Multiconductor Cable in Cable Tray

Status
Not open for further replies.
D

dummycheck

Member
Location
Texas
Occupation
Engineer
I'm working on a project where we're specifying multiconductor cable in a ladder style cable tray. As such, we'll be following NEC 392.22(A) and 392.80(A)(1). I've got a few questions about different parts of this code section. All the loads will be single phase 208V, so the cable will contain two current carrying conductors and a ground. We don't have a spec on the exact multiconductor cable yet.

For 392.22(A)(1), I think we should be able to keep everything smaller than 4/0. To get the cross sectional area, do I just add up the area of all the cables? Is there more to add for this if I don't have the exact cable we're using yet?

For example, we'll have 20 sets of (2) #2 AWG and #8 GND.
#2 = 0.067in^2
#8 = 0.017in^2
Total area = 0.067 + 0.067 + 0.017 = 0.151in^2
20 * 0.151in^2 = 3.02in^2

Per column 1 of table 392.22(A), I could use a 4in wide cable tray?

For 392.80(A)(1), there is a reference to table 310.16 which I'm familiar with, but also table 310.18 which I am not familiar with. I'm not really sure of those cable types, best I can tell they are just different insulation types? Are these common in multiconductor cables? Does the section reference this table just in case you're using these types or is there more to it?

Also, to make sure I'm reading 392.80(A)(1)(c) correctly, if you've got your cable spaced out as indicated you can use the equation in 310.14(B) which will allow you to calculate your ampacity, but this is not a requirement. It's just a way to get
 
ron

ron

Senior Member
Location
New York, 40.7514,-73.9925
As a gut check, if you were to pick a basis of design tray cable to use for example, its diameter might be ~1", which would make its cross sectional area ~0.79"^2. (20) would be 15.6"^2
Using the table from 392.22(A) col 1, would put you in a 16" tray
 
tom baker

tom baker

First Chief Moderator
Staff member
Location
Bremerton, Washington
Tray cable, 600 v insulation, is typically used in cable tray. I mostly used Belden, but sometimes other mfgs. For cable tray I liked B line
 
Last edited:
J

jusme123

Senior Member
Location
NY
Occupation
JW
....and locate the smaller cable somewhere in the tray where it will not be crushed or damaged by larger size wire.
 
D

dummycheck

Member
Location
Texas
Occupation
Engineer
cadpoint said:
:)
Yeah I read it several times myself, it's 2 conductors and a GND in a wrap, twenty times, I believe...
Click to expand...
Yep, that's correct, guess I could have been a little more clear.

To close the loop on some of these items....

I ended up using this as a reference for the multiconductor cables:

The don't show a single cable that has (2) #2 and #8 GND, but they do have on that is (3) #2 and #6 GND, so that helps the calcs be more conservative.

If you go to the Eaton link below, and scroll down halfway, there will be a link to the Cable Tray Sizing Program (NEC) that will help out with the calcs. I double checked the calcs from my example above and I came up with the same answer (18" cable tray with 6" depth).

Table 310.18 just looks like a table that can get you higher ampacities if you have different insulation types, so I'd say you'll be safe with the ampacities in table 310.16.

The equation in 310.14(B) looks like it can be used to get higher ampacities if you're spacing out the cables and if you're up to figuring out what goes in all the variables, so again you should be safe with table 310.16.
 
Status
Not open for further replies.
Top