I am trying to determine the wire size, if I parallel 250MCM alum in one 4" conduit, I come up with 368 for ampacity if I am calculating it correctly. Can I use that for a multi family/mixed use service (400 amp) or do I need to go to 300 MCM(which might not be commonly stocked). There are two 90's on a 40 foot run so I don't want to use 600 MCM alum. Thanks
Thanks, the calculated load would not be that high, 4 small apartment units and two white box retail spaces on the first floor, gas heat, hw and appliances. I am curious about the .96 factor. Is that from a regional temperature chart? The conduit is on the side of the building. I did see a chart for temp showing regions and distance above the roof figures. (for solar) If I did figure the .96 then would it be parallel 250mcm at 460a x.96 =441.6a x .8 =353a so it still would bounce up to 400, if I did it right.368A is only correct, if your ambient high temperature is 30C or less. Usually ambient temperature is slightly higher than this, so a "basic derate" of 0.96 is common to occur in addition to your bundling derate.
Unless the 83% rule comes in to play for a 1 or 2 family dwelling that is single phase serivce, you need to make sure that your calculated load is less than the actual ampacity of your conductors. Your conductor amps need at least as much ampacity as the calculated load, and they must "round up to" the breaker rating per 240.4(B). 368A does round up to 400A, and is protected by a 400A OCPD per 240.4(B), but you need to check that your calculated load is less than 368A.
Thanks, the calculated load would not be that high, 4 small apartment units and two white box retail spaces on the first floor, gas heat, hw and appliances. I am curious about the .96 factor. Is that from a regional temperature chart? The conduit is on the side of the building. I did see a chart for temp showing regions and distance above the roof figures. (for solar) If I did figure the .96 then would it be parallel 250mcm at 460a x.96 =441.6a x .8 =353a so it still would bounce up to 400, if I did it right.
Thanks again for the input. I'm going to play with that formula and see how it comes out.The standard ampacity values in 310.15(B)(16) are based on an ambient temperature of 30C.
The 0.96 factor is simply the typical value corresponding to your ASHRAE hot temperature being between 31C and 35C, which it is in most places where my projects reside. The temperature correction factor for basic ambient temperature. See 310.15(B)(2)
You can also use either the value from Table 310.15(B)(2), or as an alternative you can use the sqrt formula that you will also find in that part of the NEC. The table simply evaluates every group of 5C intervals and provides a representative result from the sqrt formula.
