Backfed Breaker Used as Main / Trip Curve

Status
Not open for further replies.

bpk

Senior Member
I have a small 20 space 100a panel that has a backfed 60amp being used as the main. On this install or lots of others like this, can you get a breaker with a slower trip curve so when there is a dead short downstream it doesnt take out the whole panel. I know there is a lot of info and calculations that go into selective coordination for large systems but I've never heard about small ones like this. This panel is a QO with a 2 pole 60 amp backfed being used as the main.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
I think you are pretty much out of luck unless you are willing to replace the 60 Amp main breaker with a separately mounted breaker with a larger frame size. Or if you can replace the main with a fused disconnect, you might be able to find 60A fuses that coordinate with the branch circuit breakers.

For a 1st estimate of coordination, I compare trip settings. A 10X difference can be expected to coordinate fairly well for low fault currents. A fault on a 20A branch circuit probably won't trip a 200 A breaker.

But the smaller 60A breaker you have is much closer to a 20A breaker. I would expect very poor coordination from that (and even worse if you had 30A or 40A branch breakers in the panel.)

For a second estimate, you can refer to something like the "Selectivity Guidelines for Square D Panelboards". (Go to Squared.com and search for that phrase.)

But they don't list anything under 100A for the main.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
I think you are pretty much out of luck unless you are willing to replace the 60 Amp main breaker with a separately mounted breaker with a larger frame size. Or if you can replace the main with a fused disconnect, you might be able to find 60A fuses that coordinate with the branch circuit breakers.

For a 1st estimate of coordination, I compare trip settings. A 10X difference can be expected to coordinate fairly well for low fault currents. A fault on a 20A branch circuit probably won't trip a 200 A breaker.

But the smaller 60A breaker you have is much closer to a 20A breaker. I would expect very poor coordination from that (and even worse if you had 30A or 40A branch breakers in the panel.)

For a second estimate, you can refer to something like the "Selectivity Guidelines for Square D Panelboards". (Go to Squared.com and search for that phrase.)

But they don't list anything under 100A for the main.

If you look at the trip curves, a 60A breaker is going to have a magnetic pick-up point at 6X rated (360A) where a 20A has a pickup point at 11X rated (220A), so they aren't that far off.

I too think it's going to have to be a fused disconnect with some careful coordination. Maybe a separately mounted breaker, but the only way to get anything you can work with is going to be a 60A 2P breaker with an Electronic Trip Unit, and I've never seen one. You would probably have to get a 3 pole 60A with an ETU and that is going to be a spendy way to go. Figure around $800+ for that option!
 

templdl

Senior Member
Location
Wisconsin
If you are luckey enough to have a fault that falls beteen the main and the branch pickup range the branch will trip. But that us highly unucommon because a bolted fault will in most cases the pickup point of both breakers and it will bepot luck as to which one will trip which the mains will often trip. Remember that faults are instantanious so the branch will not necessarily trip because of it's power pickup value than the mains. A lower pickup calibration does not guarantee that it will trip befor a breaker with a high magnetic trip calibration.
In cases like this where coordination is desirable breakers with electronic trip units that has a time delay feature are the way to go in the main in this case which would be set witth a slight delay which would allow the down stream breaker to trip. as i recakk the delay may be set up do 20ms
Of course, a time delay feature is only available in the larger industrial beaker.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Most manufacturers have 'selective coordination' guides on their websites.
http://static.schneider-electric.us/docs/Circuit Protection/0100DB0501.pdf

Ideally you will know the 'through fault current' of each branch circuit in order to determine where each breaker will trip. You also need to decide what amount of mis-coordination might be acceptable (i.e. must coordinate to .1sec, but coordination not required for bolted faults).
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
The main times I use this kind of setup is because I am feeding from an SDS and the main is the OCPD for the secondary of the xfmr and its conductors.

It would be "nice" if the main did not trip when there was a fault on a branch CB, but for these kind of circuits most of the time you really do not have much of a choice. For me most of the time, if the main trips in such a case instead of a branch, it is not any worse than if the branch trips, other than it being harder to determine just where the fault really was. Either way the equipment goes down.

Would be nice if the lights could stay on, but sometimes that is the way things work out.
 
Last edited:
Status
Not open for further replies.
Top