Adding ground wire to sub panel

Question

I have a service drop 50’ from my cabin. 2 runs of 1/0 aluminum underground wire (3 wire system) were run underground in 2” PVC conduit pulled in to a junction box and then one run was wired into the the main panel which in reality is a sub panel.

There is no ground wire back to the service drop which has the meter base and 2 100A breakers (one of which is powering the sub panel), along with a grounding electrode(2) system. I am only using one run since plans have changed.

It’s too much work to pull out the 2 runs of wire to pull in a ground wire. Can I use one of the wires from the abandoned run as a ground for the new panel since it has no ground back to the service drop?

So my question is basically, can I use an aluminum conductor as a ground wire back to the drop?

Answer 1

Let's see. You have a service and meter out on a pole, a service disconnect out at the meter which contains two 100A breakers (I'm alright with that). The enclosure with the two 100A breakers is UL-listed, but not for paralleling.

Then you have six 1/0 wires going to the cabin all in the same conduit.

Yes, with wires #4 or larger you can redesignate any wire to any purpose simply by marking with tape - green or yellow/green for ground, white or gray for neutral, anything else for hot.

So you can re-designate a 1/0 wire for ground.

The original use of a double feeder was improper - only one feeder is permitted per structure. Since this is a single feeder now, the thermal derate for multiple circuits goes away, and the wire has 120A natural capacity. However, since these wires carry the entire service to a dwelling and nothing else is powered, you are entitled to a favorable derate in NEC 310.15(B)(7) - the wire need only carry 83% of feeder/service size.

So... If you declare this to be a 145A feeder, a couple things happen. First, the 83% derate applies, and your wire only needs to be 120A ampacity (which it is). Second, on a feeder the "round up rule" NEC 240.4(B) kicks in" since 145A breakers are not made, you can use a 150A breaker. However you cannot plan to load the feeder above 145A.

Now 150A breakers are double-wide, costly and not available for every make of panel. But 125A breakers are. So you can easily go to 125A.