Step 1:Figure out if all conductors (including neutrals and grounds) are the same size, if they are, then you only have one step. 

All Conductors Same Size  Use
Appendix C. to find
out how many conductors can be in a given size of conduit. Some of the tables are as follows: C.1 EMT C.3 Flex C.7 LFMC Liquidtight Flexible Metal Conduit C.8 RIG C.9 PVC sch80 C.10 PVC sch40 Make sure you use the correct table for the correct conduit type and match the insulation correctly. Thats it, just one step. 

Step 2:Find the total cross sectional area of the wires 

Mixture
of different sizes or types of wires

Use the Table #5 of
Chapter 9 to find the cross sectional
area of each type of wire and multiply by the number of wires. Add them up and find the total cross sectional area of your wires 


Step 3:Find the smallest conduit that can fit your wires. Use Chapter9 Table 4. Each type of conduit has a different internal diameter. You can fill the conduit up 60%, 53%, 40%, or 31% depending on your application so be sure to use the correct column 

1 Wire  Normally you would use Appendix C, but just in case you wanted to know, you can fill your conduit up 53%  
2 Wires  If you have 2 wires then you can fill the conduit up 31%  
More then 2 wires  If you have more then 2 wires then you can fill the conduit up 40%  
Conduit less then 24 inches  Considered a nipple, you can fill up to 60%  
In this example a 2 in EMT can handle the wire or a 1.5 in EMT nipple can barely handle it. If you need PVC sch 80 then you'll be using 2.5in. 
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