Main panel is still overloaded by the 30A PV in the sub because that 30A will still end up on the Main panel bus and it will be overloaded by 10A

Is this correct?

Thanks
 
You are correct. 705,12(B) must be complied with in every panel between the PV interconnection and the service. It is indeed basic but you might be surprised at how many people don't seem to get it.
 
If the 100A main panel is a Main Breaker Only (MBO) panel .....you can backfeed up to 100A of solar into it.
 
You must apply the 120% at the sub panel also: so then in your case, 50A max into the 100A fed, 125A rated subpanel.
 
If it is a main panel with distribution then yes 20A max regardless of how many sub panels separate the PV from the main.
 
If there are only two breakers(100a main breaker and 100a control breaker for the 125a subpanel) in the 100a main panel, no other breakers, then you could install max 50a solar in the 125a subpanel.
 
Lynnzzz said:
If there are only two breakers(100a main breaker and 100a control breaker for the 125a subpanel) in the 100a main panel, no other breakers, then you could install max 50a solar in the 125a subpanel.
Click to expand...
By the sum of all breakers rule in a 100A/100A main panel one could theoretically push 80A of PV into it, but in 15+ years of designing PV systems I was only able to use that rule to qualify a main panel a few times, and all but one or two of them was when a main panel out at a pole with a single load/BF breaker in it was used as a cheaper alternative to a disconnect switch. For the vast majority of "normal" services this is not an option.
 
Last edited:
How likely is it for this to ever happen or do you have to try to replicate it and some more like exceeding the weight limit on most passenger elevators?

I see passenger elevators rated 5,000 lbs but you'd have to do "how many people can you fit ion a port-o-pottie challenge" to even get to this limit. It'd take twenty-eight 180 lbs occupants to hit the limit and its often sized such that it already feels crowded at 10 people.
 
jaggedben said:
Replicate what? Drawing 116% of a panelboard's bus rating?
It's not likely to ever happen. The rule is just very conservative.
Click to expand...
Overloading the bus bar by added feeding by solar PV generation. If there is a current source of 100A at the top, a current source of 20A in the first space below the main breaker. Yes, in text book theory, it's possible that 120A load could be placed at the bottom causing the bus bar to carry 120A without the main tripping.

I was just asking if this was any more probable than an elevator overload short of someone doing port-o-pottie challenge like thing, which is physically possible and can be duplicated in field lab as well as in theory, but nearly impossible in reality.

It is very conservative for sure. Is it reasonable abundance of caution or unreasonably conservative?
 
ggunn said:
By the sum of all breakers rule in a 100A/100A main panel one could theoretically push 80A of PV into it, but in 15+ years of designing PV systems I was only able to use that rule to qualify a main panel a few times, and all but one or two of them was when a main panel out at a pole with a single load/BF breaker in it was used as a cheaper alternative to a disconnect switch. For the vast majority of "normal" services this is not an option.
Click to expand...
Is it even necessary?

This documents they've fit 41 people into a port-o-pottie. https://runningmagazine.ca/the-scene/porta-potty-challenge/ If we decide on each person being 120 lbs, add the weight of the toilet itself, that's 5,000 lbs. Imagine the cost implication if it was now required to rate the ground onto which a port-o-pottie is placed must be rated to handle a loading of 3 tons, because 5,000 lbs + 20% margin of safety.
 
Flicker Index said:
It is very conservative for sure. Is it reasonable abundance of caution or unreasonably conservative?
Click to expand...

Unreasonably conservative in my humble opinion. At least for most installations.

One way to look at it...
If a panelboard has been humming along for some number of years and it's overcurrent protection has never tripped from overload, then the load is very unlikely to ever exceed the busbar rating, regardless of where the current is coming from.

Also, there are other circumstances where we allow a load calculation, rather than overcurrent protection per se, to protect conductors and busbars, notably 230.90 exception 3. Why should this be so different?
 
A couple of companies I knew had a rather cavalier attitude about downsizing a 200A MCB in a 200A resi panel to 175A without doing a load analysis in order to fit in more PV - "Call us if the main ever trips." To my knowledge, no one ever did.
 
Flicker Index said:
How likely is it for this to ever happen or do you have to try to replicate it and some more like exceeding the weight limit on most passenger elevators?

I see passenger elevators rated 5,000 lbs but you'd have to do "how many people can you fit ion a port-o-pottie challenge" to even get to this limit. It'd take twenty-eight 180 lbs occupants to hit the limit and its often sized such that it already feels crowded at 10 people.
Click to expand...
By default, the panel would already be sized for it to be statistically unlikely that you'll exceed the main breaker rating in the first place. It is physically possible for an adversarial user to deliberately turn on everything at once, and exceed the main breaker rating The idea of the 120% rule (and its 100% rule demotion for the same end of the bus), is that if such a user ever does end up turning on an unlikely amount of load, that it isn't in the main breaker's blindspot created by the panel being fed from multiple sources. The opposite end of the bus part allows us to take partial credit for current being subtractive instead of additive. The fact that it is a 120% allowance, instead of anything else between 100% and 200%, is an industry compromise, with a physical basis of mutual heating among branch breakers being the concern that limits it from being higher.

jaggedben said:
Unreasonably conservative in my humble opinion. At least for most installations.

One way to look at it...
If a panelboard has been humming along for some number of years and it's overcurrent protection has never tripped from overload, then the load is very unlikely to ever exceed the busbar rating, regardless of where the current is coming from.

Also, there are other circumstances where we allow a load calculation, rather than overcurrent protection per se, to protect conductors and busbars, notably 230.90 exception 3. Why should this be so different?
Click to expand...
The idea is that *if* it ever does get overloaded to a point where it could be a problem for the busbar and the collection of branch breakers, a breaker in some form or another, is there to shut it off. Regardless of how unlikely of an event that may be.
 
Flicker Index said:
How likely is it for this to ever happen or do you have to try to replicate it and some more like exceeding the weight limit on most passenger elevators?

I see passenger elevators rated 5,000 lbs but you'd have to do "how many people can you fit ion a port-o-pottie challenge" to even get to this limit. It'd take twenty-eight 180 lbs occupants to hit the limit and its often sized such that it already feels crowded at 10 people.
Click to expand...
Another explanation for elevator capacities being that far beyond the weight of the maximum population you could realistically fit, is that there's also cargo loads that can be significantly denser than people, even in a passenger elevator. A handtruck can take up the space of a single person, yet some can carry double the weight of a person. If you have 3x 200 pound workers on a moving day all hauling 500 pound handtrucks of cargo, that's 2100 pounds, filling about half the elevator. Maybe you could add another 4 people, and that would be 2900 pounds. So we're now above half the posted capacity, which means the posted capacity isn't really that unrealistic of a standard of design.

The last thing you want is for an elevator to get overloaded because the riders didn't bother to add up the payload, so the engineers rated it for a payload capacity that's improbable to exceed by accident.
 
ggunn said:
A couple of companies I knew had a rather cavalier attitude about downsizing a 200A MCB in a 200A resi panel to 175A without doing a load analysis in order to fit in more PV - "Call us if the main ever trips." To my knowledge, no one ever did.
Click to expand...
When you realize that's 42kVA you start to understand why that is.
 
You must log in or register to reply here.
Top