Why do I see low voltage in a repaired underground cable?

Question

An excavator accidentally pulled and cut 2 underground cables that power up our pool shed (light and pool pump).

I fixed one wire that powers the lights but I have trouble fixing the other one that powers up the pump. And this wire is more important because it's 12AWG and delivers 20amp needed for the pump.

The issue I am having is this:

Hot to ground delivers only 30v.
Hot to neutral delivers only 30v.
Neutral to ground delivers 0v.

And when connected to an outlet, hot to neutral/hot to ground delivers 5v.
I checked the cable connection in the main panel. Everything looks in place. I even move the cable to another breaker, thinking that the old one was no good. Same thing. 30V and that's it.

I thought that it's a phantom voltage and the cable is completely down. Yet, when I connected a GFCI outlet, the light in the outlet turns on, it signals red and is very bleak. The point is that there is current going through the cable but the voltage is extremely small.

It looks like a loose hot wire but there can't be any lose connections, because the wires are directly connected to the breaker and the connection is solid. There is nothing else between the connection at the breaker and the cut end of the cable.

I am puzzled. Has anyone encountered anything similar?

Answer 1

The wires are likely more damaged than you realized. For example, the excavator may have pulled the wire for a few seconds before it actually broke, resulting in hidden sections that are almost broken - "hanging by a thread". That hidden damage can result in a high-resistance connection which could result in strange readings and, more importantly, a very dangerous situation.

Even inside walls, it is generally recommended that you replace broken wire segments rather than splice them. Underground? Always replace the entire damaged section. The "entire" damaged section means from one junction box (house) to another (pool). You don't get to dig a foot in each direction and cut off the old messed up cable and replace just those few feet. It doesn't work that way. There are certain types of splices available for in wall use (i.e., not "accessible"). I would be very surprised to find any splices certified for in ground use.

I suspect your original wiring was direct bury cable. The general rules are:

• 6" for individual wires in rigid metal conduit
• 18" for individual wires in PVC conduit
• 24" for direct bury cable (the cable must be rated for direct burial - you can't use the indoor Romex)

Personally, I prefer metal all the way. While 24" does indeed get cable out of the way of most excavation work, I like that very clear indication a metal conduit provides compared to the "was that a tree root...no, I just dug through the pool cable..." situation.

Whatever you replace it with, you must do a full replacement from the wall of the house (unless there is a legitimate accessible junction box in between somewhere) to the junction box near the pool.

Also critical for safety: If this circuit is not protected by a GFCI at the source (e.g., at the breaker in the house panel), you could have an extremely dangerous current flow through the ground. Not normally an issue - usually the GFCI will be outside and protect the pool area just fine - but definitely a concern here. TURN OFF THIS CIRCUIT UNTIL YOU FIGURE IT OUT.

Answer 2

If you're measuring those voltages with no load applied then it isn't a problem of damaged wire having a high resistance (being "almost broken"): with no load current flowing there can be no voltage drop, so you'd measure full mains voltage even if a conductor had been stretched thin. More likely is that there's a problem with the splice itself, or a broken conductor in another location you haven't discovered yet.

Disconnect mains power and measure the end-to-end resistance of each conductor. It might be helpful to connect a receptacle at the shed, and then an extension cord, so that you can bring the other extension cord end around near the breaker panel. This will enable you to virtually reach both ends of the buried conductor at the same time. The end-to-end resistance of each conductor should be less than 1 ohm and resistance between conductors should be infinite.

If you find anything different, re-excavate, open up the repair splice, and measure resistance of each conductor to that point from both ends. This will help you determine whether the splice itself was faulty, or whether there's additional cable damage in either of the house-to-splice or splice-to-shed segments.