Ampacity of Direct Burial Conductors

Status
Not open for further replies.

jason sleeth

Member
Location
Illinois
I have an installation in which the customer upgraded the size of his generator. I have installed a triplex "stephens" ALUM 2 awg, 7 strand, .403 inches. I have it buried under 36" in direct earth. The ampacity from the manufacturer states that it is good for 165 amp based on the NEC 2005. I need 145 amps. The length of the cable is 175'.

Can someone walk me through the NEC Calculation to determine the ampacity of the wire? I am unclear going through 310 on how to properly calculate the ampacity.

Thanks
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
There's a basic clash between "Stephens" and the NEC as "Stephens" is a UD cable and not a recognized NEC conductor, is not listed in Art 310, and normally is not acceptable on NEC covered installation.
Under NEC installs a #2 AL would normally be a 90 amp conductor.
 

iwire

Moderator
Staff member
Location
Massachusetts
For what it is worth, 145 amps, 175' 2AWG AL results in more than 7.2% voltage drop.

Unless this is the 'main feeder' to a dwelling unit 2 AWG AL is limited to 90 amps and you can't even roll it up to 100.
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
Can someone walk me through the NEC Calculation to determine the ampacity of the wire?
This is not a "walk through" situation. Calculation of ampacity for underground installations is a highly complex mathematical problem that involves dozens of variables. The bad news is that you need a computer program (those are not cheap) and a licensed professional engineer (we are not cheap either) to perform such a calculation. The really bad news is that the results are almost always lower than the values shown in table 310.16. So if you are hoping to get a little extra maneuvering room by using a calculated value, rather than a tabulated value, it is time to give up that hope. Sorry.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
This is not a "walk through" situation. ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, and a licensed professional engineer (we are not cheap either) to perform such a calculation. ,,,,,,,,,,,,,,,,,,,,,,,,,,,,.

but , normally, worth every nickle.
 

jason sleeth

Member
Location
Illinois
The spec sheet states that it is UD but is rated for multiple insulations.

The spec sheet indicates the wire is ICEA s-105-692UL standard 854 for type USE-2 Federal Specification JC-30BNEC.

It also states that ampacity 90 degrees C, Conductor temperature 20 degree's celcius ambient, RHO50, 100% Load factor.

I am guessing the correct formula used for the engineer is 310.15 (C)

The RHO might be the value of thermal resistivity

I need to look through annex B then I will get back to you.

The amacity of conductors in free air greatly increases the amacity you are allowed on a conductor.

Direct Burial allows the heat to disapate in the ground plus the abient temp is about 57 degrees F

I do not think you are limited to the 90 amps in this scenario but that is why I am asking the more experienced electricians out there
 

jason sleeth

Member
Location
Illinois
For what it is worth, 145 amps, 175' 2AWG AL results in more than 7.2% voltage drop.

Unless this is the 'main feeder' to a dwelling unit 2 AWG AL is limited to 90 amps and you can't even roll it up to 100.

You are right. There would be an issue with Voltage Drop. Thanks.
 
Status
Not open for further replies.
Top