Sub panel bonding

marc216 · 11-30-2012, 10:19 PM

Just wondering, with all that I understand about wiring remote panels, mostly 4 wire, what makes it possible that a 3 wire is allowed to have the neutral/ground bonded to case? Under what circumstances is this possible?

mpoulton · 11-30-2012, 10:51 PM

It is no longer code compliant under any circumstances. As the diagram above says, the 2008 NEC removed the use of 3-wire feeds for anything other than a service. 2011 did not change that.

Stubbie · 11-30-2012, 11:44 PM

Well since I'm the author of the drawings I suppose it is prudent that I reply ...

To add to mpoulton 3 wire feeders were the norm to detached buildings for years and are just fine if installed properly. The feeder neutral just did double duty as 'neutral' and 'equipment ground'. The utility only brings 3 wires to your service equipment, the difference is the home owner will never likely fool with those wires.

One issue was other metallic paths existing or being brought to the building at a later date. Bonding neutral to ground allowed neutral current to use these other metallic paths to return to the transformer. Mainly because of improper grounding at the detached structure.

As an example see this drawing

marc216 · 12-01-2012, 12:03 AM

Quote:

Originally Posted by stubie

Well since I'm the author of the drawings I suppose it is prudent that I reply ...

To add to mpoulton 3 wire feeders were the norm to detached buildings for years and are just fine if installed properly. The feeder neutral just did double duty as 'neutral' and 'equipment ground'. The utility only brings 3 wires to your service equipment, the difference is the home owner will never likely fool with those wires.

One issue was other metallic paths existing or being brought to the building at a later date. Bonding neutral to ground allowed neutral current to use these other metallic paths to return to the transformer. Mainly because of improper grounding at the detached structure.

As an example see this drawing

Your drawings are very helpful, great work. So essentially now, 4 wire is the only way providing we dont have EMT, water etc... between the two structures. We still have a separate GES, effective grounding path e.g. steel building, G-rod etc...with the detached structure correct?
What "if" there is an attached metallic element between the two structures e.g. EMT. Then what happens?