wireway calculation

[p051981]

I need some help on wireway calculation. 376.22 (A) says wireways are limited to a maximum of 20% fill and 376.56 (A) says splices and taps must not fill more than 75% of the wiring space. So if I had:

3- 500 kcmil THHN = 2.1219 sq in
1- 250 kcmil THHN = .3970 sq in
4- 4/0 THHN = 1.2948 sq in

Total equals = 3.8137 sq in
Since 20% is equal to one-fifth
3.8137 sq in x 5 = 19.07 sq in
A 5x5 wireway would be sufficient.

Now my question is, how do you calculate for a 75% fill?

 

[LarryFine]

Now my question is, how do you calculate for a 75% fill?

Wireway area x 0.75, then compare that to the conductor/splice/tap area.

Don't work backwards. Calculate the occupiable area of the wireway, calculate the conductor, etc., area, and just compare the two.

 

[p051981]

thank you

thank you

thanks for the help!

 

[p051981]

Still having problems

Still having problems

I tried to do what you said but im so lost now. Can you explain what to me what you was talking about.

 

[LarryFine]

Sure. Your 5x5 wireway is 25 sq.in. in area. 75% of that is (25 x .75) 18.75 sq.in.

That's your 75% fill limit for sections where splices and/or taps make up the filling.

For the 20% limit in your original post's example, your fill limit is (25 x .2) 5 sq.in.

 

[RUWired]

Now my question is, how do you calculate for a 75% fill?

To calculate for the 75% fill area of splices or taps before you order the wireway, you need to know what method of splice or tap you are going to use.

For example, if you were to use (3) 2-hole clear tap mechanical lug (PCT-2-600) for the 500 - 4/0 tap, it has a dimension of (L)3.17" x (H)3.27". if you were to use a (PCT-2-350) for the 250-4/0, it has dimension of (L)2.75" x (H)2.65". All 4 mechanical lugs take up an area of 38.37 sq". If you were to use an 8" x 8" wireway, 75% fill would be 48 sq". A 6" x 6" would be too small at 75% fill.(27sq")

If you were to use split bolt connectors, you would have to figure the overall area of the connector with splice tape included times all 4. If you had an overall splice ball of tape that was 3-1/2" in diameter for the 500- 4/0 and a splice ball of 3" for the 250-4/0, you would have an area overall area of 35.92 sq". It would also require an 8" x 8" wireway.

Although a 5" x 5" wireway would be ok for the conductors themselves, the splices would require a larger wireway.

Rick

 

[p051981]

Thanks

Thanks

You guys are great! I finally understand the concept now. Thank you very much!

 

[LarryFine]

All 4 mechanical lugs take up an area of 38.37 sq".

Of course, there's nothing to prevent one from staggering the splices along the wireway, so one would only need to add one splice's area to that of the rest of the conductors.

 

[RUWired]

Of course, there's nothing to prevent one from staggering the splices along the wireway, so one would only need to add one splice's area to that of the rest of the conductors.

Thats a good point Larry, if you had a wireway long enough to strech the tap points out you could get away with a smaller wireway, but with the lugs i quoted and the 5" wireway, it would be a real task to try and get 3-1/4" lugs and a .949 wires to pass by one another without encroaching on the 75% fill area. With the method i used, there is no question that it will all fit.

Rick