Why do I measure high and low voltage on a shop panel circuit?

Question

I have power running to my shop, shop panel has a 100 amp main breaker, voltage on one leg reads 109 volts. The other shows less than 100 volts. If I turn 1 20 amp breaker on in the panel I then read 245 volts on one leg and 1volt on the other leg, as well as on the 20 amp breaker. I am far from an electrician so I'm at a total loss as to what my issue is and how to fix it. Service comes from a double pole 60 amp breaker from the house. Any advice would be appreciated. Was like this when bought the house a yr ago, and was working fine(it seems) until last week. Thanks in advance.

Answer 1

Watch your tail! The grounds in the shop will be energized!

This. This is why 3-wire feeders were outlawed in NEC 2008.

Anytime a neutral wire breaks, it is pulled up to the voltage of a hot wire. In a 3-wire installation, it's also pulling safety grounds up to hot voltage, because neutral and ground are bonded in the subpanel.

The only thing that tries to bring this back to earth is the grounding rods, but based on the voltages you measured I suspect some short cutter didn't bother with those. Well, if you get between anything 'grounded' (HA!) and the actual ground, you become the ground rod, an experience that will kill you.

Even if the ground rods were intact, dirt isn't a great conductor (that's why we bother to mine copper and aluminum). So your loads will often win that tug-of-war and energize your grounds anyway. This is why a 4-wire feeder is a Code requirement now.

A neutral wire connection went bad

Wire problems are almost always at the ends of the wire, i.e. the terminal or at least inside that junction box. People often "psych themselves out" into believing wire damage must surely be along the wire in an inaccessible location, thus unfixable. Usually untrue.

That said, I won't discourage you from replacing a 3-wire feeder with 2-2-2-4 aluminum. That will give you 90A instead of the 55 or 65A you have now, won't be costly, and most important, will give you separated neutral and ground! Aluminum is fine at these large wire sizes, the lugs are aluminum anyway.

So go to each of the endpoints of the neutral wire - the main panel, the subpanel, and any intermediate splice boxes. Power down the circuit, undo the connection completely, clean the wires, inspect everything, reapply No-alox "goop" if either wire or lug is aluminum, and reinstall and torque the connections to specification.

Torque matters; and science recently showed it matters on small connections also! Torque screwdrivers are very nice but overpriced, but beam-type 1/4" drive torque wrenches are "0-100 inch-pound" range, affordable, and never need recalibration. Amazon even has a $20 one that fits in the palm of your hand (though it makes you convert from in-lb to n-m).