Sub panel sanity check

[green92]

Hi everyone,

The building & electrical inspectors just visited my detached garage and the electrical inspector advised me that I've miswired the subpanel and created a ground-loop. This was a huge surprise. Would you guys mind reading through my setup and tell me what I did wrong, because he declined to clarify why the circuit is flawed and although I will make the change he requested, I really want to understand why this is incorrect.


- The subpanel is supplied by 3, #3 RW90 lines (no issues with those) from the main panel in the house.

- The subpanel's neutral bus is not bonded to its ground bus.

- There are 2, 10' long, 3/4" copper grounding rods (driven a few inches below grade) connected by a contiguous 6awg bare ground wire which then connects to the subpanel's ground bus.

Specifically, the inspector said "those rods are useless" and that I should have ran a single insulated ground line to the panel and he repeated a few times that I'd created a ground loop. But I cannot for the life of me see it - I mean, I will pull the 4th line as per his request, but I don't understand why.

Any info is much appreciated!

 

[Stuart in MN]

Is there a ground wire running from the main panel to the subpanel, or is it just connected to the two ground rods? I'm in the US so I can't comment on Canada regulations, but that may be the issue.

 

[mrb]

based on how we do it here in the US, he is correct in that you did it wrong, but he is using the wrong terminology 'ground loop' you need a 4th wire back to the main. Without this if you have a short circuit the breaker wont trip. search google 'grounding vs bonding'

 

[green92]

Hi Stuart, there is no ground wire presently running from the subpanel to the service panel. That's the wire the inspector wants me to pull, however - and then disconnect wire from the grounding rods entirely.

 

[mrb]

read this to understand why http://ecmweb.com/grounding/electric_grounding_vs_bonding_2/

 

[green92]

Ahhh, thank you MrB. I will read up on that - what had concerned me was that I thought I understood what was involved and want to make sure I understand the fundamentals so future projects are done up right the first time.

 

[VHF]

Here in the US we would typically be required (under NEC 2008) to have both the 4th wire and the ground rods for a detached garage. Your inspector wants you to run the 4th wire and disconnect the ground rods?

BTW, did he way what size wire to use for the 4th wire (grounding conductor)?

 

[green92]

Correct - run the 4th wire, disco the ground rods. He'd said to run a 6awg ground wire back to the service panel (the sub panel has 100amp main)

*edit, for clarity, the new 6awg wire would be in the same conduit as the 3, RW90 lines

*editx2, Prior to MrB's post, I phoned the head of civic inspections (provincial housing authority) to ask about this, because the whole "this is a ground loop" did not compute for me and he also said that I must remove the grounding rods, but also expressed his concern that the inspector did not explain the fault correctly (or at all). So I'm hoping we're all still friends when the inspector looks in on this again...

 

[Aceman]

With a 3 wire feed to a subpanel the neutrals/grounds ARE bonded together.

With a 4 wire feed, you seperate neutrals/grounds.

Either method REQUIRES a grounding electrode if this is a detached building. Ground rods are the most common choice.

I have no idea what the Canadian electrical code requires.