Ground Currents in Parallel Feeder Equipment Grounds
Ground Currents in Parallel Feeder Equipment Grounds
(OP)
I have an 800 amp feeder between the building main where the ground and neutral are bonded. There are 3 parallel 350 Kcmil and 3/O equipment ground per EMT conduit. With the equipment grounds connected at both ends, does this result in some circulating currents due to induced current in each of the equipment grounds.
I measure around 20 amps on each equipment ground individually. If the amp meter measures them collectively, there is around 12 amps.
I measure around 20 amps on each equipment ground individually. If the amp meter measures them collectively, there is around 12 amps.






RE: Ground Currents in Parallel Feeder Equipment Grounds
RE: Ground Currents in Parallel Feeder Equipment Grounds
See:
http://www.nexans.ca/egy/tecdoc/7.htm
This circulating current can be quite high sometimes. On one of our installations (at that time new) we've meassured over 400A circulating current when grounded at both ends. It stressed the cable a lot and eventually it failed - not even two weeks after energized.
Check also:
ANSI/IEEE std 575-1998 – “IEEE Guide for the application of sheath bonding methods for single-conductors cables and calculation of induced voltage and current in cable sheaths.”
IEC 287-1-1 1995.8 “Electric cables – Calculation of current rating, part 1: Current rating equations (100% load factor) and calculation of losses, section 1: General”
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RE: Ground Currents in Parallel Feeder Equipment Grounds
And also exlain "collective" measurement by amp meter? Is this done using a flex around all conductors?
Equipment ground condutcot is always connected at both ends to the eqipment enclosure, ground bus or the ground lug.
What is the current in phases? What does it serve?
RE: Ground Currents in Parallel Feeder Equipment Grounds
So, there is an equipment ground running in each of the three conduits and they terminate in the ground bus at each side along with bonding to the conduit at each end.
RE: Ground Currents in Parallel Feeder Equipment Grounds
Is there a neutral between the two equipment? Do you have L-N loads?
Measure the current in the neutral that is coming in the outdoor disconnet 'before' the N-G bond. That would equal the the current in all the grounds plus the N at the interior panel.
The ground rod should also be at the outdoor switch where service entrance N-G bond exist. Ground rod at interior panle is just a supplemental rod, although wont hurt, it is not required by Code.
Have someone qualified check out the installation.
RE: Ground Currents in Parallel Feeder Equipment Grounds
Detail calculation is recommended to verify the above statement.
RE: Ground Currents in Parallel Feeder Equipment Grounds
I think rbulsara may be on to something, if there is a downstream N-G connection it will cause current to flow on the ground, but that current should add rather than cancel when all three ground conductors are measured together.
Since the circulating currents tend to cancel when taken in total, I'd look for a loose connection somewhere that is causing the phase currents to be unbalanced. This unbalanced (net current) will drive current one direction in one ground conductor and the other direction in the other two ground conductors.
RE: Ground Currents in Parallel Feeder Equipment Grounds
We did identify several N-G bonds in building panels and have removed them, which has helped. However, we still have 14-16 amps on each of 3 equipment grounds. I'm wondering if we are missing a bond somewhere yet.
Back to davidbeach: It seems like I should not be getting induced current on the equipment ground conductors in these parallel feeders. And, yes I thought they should all add rather than cancel out. That's why we've been wondering about some circular currents being induced somehow.
RE: Ground Currents in Parallel Feeder Equipment Grounds
The Neutral currents in the 3 parallel feeders are:
13.4a - 14.4a - 14a
With amp meter around all 3 = 21.7a
The equipment ground currents in the 3 parallel feeders are:
17 - 16 - 14
With amp meter around all 3 = 12a.
Even though the N-G bond is at the Main Disconnect, the equipment grounds are basicaly bonded together at both ends via the grounding bus.....the length here is around 100 feet.
RE: Ground Currents in Parallel Feeder Equipment Grounds
RE: Ground Currents in Parallel Feeder Equipment Grounds
RE: Ground Currents in Parallel Feeder Equipment Grounds
The point is, have you measured the currents in the phase cables? If for some reason the currents are not balancing properly the unbalance can cause induced currents. With an unbalance condition, the induced currents in each conduit will not be in phase with each other. This will account for the currents not adding up. Check the current on the ground cables leaving the panel. You could have ground currents originating from equipment. I would suspect a combination of factors; 1> ground currents originating from in plant equipment.
2> Grounding cables shareing the neutral current.
Please let us know what you finally discover. Thanks.