Pass un-spliced circuit through sub-panel. What to do with EGC.

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westtx28

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New member here. Long time lurker needing help with a situation.
200 amp main on one side of a shop that feeds a 125 amp sub-panel via under slab pvc conduit for a small living area on the other side. New aerobic septic system on living area side of shop requires a 30 amp 120 circuit. Since the septic service will also serve the main house the owner and I think it would be wise to run the circuit from the main breaker rather than through the sub-panel. That way the shop apartment circuits can all be cut from 125 amp breaker but would not affect the septic service.
My question is: If I run the circuit from the main panel through the existing conduit, and pass through the sub-panel un-spliced with 10 awg hot and common (remember it's 120V 30 amp) to the septic disconnect, do I also need to run the equipment grounding conductor from the main panel or can I run the EGC from the sub-panel? I don't understand why I couldn't since it's a noncurrent-carrying equipment ground but thought I'd ask. The sub-panel obviously has a non-bonded grounding bar.
Thanks.
 
I don't see a problem code wise. But why could you not just run the EGC with the other conductors? Is it that far that a single conductor (assuming you're running THWN or other single conductors) would add that much to the cost?
 
Little Bill said:
But why could you not just run the EGC with the other conductors? Is it that far that a single conductor (assuming you're running THWN or other single conductors) would add that much to the cost?
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No, its $25 bucks worth of wire. I probably will run it with the other conducters. I just haven't run in to this before and it became more a question of whats the right thing to do. I was trying to wrap my head around it possibly being more correct put the EGC closer to the equipment but i don't see how that could be either.
 
I thought all circuit conductors have to originate from the same paneln dont they?:?
 
edward said:
I thought all circuit conductors have to originate from the same paneln dont they?:?
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I know that's true for a MWBC (210.4) but I'm not familiar with a Code requirement for a single circuit.
 
westtx28 said:
200 amp main on one side of a shop that feeds a 125 amp sub-panel via under slab pvc conduit for a small living area on the other side.
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westtx28 said:
My question is: If I run the circuit from the main panel through the existing conduit, and pass through the sub-panel un-spliced with 10 awg hot and common (remember it's 120V 30 amp) to the septic disconnect,
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If you add those two current carrying branch circuit conductors to the feeder conduit than code wise the feeder conductors will have to be derated.

Many people would choose to ignore this but it is a code violation.

westtx28 said:
do I also need to run the equipment grounding conductor from the main panel or can I run the EGC from the sub-panel?
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In my opinion 300.3(B) can be read to require the EGC run all the way back to the main panel.
 
In my opinion 300.3(B) can be read to require the EGC run all the way back to the main panel.

If the 125 amp circuit and the 30 amp circuit both originate in the main panel, have we not satisfied that requirement ?
 
augie47 said:
If the 125 amp circuit and the 30 amp circuit both originate in the main panel, have we not satisfied that requirement ?
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:)

I am not sure, that is why I said could be.

300.3(B) Conductors of the Same Circuit. All conductors of
the same circuit and, where used, the grounded conductor
and all equipment grounding conductors and bonding conductors
shall be contained within the same raceway, auxiliary
gutter, cable tray, cablebus assembly, trench, cable, or
cord, unless otherwise permitted in accordance with
300.3(B)(1) through (B)(4).
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Is the feeder EGC really part of the branch circuit?

In a practical sense I say yes, however in application of the NEC I have always considered branch circuit conductors as separate from feeder conductors.
 
If the branch circuit wires pass thru the panel and the equipment grounding conductor is spliced in the panel then why would this be an issue

The first conduit- I assume has an equipment grounding conductor???? If not I am not sure why that would change things.
 
EGC for fault-current

EGC for fault-current

Since you likely have (2)#1AWG +(N)#1AWG serving that 125A feed (310.15(B)(16) 75C, you only have a bit of overhead with the 130A capacity for slight derating.
As previously commented, if 310.15(B)(a) applies a derating of 80%, which many would likely ignore, you could swap out the 125A breaker for a 100A to address it. Assuming your demand load is low enough on that sub-panel.
But when you derate your septic pump conductors, it could be an issue moving to #8AWG instead of #10's.

For the GND, your existing required #6 CU EGC (250.122) that was sized for the 125A sub-panel feed through your PVC conduit would be easily capable of providing fault-current back to trip the new 30A breaker.
I don't think there's an issue once you bond your septic branch EGC to the separate Ground Buss terminal you have in that sub-panel.
 
Thanks for the help all. The existing construction for the sub-panel is 2-#2,1-#2, 1#6 with 125 amp breaker from 200 amp panel. I did a load calculation on what that panel is serving and came up with 106 amps using residential standard and 80.5 using the optional. Owner said this was done in 2001. Property is out in the country with no permits or inspections. I feel I need to replace the #2 regardless of additional conductors in the conduit.
 
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