size of conductor in conduits

[jjavier]

Hi

We check the nec about the size of the conductors (output inverter) inside banks conduits (pvc) in the trenches. We have in the same trench some pvc conduits with different circuits (some ac, sometimes dc), and we understood that:

- you must multiply amps*1,25
- you must to check the size in table B.310 (depends of the number of conduits) and use the factor temperature to adjust it.
- or Easier- use one program as Ampcal, or other

We check that really you need a very, very big size of cable using the nec.....

Is this the correct way to do it?
Why in the B.310 there aren´t 90º cable temperature?, (conductor temperature 75º)
Is there a free computer program to do it?

Regards

 

[Dennis Alwon]

You cannot use Annex B without an engineer signing off. You should be using Annex C Tables 4 and 5. Are you licensed to install the wiring in the state of maine? I would hate to see you get into trouble esp, after the trench is closed up.

 

[jjavier]

We are going to contract the job in maine to a local company

You cannot use Annex B without an engineer signing off. You should be using Annex C Tables 4 and 5. Are you licensed to install the wiring in the state of maine? I would hate to see you get into trouble esp, after the trench is closed up.

 

[pv_n00b]

The NEC is very conservative on duct bank sizing. That’s why people who do a lot of this have to use software to do an analysis and get a smaller conductor. I don’t know of any free software and they are all expensive, but if you do enough duct bank work they are a necessary expense.

 

[jjavier]

I checked this tables, and I only see number of cables per duct, not ampacities. We are an engineering that have to contract an American electricians and we need to pre calculate cables.

You cannot use Annex B without an engineer signing off. You should be using Annex C Tables 4 and 5. Are you licensed to install the wiring in the state of maine? I would hate to see you get into trouble esp, after the trench is closed up.

 

[iwire]

I checked this tables, and I only see number of cables per duct, not ampacities.

Correct and then you use the correct ampacity table in article 310.

We are an engineering that have to contract an American electricians and we need to pre calculate cables.

You have two choices.

Use the ampacity tables in article 310 or employ an electrical engineer to do the calculations.