On Sat, 26 Dec 2015 00:58:19 -0500, wrote:

Clare pegged it, You haver a loose neutral. Try checking it at the
main lugs. If it is bad there call the PoCo. If it is bad at your sub,
investigate your neutral connections or look for a serious load
imbalance.

It's possible there is a neutral problem, but you could just have a pole
transformer that is not exactly center tapped. There could also be some
load on one leg, pulling down the voltage. Measure the voltage at the
MAINS on both legs with everything turned on. Then do it again with
everything turned Off.

If it's still unbalanced with everything OFF, it's just a transformer
that's a little unbalanced. Not to be worried about, if that's all it
is.

You did not say what the device is, which is using 23A, but you'd be
wise to run #10 wire and use a 30A breaker for that device. It would
help of we knew what it is, because a motor load can vary from when it
starts up to when it's running, while a resistive load, such as a
heating device has a pretty continuous power draw.

And if it's something like an air compressor, they can use a bit more
current after the tank gets full, causing a heavier load on the motor
(and when it starts in cold weather). I had to repair some air
compressor wiring for a guy who said in the summer it worked fine, but
in cold weather, it tripped the breaker. It was obvious that the cold
compressor would not allow the motor to get up to full speed when it
first started. The solution was to either move the compressor into a
heated space, or use a bigger motor and upgrade the wiring to a heavier
gauge wire and bigger breaker. A motor was costly, and the wiring
upgrade even more money. He said he could not afford to heat his garage
all the time. I suggested moving the compressor to his basement and just
running a longer air hose to his garage thru a small piece of pipe in
his foundation, and cauking around it, or even putting an air chuck on
the foundation wall.

He decided to move the compressor to the basement, so all I had to do
was run a dedicated outlet for it from his panel. He said he's buy more
air hose and just stick it thru a basement window because it was just
for occasional flat tires.