2005 NEC - Can a covered conductor be used without engineering supervision

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Davidh

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Ukiah, California
Section 310.15(A)(1) states that ampacities for conductors shall be permited to be determined by tables as provided in 310.15(B).

Section 310.15(B) states that apmacities for conductors rated 0 to 2000 volts shall be as specified in the allowable ampacity table 310.16 thru 310.19, and ampacity table 310.20 and table 310.21 as modified by (B)(1) thru (b)(6).

Table 310.21 states Ampacities of Bare or Covered Conductors in Free Air Based on 104 F Ambient, 176F Total Conductor Temperature, 2ft/sec Wind Velocity.

The question is: Can triplex (covered conductor) be used for overhead conductors from a service to a subpanel at an outbuilding for a residence?

According to the code sections listed above it looks to me like the code allows this use without engineered supervision. One of the issues that was brought to my attention is how do you figure the ambient air and total conductor temperature and how do you figure the wind velocity without an engineer. Any help would be appreciated.
 
Davidh said:
1) . . . Can triplex (covered conductor) be used for overhead conductors from a service to a sub-panel at an outbuilding for a residence?

2) . . . how do you figure the ambient air and total conductor temperature and how do you figure the wind velocity without an engineer. . .
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1) In my opinion, sure. I see no real problem with using duplex, triplex, or quadraplex in either a residential or commercial occupancy.

2) You can't unless you have engineering help. However, as conservative as the Code is with tables, I would have no problem with approving this type of installation. Consider that the triplex is outside and elevated so that no damage will be done if the installation fails. Also consider, IPL uses #4 Al triplex for 100 ampere services and #2 Al triplex for 200 ampere services. We normally don't have a failure unless trees are involved that take the service drop down.

If you wish to force the issue, you may require engineering calculations but, in my opinion, you will be wasting the customer's money. :smile:
 
charlie hit the nail on the head.

The calculation is a little involved and requires more than a simple plug-and-chug formula with some wind speed numbers. There are calculations for internal heat, convection heat loss (includes the wind stuff), solar heat gain, and radiated heat loss.
 
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