I need to maximize the amperage that I can get into a 1" pvc conduit for temp power at a job site. I can fit (3) #2 THHN into a 1", but would have no room for a ground. What is the formula for the calculation to downsize as much as possible the neutral so that I can also pull a ground?
How much can I derate a neutral to fit conductors in an existing conduit?
- Thread starter chappy707
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- Lockport, IL
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- Semi-Retired Electrical Engineer
It is not a "formula," but rather a "load calculation." See article 220.61 for possible reductions in the calculated neutral feeder load. Once you know the total neutral load that you must take into account, you can select a neutral wire size to handle that amount of current.
You might be able to learn something useful by "reverse engineering" the neutral size. Take the internal cross-sectional area of your conduit, calculate 40% of that value, subtract out the cross-sectional area of the ungrounded conductors and the equipment grounding conductor, and that will tell you how much room you have for the neutral wire. You look up the conductor sizes in Table 8 (back of the NEC), and this gives you the maximum size neutral wire you can fit into the conduit. Going next to Table 310.16, you will get the amount of current you can pass through the neutral (i.e., as unbalanced load). Finally, you can compare that value to the known load on the ungrounded conductors, and see what percentage of a reduction you need, in order to make this project work.
You might be able to learn something useful by "reverse engineering" the neutral size. Take the internal cross-sectional area of your conduit, calculate 40% of that value, subtract out the cross-sectional area of the ungrounded conductors and the equipment grounding conductor, and that will tell you how much room you have for the neutral wire. You look up the conductor sizes in Table 8 (back of the NEC), and this gives you the maximum size neutral wire you can fit into the conduit. Going next to Table 310.16, you will get the amount of current you can pass through the neutral (i.e., as unbalanced load). Finally, you can compare that value to the known load on the ungrounded conductors, and see what percentage of a reduction you need, in order to make this project work.
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