3 Phase Problem

[piston]

Hello I am new to the forum. I have a question about three phase. I am a student also so i will show my work and then ask specific questions at the bottom.

The problem i am working on:

Given data for the problem:
-IMC installed in a room with ambient temperature of 110 F
-thhn/thwn conductors are used
-growth allowance of the computed load is 10%
-the power factor will be maintained at 90% or higher
-conductor length is 180 feet
-voltage drop at max load shall not exceed 2%

Continuous load=20,000 va
Non Continuous load/receptacles=12,000 va
highest motor= 8,000 va
other motor= 16,000 va
nonlinear= 20,000 va
balanced= 24,000 va

Non continuos load

Non continuous/receptacles = "12,000" va
largest motor = "8,000" va
25% of the largest motor = "2,000" va
Other motor = "16,000" va
"38,000" va non-continuous subtotal
Continuous load

Continues load = "20,000" va
Continues load + 25% = "5,000" va
"25,000" va continuous subtotal

non-continuous subtotal = "38,000" va
continuous subtotal = "25,000" va
subtotal = "63,000" va
Plus 10% growth = "6,300" va
TOTAL VA = "69,300" va

Convert VA to I
"69,300/(208V*1.732)" = 192 amps

Conductor chosen for use
THWN at 75? C = 300 kcmil rated at 285 amps

Conductor adjustment
I * conductor count factor = 285a * 80% 228 amps

Conductor correction factor
I * temperature factor = 228a * 95% 217 amps
This would be a 250 kcmil thwn
Neutral Conductor sizing
125% of the Total load = Maximum neutral current in VA
192amps * 1.25 = 240 amps

Adjustment/Correction of the neutral 182.4 amps

Questions:
Did I calculate the ungrounded conductors correctly?

Did I calculate the grounded (neutral0 correctly)?

I have been working on this for hours on end each day and the code is very vague on the neutral conductor calculations or even 3 phase (in my opinion). But I am a beginner so what do i know. :slaphead:

 

[Julius Right]

I see you take 300 MCM from Table 310.15(B)(16) for 30 dgr.C ambient. Now you have to
use Table 310.15(B)(2)(a) Ambient Temperature Correction Factors Based on 30?C (86?F) and for 110 dgr.F[43.33 dgr.C] the adjusting factor is 0.82.
What :
"conductor count factor" of 80% and
"Adjustment/Correction of the neutral" is it ?

 

[piston]

I see you take 300 MCM from Table 310.15(B)(16) for 30 dgr.C ambient. Now you have to
use Table 310.15(B)(2)(a) Ambient Temperature Correction Factors Based on 30?C (86?F) and for 110 dgr.F[43.33 dgr.C] the adjusting factor is 0.82.
What :
"conductor count factor" of 80% and
"Adjustment/Correction of the neutral" is it ?

Sorry for the confusion...I have read so much on this topic my knoggin' hurts, LOL

As for the conductor count factor, I used this term to say that there is 4 current carrying conductors,310.15(B)(3)(a) 80% de-rating, and I got the 95% temperature factor from 310.15(B)(2)(b). However, I used the 90C column when i should have used the 75C column. Since I selected a 300MCM at 75C. I think?!

Does this make any sense?

Does the neutral get counted? Do I use the full load VA when figuring the neutral? I should also mention if i haven't that this is a 3 phase 4-wire wye. :dunce:

 

[Dennis Alwon]

If the conductor is a 75C cable and not a rated cable such as thwn-2 then you must start at the 75C rating. If the wire is dual rated then you could use the 90C rating to start. Most all thwn wire is now dual rated however if this is a homework problem then you need to use what is given

 

[piston]

I see your point with the dual temperature rating but I don't think it applies to this application. Aside from using the wrong rating does anything else jump out as being incorrect?

I chose a 250 mcm for the neutral based---??? right/wrong?:?

 

[Dennis Alwon]

I see your point with the dual temperature rating but I don't think it applies to this application. Aside from using the wrong rating does anything else jump out as being incorrect?

I chose a 250 mcm for the neutral based---??? right/wrong?:?

I haven't had time to check and am heading to bed. Perhaps someone here can check it.