What is the minimum alum. Conduit size for 10-3 wire ?

What kind of 10-3 ? This is very important.

Are you sure about AL conduit? That is very rarely used.

Also, why are you running cable in conduit?

10-3 separate copper wire in alum conduit
Running in garage outside of the wall.
To supply power to a cloths dryer.

Well to start with, I echo Stan's question on Al conduit.

The better choices are RMC or RNC conduit.

We need to know the type of insulation of the cable. Different insulations have different thicknesses.

No use to me

Dbunch,

I saw your disagreement with my posting. You are not helping yourself by not supplying information.

All of the information you require can be found in the NEC Code. I'm using NEC 2008.

Annex "C" provides conduit fill information by Insulation Type and Conduit size (providing all of the conductors are the same size) If they are different sizes, then an entirely different process must be used. Table 310.15 (B) (2) (a) provides the adjustment factors for the number of current carrying conductors in a conduit. Table 310.15 shows the correction multipliers for heat related corrections.

Some examples:

RMC - conduit has thicker construction, meaning that it will hold less conductors.

RNC - (Sch. 80) Is thinner than RMC.

RNC - (Sch 40) Is much thinner than RMC or RNC and not as strong.

Do you need the conduit for above ground or below ground. Different types of conduit have to be used.

I already gave you a response for the type of insulation on the conductors.

If you just want a generic answer then:

For type THHN/THWN/ THW-2 you can put six # 10 AWG conductors into trade size 1/2 RMC conduit.

For type THHN/THWN/THW-2 you can also use 6 #10 AWG conductors in Trade size 1/2 RNC (Sch. 80).

For RNC (Schedule 40 PVC) you can use 5 THHN/THWN/THW-2

Then you must correct the ampacity of the conductors by 40 % if you have 4 to 6 current carrying conductors in conduit. For example if you have six Current carrying conductors (Neutral conductors carry current) you would need to correct the ampacity. 20A * 0.8 = 16 Amps. That means that these conductors can only serve devices that use 16 or less amps.

If there are only 3 current carrying conductors then you do not adjust the ampacity.

Also, if you need to run these conductors and conduit in a hot area, then you need to adjust the loads further by using the table at the bottom of T 310.15. Knock yourself out. Oh and by the way, you did not give a distance on the conductors so we have no idea if the cable size has to be adjusted for voltage drop considerations. But you know best.

dbunch, thanks for disagreeing with my questions. I am not sure how that helps the situation.
Does that mean you don't know the answers, or do you want us all to guess at an answer to your cryptic question?

I'd say 2" is plenty big enough. How's that?

Don, you post is very helpful, but the OP asked about "10/3", which implies cable, not individual conductors.

Stan,

Yes, that is correct, but I tried to read his mind telepathicaly and found that he really wanted individual conductors inside the conduit.

Also, he wants the inside of the conduit filled with foam to protect the conductors. Oh, did I mention that he plans on using plumbing conduit because white looks so much better than gray!

:)