Electrical project

I am working on a project. I have much 3/4 PVC and fittings conduit already in possession from a salvage job. I cannot interpret the rules about current ratings. I found charts for conduit fill. It says up to 15 current carrying conductors for 12 guage wire. I want to mix some 8 and 10 and 12 guage in some places. How do I determine how many amps the conduit can handle?

In my mind 14 seperate 12g ccc would be 7 circuits, each with a 20 amp breaker gives me 240 amps of load. 8 separate 10g ccc would be 4 circuits with a 30amp breaker is 120 amps of load. Is this the correct calculation or am I out in left field. The 240 amps seems way high for a 3/4 conduit?

You are kinda out in left feild . I actually have an electrical contractors license ,but I don't know how to even start to answer this one.

Right knoiw your only concern is the fill. Calculate the circular Mil area of each conductor and don't exceed the percentage of fill allowed.This is done when various conductor sizes are mixed.PVC runs take expansion joints. How long are your pipe runs ?

This is actually indoors but high humidity so I am using the PVC. Runs are short. 6-8ft between junctions. Its climate controlled so expansion/contraction will be minimal.

Is there a chart to give the circular mil area of each size of THHN wire?

Just to help me comprehend, How many 110v circuits using 12g wire and 20amp breakers would be allowed in a 3/4 PVC.

Thanks for your help.

It does not sound right. Best to ask your local building inspector before you start, especially if it needs to be inspected or insured.

The tables in the back of the code book will tell you the allowable area of 3/4" PVC schedule 40 and 80. In another table it will tell you the area in circular mils of the wire depending on insulation type. If you tell us how many wires of what size you want to run we can probably figure it out and explain it better using your numbers.

On the longest run I want to run 1 110 v circuit with 12g and 20 amp breaker, a 220v 20 amp with 12g, and a 220v 30 amp with 10g wire. 25-30ft from breaker to outlets.

3/4 conduit inside dia is .824. Square that is 696,280. 40% of that area is allowable since more than 3 CCC leaves 278,512. Is this the correct math?

You will have to de-rate the ampacity of the conductors based on how many are in your conduit. Over three conductors will require de-rating according to the NEC. Without knowing how many conductors are going to be in your conduit, I can't give you the de-rating factor.

your conduit doesn't care how many amps are in it. All you need to worry about is fill and how to derate the ampacity of the wires accordingly.

Yes. I have found some good info online now that I know the terminology used in the process.

First, let me state I am NOT an Electrician, I'm a BUBBA, - but it seems to me that you are trying to make this 'way too complicated'. You state that you have lots of PVC pipe & fittings that you've salvaged, - so why not run a separate 'dedicated conduit' for each circuit; since as you have stated, you will be having both 120 & 240 volt circuits.


:>)

I think what you are trying to get at is how many current carrying conductors can you put in a conduit? The fill chart you found just says how many will physically fit......but there are other considerations......

The 2014 code book (just what is sitting close here) says up to (3) current carrying conductors (this includes neutrals) at 100% rating. 4-6 at 80%, 7-9 at 70%, 10-20 at 50%.

Basically, the more CURRENT CARRYING CONDUCTORS you put in a conduit, the bigger the wire. Now, if you use #12 wire on a 15A breaker, you have effectively de-rated it.

Generally speaking, this is not a huge issues in most installations (although it can be)

IF these circuits are for general purpose receptacles, the likelihood of them all being operated at "capacity" is small.

If these circuits are for dedicated appliances/machines/equipment that will operate at or near capacity.....you better look at it.

I have seen a guy TRY to run a 2" PVC through a hog barn and pull EVERY branch circuit through it (trunk line with lots of boxes) Most of the circuits were loaded at capacity......when that mess was over, the pipe came out with the wire in it still......all melted together in one gigantic mess.....

OK, a #12 has the area of .005 cubic inches, a #10 has the area of .008 cubic inches. You will need one #10 ground and two #1) hots for a combined .024 cubic inches. You will also need one #12 neutral and three #12 hots. This means using the #10 ground to ground everything. So five #12 at .005 gives you .025. Add that to .024 for a total of .049. These figures bare from table 8 chapter 9. In table 4 it lists 3/4" schedule 40 PVC allowing .203 cubic inches if there is over 2 wires. You should be ok.

I didn't have the heart to tell him about the derating. Hey if you keep your pipe nipples less than 24 inches you don't hjave to derate.LOL

I goofed and used the wrong table for the wire. Everything will work with what you have

I can derate the 220v 20amp to a 15(lights and outlet). I can derate the 110v 20amp to a 15(small heater). My 220v 30 amp needs to stay(large heater). Will this setup work safely?

Doing it like gregK described above.

Thank You for the help.

Nathan, you'd be fine with what your talking about.....no issue