I was wondering if anyone could explain the need for a four wire feed to a separate building apart from the house. In the case of the house I looked at today there was a separate garage with a subpanel fed by a three wire overhead feed. The grounds and neutrals were separate. To my knowledge there wasn’t any water or gas line going out to the garage. This should be a four wire feed, correct? And wasn’t this a relatively recent change to the code?
Sub panels require a 4-wire feed. 2 hot, neutral and ground. If only a 3-wire is present, a separate isolated ground can be used at the sub panel location. (i.e. grounding electrode with ground conductor)
Up until the 2008 NEC code cycle 3 wire feeders were permitted if there were no metallic paths between the two structures. From 2008 and beyond a 4-wire feeder is required. If there is an old 3-wire feeder you must bond the neutral just like in a service and connect the GEC from your grounding electrodes to the neutral.
With a 4-wire feeder the EGC’s and neutrals are separate and the GEC goes to the EGC bus which is bonded to the enclosure, the neutral is left floating.
You’re welcome, and depending on when a jurisdiction adopted the 2008 NEC you might even see this on new installations beyond 2008 which may have been code compliant when installed.
Excellent clarification. Thank you everyone. This one did have a separate ground from the panel. It looked fairly recent but I don’t know when it was done exactly.
Actually they cannot be. If you isolate the EGC and neutral there will be no return path for the current to open an OCPD and the system will be ungrounded.
If you think of it like a service the reason that a ground fault to an EGC trips the OCPD is because the EGC’s and the neutral are bonded together. With a 3-wire feeder you would also have to bond the EGC to the neutral for a ground fault to open the OCPD. For separate structures this was permitted until it changed with the 2008 NEC.
Also note that a detached structure requires a grounding source (ground rod) whether it is a 3 or 4 wire remote panel. The ground rod can not be used as the only source of grounding and must be tied to the neutral if only a 3 wire feed. Best to upgrade to 4 wire but how many of us would do that.
2017 NEC would be the basis for my recommendation that it does not meet current safety standards. I dont care when it was built, wrong is wrong. My job is to point it out not to decide if it will or will not be repaired. I take this position because like Dan we have areas that dont enforce anything code wise.
I think that it’s more of a question as to whether or not it’s unsafe. Unless a parallel metallic path has been added between the structures IMO it’s not unsafe.