I want to run a feeder line from one building to another in a 2 in. underground conduit.about 100 ft. run what size alum. Wire will I need for 125 amp sub panel

If they are not on the same property, you can't do it.

If they are on the same property, best to use 4" PVC (versus 2"). I would suggest of the top of my head,3C 3/0 but you'll have to check voltage drop (or be safe and go with 3C 4/0). Also watch trench depth (deeper over travelled areas) and make sure it is marked.

Assuming the typical 120/240 volt single phase service, #2 XHHW aluminum, will be fine. This also assumes the 125 Amp circuit is loaded to the max allowed of 80% of the full circuit rating, which is 100 amps total max load.

You can use three #2 XHHW, for 2 hots and 1 neutral, and plus,either a min of #4 Al or #6 copper, colored green for the equipment ground.

Using #2 Al , 100 feet, with a 100 amp load, will result in a voltage drop of 6.38 volts, or 2.65 %. The max recommended allowed Vd for a feeder is 3%, so #2 is fine as a minimum size aluminum conductor.

You certainly can run larger wire if you like, but if money is tight, my design will pass inspection, and be safe.

If you like, PM me so I can show you how this calculation is done.

The conduit needs to be at least 18" below grade generally, at least 24" if running under a road or driveway.

Best to trench 6" deeper to lay a bed of sand first, lay the conduit, and then another 6" of sand. This is recommended for areas with rocky soil conditions. 12" below grade, lay the warning tape, that bhayne mentions, the entire length of the conduit, before final backfill to grade.

Be sure to terminate the Aluminum conductors according to the manufacturers instructions, which calls for wire brushing and applying an anti-oxidant compound at each terminal.

Bhayne, I realize that your Canadian code is somewhat different that the NEC we use here in the states, and I am not familiar with your code at all. Allow me to explain my point.

I would had preferred to discuss this with you here in the forum, rather than through comments, as I would appreciate a good code discussion with a brother electrician,but here goes,

I am not using the 80% rule to size these conductors, directly.

I am applying the 80 % to the rating of the overcurrent protection of the circuit, which has been stated by destroy to be 125 Amps,so as to arrive at the maximum allowed current on the circuit.

This then gives me the maximum possible load in amperes, since the amperage is one variable that is needed to arrive at the total voltage drop, by using the formula:

Vd= Length x Volts x Amps x Resistance per 1000 foot of wire chosen / 1000

To arrive at the minimum size circuit conductor, however, here in the US, we must , generally speaking, increase the load in current by 125% . So, using this poster's info as an example, I derated the load on the 125 Amp panel/circuit by 80% to arrive at 100 Amps.

And then used the 100 Amps load to calculate the voltage drop, having the length known, and choosing a reasonable size of aluminum conductor, to arrive at what the Vd% would be, to see if it falls within the maximum recommended allowance of voltage drop.

That is how I arrived at the answer I gave

Now, if I knew first that I had a 100 Amp load, and needed to size the circuit conductor, the conductor shall be rated 125% of the load:

100 Amps x 1.25 (125%) = 125 Amps

Which, not by coincidence, is 80% of the rating for overcurrent protection device.

So, in this case I was not using the rule you were referring to, however, in the US, conductors can be sized by derating 80% in lieu of the 125% rule, only when the 125% increase applies.

So, your disagreement is incorrect, at least here in the states.

The Canadian Electrical Code and NEC are basically made of identical information arranged in a different order.

I am also a professional member for the State of Washington Department of Licensing for Engineers and Land Serveyors. I hope that you have not been making a general practice of derating conductors to 80% of protection devices for 30 years. A 125A breaker is designed to allow 125A (and probably more) of continuous current. If that is flowing in a conductor which is not designed to handle 125A, you'll have heating and further voltage drop giving the load a faulty power supply.

Is the subpanel 125A or 100A? If it is 125A and is protected by a 125A breaker then I am right. If it is 125A panel and is protected by a 100A breaker then the conductors can be smaller (however, not as small as you have indicated because they are randomly spaced in conduit and must be derated for a 3C run).

Actually, for direct bured conduit, I would hesitate to use single conductors and go with 3C teck cable for the added protection- especially if not using steel conduit.

I think there is a bit of confusion.

First I never said I was derating conductors to 80% of protection devices, so I am not sure how that is the understanding.

I do have some questions thou:

Can you provide the NEC article and section that allows overcurrent protection devices, such as you example that a 125 amp panel/breakers can handle 125 amps?

Can you also show where 3 conductors in a raceway needs to be derated?

How is that wires Randomly placed in a conduit a reason for derating?

Why 3C teck cable? This is expensive 600 volt mill and 5000 volt cable.

When is 80% derating applied?

I hope to gain a better understanding of your perspective and possibly learn something.