What is the cause of this voltage difference between my breaker panel and AC unit?

Question

I've got a 3ton AC condenser unit on my roof, with a blower system nearby in attic space supplied by 20A 240v circuit.

• The condenser unit is supplied by a 40A 240v circuit, 2 8ga wires
• The blower unity is supplied by a 20A 240v circuit, 2 12ga wires
• Both circuits are run through the same steel conduit from my outdoor breaker panel.
• There are no ground wires in the conduit.
• The conduit is a little corroded in places
• The total span is about 45'
• Measuring voltage in the panel, I get 118v to the ground bar on each wire and 236v wire to wire on each circuit.
• Measuring voltage on the roof (with the conduit completely disconnected from the AC unit, so only connected to the panel), I get 95v between the wire and the conduit on each of the 4 wires, but I still get 236v wire to wire on each circuit.

Why would there be so much voltage drop? It's like the conduit at the roof is at a higher potential than at the panel. Do I need to replace the whole run with new conduit, and circuits with a shared 8ga ground wire? Or run a new ground through the conduit? I can't see being able to easily do that. What size conduit do I need to carry those wires? Thanks

Answer 1

If you think the conduit is not providing an adequate ground-fault current path, you can run a separate grounding conductor from the panel to the A/C unit. Current code no longer requires you to run the grounding conductor with the other circuit conductors. So you can run an appropriately sized grounding conductor any way you'd like.

If it's easy enough to do, you could run the grounding conductor along the outside of the conduit. If that's not possible, you could simply run it up the side of the building (providing protection if necessary) .

Answer 2

Conduit is a perfectly acceptable grounding path

EMT conduit is a valid grounding method. I would be surprised to see a ground wire inside EMT - nothing wrong with it, just would be "belt AND suspenders".

However, it may be appropriate if you are in weather conditions where the EMT is rusting - it's possible for the clamps/bonds to rust out and give you a poor grounding path. You can go with green stranded wire (which is easier to pull) or bare solid wire (which is harder to pull, and will make future pulls harder, but is better under "conduit fill rules".)

When I say "pulling" I mean most people run a stiff "fishing tape" down and use it to pull a wire through. I usually don't bother, or don't want to push my metal tape into something hot. So I just push the wire down the conduit and do a whole lot of coaxing and sometimes a little light disassembly of a conduit joint. Intermediate access points are very helpful, e.g. that junction box you mentioned.

Since you disconnected the conduit from the A/C unit, you have broken the grounding path if it was good before. If you had a ground fault, it would be "pulling the chassis toward one pole or the other" and the two hot-chassis measurements would total ~240V. However since it is not, and they are equal, I would say you are detecting the weakness of your grounding path.

I would not tolerate that for a minute; disassemble and wire-brush it as much as you can, and consider running that ground wire. #10 should suffice for all circuits, and hardware stores sell bare #8 solid. I would use green stranded #10.

You can also replace rusted sections of the conduit, but that requires pulling the wires out.