Rich Grise said:

Don't forget the 900' of pipe running parallel to the hiline upstream

of the transformer.

My readings of the OP's description gave either 1000', 100' or 900'

of parallel pipe and power. My remarks apply to any of them. For

sake of discussion, suppose the power flows thru a pair of wires,

one located 11 inches from the pipe and the other 12 inches. (an

assumption corresponding to Romex with about 2 inch diameter)

For the moment, assume they maintain these distances along their

length. (no twisting) Assume a current +I at 11 inches, -I at 12.

The magnetic field in free space encircling one wire at radius r is:

B(r) = u0 * I / (2 pi r)

for an infinite straight wire where u0 = 1.26e-6 and r is in meters.

(Except near the ends of the run, this *closely* approximates the

situation given the assumed geometry.)

Evaluating for +I at 11/39.37 m and -I at 12/39.37 m, yields

B = (1.26e-6 * I / (2 * pi)) * (39.37 / 11 - 39.37 / 12)

B = I * 5.98e-8

Considering I to be the peak value of a 60 Hz AC current, and

so taking B(t) = sin(2 * pi * 60 * t) * I * 5.98e-8 , then

d(B(t))/dt = 2 * pi * 60 * cos(2 * pi * 60 * t) * I * 5.98e-8

= I * 2.25e-5 cos(...) Tesla/Second

This converts directly to Volts/Meter induced along the pipe.

(Doubters can start from F = q V x B, noting that the B field

changing in and out w.r.t. the wire pair is equivalent to the

V x B term. This is too basic to be spelled out here in detail.)

Taking the parallel run as somewhere between 30.5 and 305

meters, the induced peak voltage is 685 uV to 6.85 mV per

Amp of current flow.

With any twisting, that number will go down. If a twist were

applied to one end and the intermediate cable were follow in

proportion along its length, the coupling would be multiplied

by sinc(a), where a was the twist in radians.

Unless they are using thousands of Amps at that construction

site, the 30 VAC observed by the OP clearly has to be due

to some other effect or a markedly different geometry.

Some may opine or proclaim that only the integral form

of the induction law can be validly applied, or that twist

could be non-uniform, or the spacing may vary in that

trench. Except for the latter, none of it changes the

result because my use of the differential form already

assumes the worst possible return path (whereas the

real return path is coincident with the pipe, reducing

the net coupling between earth and pipe). Twisting

of any kind can only reduce the coupling; uniformity

simply causes more reduction, on average. If the OP

buried separate conductors willy-nilly with respect

to the pipe, that could be a problem. In Washington

state, that would never pass muster with the inspector

and I doubt local codes on this issue vary much.

If the OP's problem arose from mixing the power and

pipe badly, the result shows (in part) why that code

exists, and he has some digging to do. But I think it

much more likely that there is an exposed portion of

a hot power conductor leaking *a lot* of power into

ground and the pipe. That is an unstable situation,

dangerous as I've stated, and one the power company

will want fixed, if only because their meter is going to

be downstream from a ground-heating fault.

Given the OP's assertion that the 1000' pipe is buried

in red clay, I don't see any way for it to be anything

but a fault. Driving a well grounded conductor like

that to 25 VAC would take more power than even

an accidental air core transformer could deliver.

You bet!