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Kevin, Licensed Electrical Contractor
Category: Electrical
Satisfied Customers: 3674
Experience:  30 years Licensed Electrical Contractor in Illinois, Adjunct College Electrical Instructor, Former Electrical Inspector, Diploma: Digital Electronics, FCC Technician License
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I have a question about a 100amp 240v sub panel mounted in a

Customer Question

I have a question about a 100amp 240v sub panel mounted in a separate steel building. The sub panel has a 3 wire feeder to it and the panel is mounted to the steel support members of the building. The panel has an insulated neutral bus and a separate grounding bus connected to the building and two 5/8" grounding rods. Now back to the main with the 100amp breaker, the ground and neutral is bonded.
My question is: Do I bond the neutral and ground in the sub panel of the metal building or keep the separated?
Submitted: 1 year ago.
Category: Electrical
Expert:  Kevin replied 1 year ago.

Hello and welcome to Just Answer. My name is ***** ***** I will be happy to assist you with your electrical question.

1) Sub-panels are never bonded. The neutral feeder terminates to the neutral bus bar and the green main bonding screw or main bonding jumper strap is never installed in a sub-panel. By not installing the main bonding screw or strap, this is how the neutral feeder remains isolated from the panel metal enclosure.

2) The grounding electrode conductor from the ground rods will land on a separate equipment ground bar that needs to be installed in the sub-panel. At the equipment ground bar, only the grounding electrode conductor and any bare copper grounds will land there.

3) At the neutral bus bar, only the neutral feeder and the individual white branch neutral conductors will land on the neutral bus bar. Never inter-mix neutrals and grounds on any sub-panel as it will not only result in a code violation, but will also create a potential safety issue.

4) Only at the main service are the neutral and grounds bonded together, but never on a sub-panel.

5) If the sub-panel feeder circuit is a new install, the feeder circuit is required by code to be a 4 wire circuit and not a 3 wire circuit. Sub-panel feeders for new installations require 2 hots, 1 neutral and 1 equipment ground.

If you have any additional questions, just let me know and I’ll be glad to answer them for you.

Otherwise, don’t forget to rate me before you log Off.


Customer: replied 1 year ago.
Thank you! This is a big help!So now that my pipe is set and I have only a 3 wire feeder. (2 hots, 1 neutral) Also, providing I keep neutrals and grounding separate at the sub panel and only bonded at my main. Will I encounter any issues with only having a 3 feeder wire to my shop?
Expert:  Kevin replied 1 year ago.

Thank you for the replies.

1) If the conduit is rigid metallic type, then the metal rigid conduit acts as the equipment grounding conductor and a separate equipment ground wire is not necessary.

2) If the conduit is gray electrical PVC conduit, per code, it should contain a separate equipment grounding conductor.

3) By not having an equipment ground, the feeder circuit is always subject to potential shock hazards. Is the sub-panel feeder circuit a new installation or did it exist prior to 1996 or so and was only installed as a 3 wire feeder?

Based on your description, sounds like the sub-panel feeder circuit is a new install, is that correct?

4) That's correct. Only bond at the main service and isolate neutrals and grounds at the sub-panel.... ie........ no bonding at a sub.

Customer: replied 1 year ago.
The 3 wire feeder is in all 2inch gray 40 PVC. The main panel is an individual box with an 100amp breaker. See attached picture. This is my nuetral to ground bond area including this case inclosure being grounded.Yes, this is a new installation and I now know my mistake of only running the 3 wire in the 2 inch PVC. I know I will not be able to pull a ground wire in the same 2inch feeder as the pipe takes a quite a few turns.I guess at this point, I should be certain my grounding at the sub panel is outstanding.Any suggestions baised on my current situation?
Expert:  Kevin replied 1 year ago.

Thanks for the replies and the picture.

1) Since the feeder circuit is a new install, there should have been a 4th wire for the equipment grounding conductor. If the sub-panel feeder breaker is a 100 amp feeder, then the EGC should be sized as a minimum of an 8 AWG copper conductor depending upon the distance from the main panel to the sub-panel.

2) You basically have 2 choices. Install the equipment grounding conductor as 4 wires will easily fit into a 2 inch PVC conduit. Per code, there should be no more than 360 degrees between pull points on any conduit system..... ie.......a maximum of (4) four 90 degree elbows.

3) The other option is to leave the 3 wire feeder circuit "as-is", but now you are subject to any potential shock hazards since no equipment ground would exist. The other thing to keep in mind is that if this install will be inspected by a local electrical inspector, it will not pass code.

In addition, when it comes time to sell the home and the future home buyer retains a state licensed home inspector, a good chance that the lack of the equipment ground will be written up and not in your favor. The future home buyer will use this to their advantage to low ball the price on the home when it comes to re-sale time. So in summary, the choice is basically your choice as to whether install the feeder circuit in the safe/correct code compliant manner or not.

Expert:  Kevin replied 1 year ago.

I was just looking at your picture again. What is the large bare copper wire being used for? Looks like it originates from the sub-panel feeder circuit?

Looks like that is the ground rod wire?

Customer: replied 1 year ago.
That additional ground wire is a #4 that is grounded with two 8 foot 5/8" rods spaced 6 feet apart. That ground is for that 100amp single space breaker closure (pictured) that comes from the meter. That 100amp breaker then feeds the 3 wire feeder 127 feet to the separate building. That ground in that inclosure is the my nuetral to ground bond.At the separate building, I have the same grounding method set up. #4 wire from the sub panel grounding bus to two 8 foot rods spaced 6feet apart. All rods have two acorn attachments. Also the metal building steel frame has #4 ground tied into those same rods.I am looking into now feeding a #4 ground wire in the 2" PVC as recommended.
Customer: replied 1 year ago.
Also to note. This service comes directly off the meter itself. My House 200 amp service also comes from the meter itself and has its own neutral to ground bonding and it's own set of grounding rods. Both of the services come from a twin lug 320 amp Milbank meter base.
Expert:  Kevin replied 1 year ago.

OK,very good. Thanks for confirming.

Yes, the exterior of the home always requires 1 or 2 ground rods. so you are good there. Since the Milbank is a twin lug type, then no problem there and you are also good.

Expert:  Kevin replied 1 year ago.

If you have any additional questions, let me know and I’ll be glad to answer them for you.

Otherwise, don’t forget to rate me before you log Off.


Expert:  Kevin replied 1 year ago.

If you have any additional questions, let me know and I’ll be glad to answer them for you.

Otherwise, don’t forget to rate me before you log Off.