Electrical Circuit Capacity
By Dave Rongey
Summary: A shop is set up in a storage unit and they have tapped their electrical into one of two 20A GFI circuit breakers starting from
a sub panel at the building.
Electrical Question about the capacity of a Shared Circuit
Question From Michael P-
I have a woodworking shop set up in a storage unit and have
tapped my electrical into one of two 20A GFI circuit breakers stemming from
a sub panel on my building.
Upon investigation of the electrical set up, I
have looked into the sub panels of my building and the two previous(Out of 7
buildings between myself and the main breaker panel) and found that each
building has a panel that has a breaker linking it to the next in line.
buildings before mine feeds 2-20A circuits, one GFI for the receptacles and
one for the 70W HPS lighting circuit, and one 50 A breaker that feeds the
building just before me. The building just before me has the same 2 20A
circuits as the previous and then has a 40A breaker that feeds the panel on
My building has instead of 2 20 circuits, 4 20A circuits, two
of which are GFI protected. And all of this is being fed from a 40A breaker
in the previous sub panel? I don't have a complete electrical
understanding, but his couldn't be right could it?
We are having trouble with
having enough power on our circuits in the building sometimes depending if
someone is using electricity further down in the facility. Is there any fix
by increasing the size of the breakers down the line?
Hi Michael - Great Question!
No - increasing the size of the circuit breaker is not the answer.
Increasing the capacity of the circuit by installing larger wire is, but
this must be supported all the way back to the main panel that supplies the
power for all.
It sounds like the whole system needs to be reconfigured based
upon actual loads that are being connected throughout and then upgrading the
From my experience, storage units supply only enough power for lighting to
each unit. If the breaker size is increased without increasing the
size of the wire then there will be a high risk of fire due to overheating
the existing wire size.
Thank you for your response, that was what I had kind of figured but didn't
want to hear due to the inconvenience. They have several 70w HPS High Pressure Sodium ballasts
that have been burned out and not replaced for a long time now, so I am sure
they WILL NOT spend any money money on an upgrade. So the 40A on the
previous panel feeding the 4-20A circuits on mine is not a problem?
A first hand inspection of the panels and the circuits would need to be done to fully determine if there is a problem with the 40 amp circuit.
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