New Garage Existing Feeder

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Braden8898

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On a recent Inspection, I came across a New Garage that had the power supplied by an Existing underground Feeder to a Sub-Panel. The panel was feed with 3 Wires Black,Red,White. NO Green. (Existing installation) The Sub Panel had a Separate grounding bar with a grounding electrode conductor running outside to 2 newly installed ground rods, The Neutrals and grounds were separated in the panel. I Have passed this information around to my local resources and all agree this is an acceptable installation???? I disagree. As this installation is missing one crucial component. An EGC back to the main panel. This installation is using the ground rods to clear a fault. Which a 120 volt line to case fault will not trip a 15 amp breaker unless the resistance of the ground rods is very low. Code requires a minimum of 25 ohms or an additional electrode. How can a New Garage be supplied by an existing Feeder and be compliant without an EGC? I appreciate any input in this discussion.
 
Braden8898 said:
On a recent Inspection, I came across a New Garage that had the power supplied by an Existing underground Feeder to a Sub-Panel. The panel was feed with 3 Wires Black,Red,White. NO Green. (Existing installation) The Sub Panel had a Separate grounding bar with a grounding electrode conductor running outside to 2 newly installed ground rods, The Neutrals and grounds were separated in the panel. I Have passed this information around to my local resources and all agree this is an acceptable installation???? I disagree. As this installation is missing one crucial component. An EGC back to the main panel. This installation is using the ground rods to clear a fault. Which a 120 volt line to case fault will not trip a 15 amp breaker unless the resistance of the ground rods is very low. Code requires a minimum of 25 ohms or an additional electrode. How can a New Garage be supplied by an existing Feeder and be compliant without an EGC? I appreciate any input in this discussion.
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I agree that the installation is not safe/correct for the reason you stated. At one time a 3-wire feed was allowed and you grounded the neutral at the garage disconnect. Not allowed now.

So, a new garage? Sounds like it might require an updated feeder circuit.
 
As long as the neutral is bonded and there are no additional metallic paths between the garage and the main building then it's perfectly safe and before 2005 code it was perfectly legal. There is an exception in art. 250 that allows existing feeders like you describe.
 
Take a look at 250.32(B), and its Exception 1. That should tell you what you need to know. I don't think that exception will apply to your situation, as the "new garage" was not made in compliance with the earlier version of the code. A new building has to comply with the current code. So I also agree with you.
 
ActionDave said:
As long as the neutral is bonded and there are no additional metallic paths between the garage and the main building then it's perfectly safe and before 2005 code it was perfectly legal. There is an exception in art. 250 that allows existing feeders like you describe.
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How will this work to clear a fault? It seems to me that the issue is the separation of the grounds and Neutrals at the sub panel would create an issue yes?
 
Current would leave the main house panel on phase A, travel to the garage sub panel and to a load within the garage. A short takes place between the phase A wire and the metal case of that load. Current flows to the case, and along the green wire EGC to the ground bus of the sub panel. From there it flows over to the neutral bus of the sub panel (using the N-G bond as the path). Finally, current returns to the main house panel along the feeder's neutral wire. That will result in a trip of the feeder breaker to the sub panel. As ActionDave mentioned, this was allowed in an earlier code. But it required there to be no metal components that are common to the house and to the garage. If there was a metal water pipe connecting the two, then you would have a parallel path for neutral current to flow along that pipe.
 
charlie b said:
Current would leave the main house panel on phase A, travel to the garage sub panel and to a load within the garage. A short takes place between the phase A wire and the metal case of that load. Current flows to the case, and along the green wire EGC to the ground bus of the sub panel. From there it flows over to the neutral bus of the sub panel (using the N-G bond as the path). Finally, current returns to the main house panel along the feeder's neutral wire. That will result in a trip of the feeder breaker to the sub panel. As ActionDave mentioned, this was allowed in an earlier code. But it required there to be no metal components that are common to the house and to the garage. If there was a metal water pipe connecting the two, then you would have a parallel path for neutral current to flow along that pipe.
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A sub panel is required to have a separation of the neutral and ground as well as the N-G Bonding screw of the panel removed. So in a situation like this that should not happen? As doing so creates this issue of safety as well as compliance
 
I would look at the situation and difficulty of running a new feed if I were the inspector. I might allow the existing feed if, as mentioned, there were no metallic paths between the buildings. I would also require the neutral to be bonded in the panel with the GEC landing on the neutral bus. Basically the same as a service. But as is, if the neutral is floating, I wouldn't allow that.
 
To answer the OP directly, as it is now, the bonding screw should immediately be installed (after de-energizing the feeder.) That would be far, far safer than leaving the installation unbonded as it is now.

As for using an existing feeder to a new structure, I think a fair way to look at it is if the original structure was effectively demolished and the feeder and panel were effectively left standing, and the new garage effectively built around it, it can be used to supply the new wiring.
 
Braden8898 said:
A sub panel is required to have a separation of the neutral and ground as well as the N-G Bonding screw of the panel removed.
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It is required now. But in the earlier versions (pre-2005???) of the NEC, when it was allowed to run a feeder to a separate building without including an EGC in the feeder, you had to bond the N and G at the remote panel. Otherwise, there would not be a return path for fault current.
 
Braden8898 said:
A sub panel is required to have a separation of the neutral and ground as well as the N-G Bonding screw of the panel removed. So in a situation like this that should not happen? As doing so creates this issue of safety as well as compliance
Click to expand...

You are correct that the garage panel (as inspected) overcurrent protective devices (OCPD) won't have enough fault current flow on a Line to Equipment Grounding Conductor fault, UNLESS there are other metallic pathways back to the house (is there a water line, gas line, compressed air pipe, telephone line, internet cable, TV cable, intercom wiring, Satellite cable, photovoltaic hookup, security system wiring or house doorbell wiring, or etc. ? ? ?).

IF there is a metallic path (other than the subpanel feeder conductors), and that metallic path is connected to metallic surface(s) that have a Branch Circuit Equipment Grounding Conductor connection, if the metallic path impedance is, by chance low enough, then the subpanel OCPD will trip, clearing the fault. This is not designed by the NEC to be an "Effective Fault Clearing Low Impedance Path".

Placing a Main Bonding Jumper in the garage subpanel provides a Ground Fault current a path (on the three wire garage feeder neutral) that IS effective in a fault situation in clearing the OCPD.

However, placing the Main Bonding Jumper allows normal neutral current to split off onto any present, or future, other metallic pathways,

Now, requiring the existing garage feeder to be upgraded to four wire will keep the neutral currents in the insulated conductor path, and Ground Fault current would have a low impedance path on the newly added fourth feeder wire, the EGC.

Whether the existing three wire garage feeder is allowed to be reused is a judgement call for the local AHJ.
 
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