My experience seems to be limited with the project I am currently bidding. I am about to show my ignorance and embarass myself with this question but I have to have it answered even if I loose face with this question. I am a residential repair electrician, mainly fire damage for insurance companies, I am bidding a project that has to have a new service. It is an 8 pack apartment building. I propose to install two 4 gang meter packs with seperate feeds to the power companies pad mounted transformer 75 linear feet from the location of the meter packs. I proposed running two seperate parallel service feeders to these banks using 4/0 THHN conductors. The inspection department has failed my drawing because I didn't figure in the Temperature Correction Factor. 4/0 THHN is rated at 260 amps. Miltiply that by 2 (parallel) and you get 520 amps. I'm in Southeast Texas and it gets hot down here. It's not uncomon for temperatures to reach 105 in the summer. I honestly have never had to figure a service like this before. I know how to do the calculation using 310.15 (B) 16. But how do I determine the Ambient Temperature of my particular location in order to satisfy the electrical inspection department? Any correction factor that I use at this point will be a guess. Could someone point me in the right direction?
500 amp, 4 meter, meter pack
- Thread starter Talmadge
- Start date
- Status
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Does the temperature actually reach 105 degrees at the depth you will bury these conductors? exception to 310.15(A)(2) will allow the higher conductor ampacity if the portion that is at higher temperature is 10 percent of circuit length or 10 feet which ever is less.
Is your calculated load 500 amps or is the meter unit rated 500 amps. You don't necessarily need to supply the unit with 500 amp conductors if the load is less than 500 depending on some other factors, like is there a 500 amp main or 4 individual mains which brings up another question...is there a main in or ahead of each meter bank? If not you will have 8 service disconnecting means and you can not have more than 6.
- Location
- Massachusetts
Exactly what I was thinking.
At 105 degrees, that 4/0 would have a .87 temperature multiplier, from the 90 degree column. That gives 226 amps. Unless your terminations are listed for 90 degrees you need to use the 75 degree column anyway. At 75 degrees the ampacity is 230. Since 226 is lower, that is the available ampacity of the conductors (x2 for parallel makes for 452 amps). If you go up to 250MCM you will have just over 500amps.
Four further points, to agree with the previous posters:
1. If the service laterals are buried properly per 300.5, then there should not be a temperature issue.
2. Even without the temperature issue, 4/0 would only be rated 460 (2x230).
3. You would need to have a main breaker for each 4-pack to comply with 230.2
4. Remember to do a calculation for the apartments. If the calculated load is 113 amps or less, then a 452 amps feeder is plenty.
Four further points, to agree with the previous posters:
1. If the service laterals are buried properly per 300.5, then there should not be a temperature issue.
2. Even without the temperature issue, 4/0 would only be rated 460 (2x230).
3. You would need to have a main breaker for each 4-pack to comply with 230.2
4. Remember to do a calculation for the apartments. If the calculated load is 113 amps or less, then a 452 amps feeder is plenty.
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