5

looking for some clarity on sub-panel/EMT grounding. Been doing some research but not fully clear on some things.

Going to feed a sub-panel/loadcenter from a 60A breaker installed in a 200A main panel (no main breaker in sub-panel), using ACWU 4/3 AL cable. Two hots with 1 ground and 1 neutral separated at sub-panel (each into their own bar; neutral bar not bonded to sub-panel). No additional ground rod needed at sub-panel because located in same structure.

From the sub-panel, I will be running T90 in wall/surface mounted EMT to all planned electrical boxes. I understand that EMT is an acceptable ground run but not sure if grounding bushings are needed at the sub-panel?

Going into the sub-panel (for the ACWU 4/3 AL), I will be using an aluminum 3/4" 1-screw clamp fitting using full knockout.

Coming out of the sub-panel (to the electrical boxes) will be 1" EMT with steel or zinc alloy compression connector using concentric knockout.

Do I need a grounding bushing at the 'in', 'out' or both points of the sub-panel. If so, I can only find aluminum grounding bushings. I assume that would be fine for the 3/4" aluminum clamp fitting but is it ok to use the aluminum grounding bushing with the steel or zinc alloy connector on the way out of the sub-panel? Again, this is if the bushings are needed at all?

Also, I've read that any concentric/eccentric knockout used at the sub-panel and along the EMT run to the electrical boxes would also need grounding bushings. Or the alternative is to remove the entire concentric/eccentric knockout and use two reducing washers. This, supposedly, is good enough to keep the ground going through the EMT (reducing washers not applicable at sub-panel for grounding purposes, I believe; only at electrical boxes?).

Sorry for the length of my question but trying to be as detailed as possible.

Amazon Dies In Darkness
  • 2,749
  • 11
  • 45
  • 70
JBS
  • 53
  • 2

1 Answers1

6

Extra grounding help isn't applicable downstream of the service entrance for most folks

The NEC only requires grounding/bonding bushings in two cases:

  1. When service conductors are involved (which isn't your situation), and
  2. When the nominal system voltage to ground is over 250V and you're dealing with concentric or eccentric knockouts that are not listed for bonding at over 250V. (In practice, most steel junction boxes from reputable manufacturers nowadays have listed knockouts, so the only place where this matters is if a panelboard cabinet in a 480V system has pre-punched KOs, although many commercial panelboard cabinents don't.)

If you're up in the Great White North, then the Canadian Electrical Code applies, and apparently only requires bonding bushings in case 1 above.

So, unless you're working on a 480V or 600V system, you don't even need to think about this, and even then, it may not be necessary work in Canada.

ThreePhaseEel
  • 87,685
  • 36
  • 144
  • 243