Will a 125 Amp wire fit through a 3/4" conduit? Thanks

There are no 125 Amp wires. There are copper, aluminum, and alloy wires of different diameters.
You can use a #2 copper wire for 125 Amp.
;)

Office Lady,

Okay, I take a shot at this one. What are you going to do with a cable that carries 125 amp? Do you need Black, White, Red, Green to be inside the cable?

I can take a look in the NEC code book and see if there is such a beast, but I'd really like to know how it is going to be used. For example, if the unit is going to supply an X-Ray machine that you expect to run more that 3 hours a day, then your line needs to uplifted by 125% to get the correct Amperage.

We need the voltage level. We also need how you are going to get the supply to put on this conductor. What size breaker are going to use.

We are not trying to bore you with nit-picky questions. Just as a passing, with the permits and planning that would be needed why haven't you gotten several estimates from local licensed electrician.

Office lady,

What type of conduit are you speaking about? EMT (Thin-wall steel) or schedule 40 PVC? Its important because there is a difference between the inside diameters.

How did you measure the diameter of the conduit? Inside diameter or outside diameter?

As to the cable, is it single conductor or a three or four conductor? Are the conductors solid core or filament based (better to work with). Can you get me any type of specs. On the cable or the vendors web address?

If this is truly a power cable, then we need to know the needed voltage and the number of conductors needed to connect to the device. For power at 125 amps, I would expect that you would need at least a Three (Black, White and Red) #2 AWG and a #4 AWG Green ground wire.

The sizes of conductor also vary because of the outer shell of insulation.

For example one copper conductor size #2 (THW) has a diameter of 0.412 inch. Your 3/4 PVC conduit has an inside diameter of 0.760.

As you can see, if you only need one conductor, the cable should fit, however if you actually need 3 or 4 conductors, the diameter of the cables would be 1.7 inches. Which will never fit into the 3/4 tubing with the clearance required by the NEC