Conductor Sizing in conduit banks

[ckinney02]

I'm curious on how to determine the additional derating for conductors in a 10 conduit underground bank. Assume 3 current carrying copper conductors per conduit. The detail figures for 3 current carrying conductors per conduit (Table B.310.15(B)(2)(7)) only has a max of 6 conduits. A duct bank of 10 conduits will obviously have less ampacity per conductor than a duct bank with 6 conduits.

 

[charlie b]

In my neck of the woods, the inspection authorities long ago decided that if the load is calculated per the article 220 requirements, then we can use the ampacity values in table 310.16, even for underground ductbanks of any configuration. The notion is that the additional conservatisms built into the calculation methodology would be partially, but not completely offset by the impact of nearby conduits on the temperature inside any single conduit.

But if you want a calculate ampacity that can be defended as authoritative, then you need to hand the problem to an engineer who has access to a software package that uses the Neher-McGrath methodology to determine the ampacity of underground installations. You are right in stating that the more conduits are installed in close proximity, the lower the calculated ampacity.

 

[mayanees]

Neher-McGrath methodology...

Neher-McGrath methodology...

.... or use Figure 310.60 and th e associated tables.

 

[ckinney02]

In the past, my previous boss used Table 310.16 nationwide for an 3000A underground duct bank. On a specific project, we ran across an engineer that questioned the conductor ampacity.

I use ETAP for breaker coordination and it will also calculate the derated value of conductors. But if I were to design a conductor run and the NEC standards are selected and 'with grouping effect' active, ETAP doesn't allow the user to input 10 conduit bank. In the 'help', it says that NEC doesn't allow for grouping effects. If this is the case then what is the purpose of all the Tables B.310.15(B)(2)(1) through (10). For example, Table B.310.15(B)(2)(7) specific pupose appears to be derating based on the number conduits in a duct bank. Can anyone explain?

So say I have a 6 conduit bank at 480V, Table B.310.15(B)(2)(7) has figures to follow and, as you say, maybe so does Table 310.16. NEC also states that, in case they screwed up and have given multiple values or multiple ways to calculate something, always use the lower value.

Thanks for your responses!!

 

[charlie b]

The ampacity of a conductor is based primarily on its cross-sectional area. But it is also influenced by the ability of the insulation system to endure high temperatures. That, in turn, is influenced by the nature of the surrounding environment, and in particular by the ability of the surrounding environment to carry heat away from the conductor. Dirt is not very good at carrying away heat. The presence of nearby heat sources (i.e., other conduits with current-carrying conductors) will also reduce the ability of any given conductor to reject its heat. So therefore if you install more conduits in a ductbank, you will reduce the ampacity of any conductor in that ductbank.

The key question is, do we have to go through the trouble of calculating a value of ampacity, taking into account the number and layout of conduits, the depth of the ductbank, the physical properties of the soil, and a host of other factors? As I mentioned earlier, our state authorities have given us a specific set of circumstances in which we do not have to do the calculation. I will say that if you get a program that is capable of doing that calculation, you are not going to get any answers that are higher than the values in table 310.16.

I have not tried to use ETAP for this calculation. The software I have used, and have found very easy to use and very flexible, is this one:
http://www.calcware.com/http://www.calcware.com/