Annex B Allows More Amps

[Bob NH]

There is an occasional question about using 310.15(B)(6) for feeders that don't meet the requirement for "Main power feeder".

Here is an example of how the ampacities compare if you use Annex B.

Let's take the case of Aluminum 2-2-2-4 SE R.

As a main power feeder, it would have an ampacity of 100 Amps per Table 310.15(B)(6).

As a general feeder it would have an ampacity of 90 Amps per Table 310.16, at an ambient temperature of 26-30 C.

Using Table B.310.3 it would have an ampacity of 92 Amps at 40 C ambient, but to put it on the same basis as 310.16, the ampacity must be adjusted by applying the temperature factor at the bottom of Table B.310.3. The factor for 26-30 C ambient is 1.13, which results in an ampacity of 103.96 Amps.

I also checked the case of 2 AWG copper, with virtually the same answer.

Based on the calculation for 2 AWG copper and aluminum, I conclude that the ampacities of Table 310.16 would have to be increased by about 15% to match the ampacities calculated from Table B.310.3 for the same ambient temperature.

I think that answer supports the comment by Bob (iwire) at Post #9 of the following link that the NEC should take the conservatism out of their calculations. http://forums.mikeholt.com/showthread.php?t=83883

Does anyone else have any comments on, or experience using, the ampacities calculated from the tables of Annex B?

 

[Dennis Alwon]

sorry for being confused but B.310.3 is based on cables in free air whereas table 310.16 is not. What am I missing??? They don't seem to be comparing apples with apples.

 

[Bob NH]

sorry for being confused but B.310.3 is based on cables in free air whereas table 310.16 is not. What am I missing??? They don't seem to be comparing apples with apples.

Table 310.16 is based on 3 conductors in raceway, cable, or earth, based on 30 C ambient.

Table B.310.3 is for "Multiconductor cables with not more than 3 insulated conductors, in free air at 40 C ambient.

It appears to me that a "3 conductors in a raceway, cable, or earth " of Table310.16, and "Multiconductor cables with not more than 3 insulated conductors" of Table B310.3 would both apply to the example of SE R that I used.

338.2 defines SE cable as a single or multiconductor cable with or without an overall covering.

The uncertainty would be in whether he jacket of SE R disqualifies use of Table B.310.3.

 

[don_resqcapt19]

First the information in Annex B can only be used under engineering supervision and second, the maximum permitted ampcity of the conductor is limited by 110.14(C)(1).

(1) Equipment Provisions The determination of termination provisions of equipment shall be based on 110.14(C)(1)(a) or (C)(1)(b). Unless the equipment is listed and marked otherwise, conductor ampacities used in determining equipment termination provisions shall be based on Table 310.16 as appropriately modified by 310.15(B)(6).

Don