Electrical service question

[Chris Rosenberger]

I know that the electrical code for sub panels calls for 4 wires to be run from the main panel to the 220 volt sub panel (2 hots, neutral & ground), isolating the neutrals & grounds in the sub panel and no ground rod at the sub panel.

What is the reason for using the ground at the main panel & no ground rod at the sub panel?

[Art Mann]

I do not believe it is a violation of the NEC to use a local ground rod at the sub panel. I certainly did because it is safer than counting on a ground reference that is 150 feet away (in my case). The important thing is to keep this ground separate from the neutral, because it is current carrying and the ground is not.

[Richard Shaefer]

Agreed.
no restriction against using a local ground rod. Depedning on the mood of your local inspector, he may either demand that you install a local ground rod or remove one that you already have installed. Only reliable adviec that I can offer you is that however you ground it, the inspector will likely want it the other way.

[Thomas Bank]

You don't state it, but my experience is for a local ground rod if the sub-panel is in a separate structure and using the existing grounding if it is in the same structure as the main panel.

[Chris Hachet]

Bingo...I am a state listened master electrician and this is indeed the correct answer.

[Joseph Tarantino]

in which state?

[Art Mann]

The information that has been offered in the most recent posts, which Chris Hachet endorsed, is based on the National Electrical Code. State codes are additions to the NEC.

[John Coloccia]

FWIW, imagine what would happen if you ever lost the neutral in the sub-panel and it happened to be bonded to ground in the sub-panel. Your ground wire would suddenly become a current carrying conductor with a subsequent voltage drop all the way back to the service panel, and you would have stray currents energizing every ground connection on that panel, and you would never know it until it caused problems.

Even worse, if that cable were ever damaged, say by accidentally drilling through it...or a fire....or an earthquake....or anything like that, and you happened to loose the neutral AND the ground, you would then have a situation where the return path because the metal chassis of equipment. Every piece of metal in the shop suddenly becomes deadly, including the sub-panel itself. Keeping the ground and neutral separate insures this can never happen.

[David C. Roseman]

Some of the discussion in this thread does not reference a distinction between a sub-panel in the same structure as the service panel, and one in a separate outbuilding, such as a detached garage, shop or barn. Just want to be sure my understanding is correct: That is, in both cases, the sub-panel should be bonded back to ground at the service panel through the ground conductor in the feeder cable, and, if in a separate structure, it must also be bonded to its own ground rod (or other approved electrode) at that structure. Also, at least if residential, in both cases the neutral should be bonded to ground at both the sub-panel and service panel (through bonding of the neutral and ground busses).

Is that right? Not an electrician, but I take this stuff very seriously.

David

[Matt Marsh]

Some of the discussion in this thread does not reference a distinction between a sub-panel in the same structure as the service panel, and one in a separate outbuilding, such as a detached garage, shop or barn. Just want to be sure my understanding is correct: That is, in both cases, the sub-panel should be bonded back to ground at the service panel through the ground conductor in the feeder cable, and, if in a separate structure, it must also be bonded to its own ground rod (or other approved electrode) at that structure. Also, at least if residential, in both cases the neutral should be bonded to ground at both the sub-panel and service panel (through bonding of the neutral and ground busses).

Is that right? Not an electrician, but I take this stuff very seriously. David

David,


The only time you bond the neutrals and grounds together is where the panel in question is fed with a 3-wire feeder. The neutral bus in a sub-panel fed with a 4-wire feeder must be isolated from the grounding system, whether it is in the same structure, or a seperate structure. A panel in a seperate structure, whether it is fed with a 3-wire or 4-wire feeder, must have it's own seperate grounding electrode system. The only exeption to this rule, is when the seperate structure is fed by a single branch circuit. A multi-wire branch circuit is still considered a single branch circuit.

[David C. Roseman]

David,


The only time you bond the neutrals and grounds together is where the panel in question is fed with a 3-wire feeder. The neutral bus in a sub-panel fed with a 4-wire feeder must be isolated from the grounding system, whether it is in the same structure, or a seperate structure. A panel in a seperate structure, whether it is fed with a 3-wire or 4-wire feeder, must have it's own seperate grounding electrode system. The only exeption to this rule, is when the seperate structure is fed by a single branch circuit. A multi-wire branch circuit is still considered a single branch circuit.

Thanks, Matt! That explanation is very clear, and makes complete sense.

David

[Julie Moriarty]

From 2011 NEC:

250.24 Grounding Service-Supplied Alternating-Current Systems.
(A) System Grounding Connections. A premises wiring system supplied by a grounded ac service shall have a grounding electrode conductor connected to the grounded service conductor, at each service, in accordance with 250.24(A)(1) through (A)(5).

(1) General. The grounding electrode conductor connection shall be made at any accessible point from the load end of the service drop or service lateral to and including the terminal or bus to which the grounded service conductor is connected at the service disconnecting means.

Informational Note: See definitions of Service Drop and Service Lateral in Article 100.

(2) Outdoor Transformer. Where the transformer supplying the service is located outside the building, at least one additional grounding connection shall be made from the grounded service conductor to a grounding electrode, either at the transformer or elsewhere outside the building.

Exception: The additional grounding electrode conductor connection shall not be made on high-impedance grounded neutral systems. The system shall meet the requirements of 250.36.

(3) Dual-Fed Services. For services that are dual fed (double ended) in a common enclosure or grouped together in separate enclosures and employing a secondary tie, a single grounding electrode conductor connection to the tie point of the grounded conductor(s) from each power source shall be permitted.

(4) Main Bonding Jumper as Wire or Busbar. Where the main bonding jumper specified in 250.28 is a wire or busbar and is installed from the grounded conductor terminal bar or bus to the equipment grounding terminal bar or bus in the service equipment, the grounding electrode conductor shall be permitted to be connected to the equipment grounding terminal, bar, or bus to which the main bonding jumper is connected.

(5) Load-Side Grounding Connections. A grounded conductor shall not be connected to normally non–current carrying metal parts of equipment, to equipment grounding conductor(s), or be reconnected to ground on the load side of the service disconnecting means except as otherwise permitted in this article.

Take from that what you want, but every sub-panel I have ever installed (and I've installed a lot) is grounded back to the main panel. That may be through metallic conduit or through a grounding conductor. Some inspectors have required a grounding rod in addition to the grounding conductor. There's some controversy regarding potential differences created by two separate grounding systems on the same feeder but I've never seen that issue resolved. Grounding is the grey area of electrical systems.

The easiest way to know what you need to do in your area is to call the inspector.

[Michael Dunn]

This most certainly will not apply to your situation, but multiple paths to ground is an absolute nuisance when dealing with professional audio. The potential difference between the paths to ground creates a ground loop and shows itself as a terrible 60Hz hum with a buzz several harmonics above the fundamental.

For that, I would personally prefer to have one path to ground. But I'm also an audio engineer in addition to being a woodworker.

[David L Morse]

What is the reason for using the ground at the main panel & no ground rod at the sub panel?

Short answer: It makes it less likely that lightning ground currents will take a shortcut through your house and fry all of your line powered electronics.

Google "lightning ground potential rise" for some really scary numbers.

[Chris Rosenberger]

Thank you for the replies. I will check with the inspector.