If I have a subpanel in the same room or same building as the main panel, then I need a grounding conductor between the main panel and the subpanel. No separate conductor from the subpanel directly to a ground rod is required.
If I have a subpanel in a different building then the main panel with a 4 wire feed, then: Besides the 4 wire grounding conductor, I also need a grounding conductor to a ground rod located at the separate building.
I do know: If I have a 3 wire subpanel feed in a separate building, then I need a grounding conductor to a ground rod located at the separate building.
In all cases, the subpanel common (white) and grounding conductors are isolated from each other, common not bonded to the panel.
That sounds about right Michael, except for terminology.
The “grounding conductor between the main panel and the subpanel” is called the Equipment Grounding Conductor(EGC), the “grounding conductor to a ground rod” is called the Grounding Electrode Conductor(GEC) and the ground rod is called the grounding electrode.
Is a sub-panel required to be bonded to the disconnect panel?
Certainly! Let’s delve into the world of electrical panels and bonding:
Main Panel vs. Subpanel:
Your home’s electrical system typically consists of a main panel (also known as a service panel) and one or more subpanels.
The main panel is where you have complete control over the power within your home. It contains the main “service disconnect” (shut-off) switch.
Subpanels, on the other hand, are additional electrical boxes that control specific rooms or appliances. They receive power from the main panel.
Wiring in Panels:
In both main panels and subpanels, you’ll find four types of wires:
Two hot wires: These carry energy to fixtures (e.g., wall plugs, stoves).
One neutral wire: It carries current back to the panel, completing the electric circuit.
One ground wire: This wire provides safety by preventing short circuits and fires.
Bonding and Grounding:
Here’s the crucial point: Ground and neutral wires must bond (connect) together, but only in the main panel.
Never bond ground and neutral wires in a subpanel or anywhere else in the home.
Improper bonding in subpanels is a common mistake and can be dangerous.
Why Separate in Subpanels?:
When you bond ground and neutral wires in a subpanel, it creates a parallel path for current flow.
This can lead to:
Electromagnetic interference.
Overheating due to increased current.
Fire hazards.
Starting from the 2008 National Electric Code, subpanels must be wired with a four-wire feed: two hots, one ground, and one neutral wire. The grounds and neutrals must be isolated.
Safety First:
Always follow proper wiring practices to ensure safety.
If you encounter improper bonding in a subpanel during an inspection, it’s essential to address it promptly.
With the exception of some wording such as " Ground and neutral wires must bond (connect) together, the word connect maybe shouldn’t be there, what else do you see that’s wrong?
Yes an EGC is required to be installed with the feeder to the sub-panel. It can be a metallic raceway if it qualifies under NEC 250.118. Connection via a GEC or bonding jumper to a grounding electrode system (GES) is not required for a sub-panel within the same structure.
Yes the 4-wire feeder needs to have an EGC. The sub-panel within a separate structure requires a connection via a GEC to a GES, typically two ground rods.
An existing 3-wire feeder (Pre-2008 NEC) would require a connection with a GEC to a GES and also that the neutral is bonded to the enclosure.
“also that the neutral is bonded to the enclosure.” Meaning, for a 3 wire feed to a separate building, the bonding screw or jumper is in place?? Neutral (white) and GEC (bare) are connected on terminals that are bonded to the enclosure??
For a 3 wire feed sub panel in a separate building, that’s the same condition as a main service panel!
Thanks! I’m finishing a report for an electrician/buyer. There is a subpanel in the same building as the main panel, plus a subpanel in a separate building; 4 wire feed on this one. The separate building has no GES. Got it right!
I did a lot of inspecting around Portland, OR and there is a HUGE Intel chip plant west of town and I’d regularly work for, “electrician” buyers that would laugh when I asked if they worked on (or even knew much) about residential wiring.
If the path were paralleled it would not result in increased current flow. Also left out the shock hazard created. Ignored that a 3 wire feed in a detached structure requires bonding like a service.
I agree with your take on the shock hazard and I’ll add electrocution hazard, and IMO they could have worded some aspects better, but also IMO the gist of the information is correct.
Here’s another take I found on Mike Holt chat board that I consider a good read.
Welcome. The exact hazards depend on the exact conditions of the installation; they are not the same every time under every failure mode. The worst I would say is a 3 wire feeder (2 hots, ground/neutral) subpanel losing its ground/neutral, if all the branch circuits have un-separated grounds/neutrals in the panel, every single grounded surface will sit at ~120V, waiting to shock/kill someone.
For me, I will call out ANY and ALL sub-panels, regardless of the date of installation, if there is not a 4 feed wire from the disconnect/service panel.
Gotta say that conversations like this thread are why so many HIs say that electrical is tough. I agree.
I expect to see 4-wire feeders in post 2008 separate buildings, but as I research this… It looks like a 3-wire feeder (2 hots & neutral) to a separate building is still allowed, if there is no conductive grounded connection between the “house” and the building like a metallic raceway or buried metal water pipe.
And also, if a simple 120v circuit is extended to an out-building, then there is no separate grounding requirement or 4-wire requirement.
And then, I do a lot of rural inspections with large separate shops, arenas, and barns. There are some additional rules for those like the grounding conductor for a 4-wire feeder to a barn has to be the same gauge as the ungrounded conductors.
Am I correct on these comments?
It’s allowed if it was installed on a code version before the adoption of the 2008 NEC. From the 2008 going forward it is not permitted. In the 2008 NEC the rule was changed to an exception to allow a 3-wire feeder only for existing installations.
I’ve made some notes for me, not necessarily using the correct terminology. It would be very helpful if NACHI could make one or two graphics of these conditions. Helpful both for when inspecting (I keep a clipboard with a few notes and graphics), but also for reports.
IMO the code was changed for a reason, most likely because someone died and that’s when someone realized that we should fix this, put in for the change and stated the reason why, and the code was changed.
