HELP!!!!!!!

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jatindall82

Member
Location
troup, texas
What size branch circuit overcurrent protection device and conductor (THHN) is required for a 19kva of non linear loads (75 degree terminals). The branch circuit is supplied by a 208/120v, 4 wire, 3 phase, wye connected system.

What I have come up with:
I converted the kva to amps

va / (volts x 1.732)
19,000 /(208 x 1.732) = 52.74 amps rounded to 53 amps

branch circuit OCP has to be sized @ 125% of the 53 amps
53 x 1.25 = 66.25 = 66 amps. So a 70 amp OCP

To select the conductor I believe I have to increase 125% also.

66 amps in the 75 degree column requires #4 @ 85 amps

Since there are more than 3 current carrying conductors in the race way I have to correct the #4 THHN.

The instruction at this point say use the 90 degree column for the ampacity and then correct it at 80%

My question is why am I using the 90 column at this point?
 

Martin B

Member
Location
Nebraska
What size branch circuit overcurrent protection device and conductor (THHN) is required for a 19kva of non linear loads (75 degree terminals). The branch circuit is supplied by a 208/120v, 4 wire, 3 phase, wye connected system.

What I have come up with:
I converted the kva to amps

va / (volts x 1.732)
19,000 /(208 x 1.732) = 52.74 amps rounded to 53 amps

branch circuit OCP has to be sized @ 125% of the 53 amps
53 x 1.25 = 66.25 = 66 amps. So a 70 amp OCP

To select the conductor I believe I have to increase 125% also.

66 amps in the 75 degree column requires #4 @ 85 amps

Since there are more than 3 current carrying conductors in the race way I have to correct the #4 THHN.

The instruction at this point say use the 90 degree column for the ampacity and then correct it at 80%

My question is why am I using the 90 column at this point?

I believe you are increasing the ampacity by 125% one to many times. You only have to increase it once if its a continuous load. So you would size your OCP and Conductors at 53 amps.

I think the reason you would use the 90 deg. collumn is that when derating for the number of conductors in a raceway you are talking about that portion of the circuit that is in between Point A and Point B. So its in the conduit. The insulation, at this point is what you are trying to protect.

Hope this makes sense, Good luck.
 
I believe you are increasing the ampacity by 125% one to many times. You only have to increase it once if its a continuous load. So you would size your OCP and Conductors at 53 amps.

I think the reason you would use the 90 deg. collumn is that when derating for the number of conductors in a raceway you are talking about that portion of the circuit that is in between Point A and Point B. So its in the conduit. The insulation, at this point is what you are trying to protect.

Hope this makes sense, Good luck.

No I think he's good look over it again carefully, the flow is a bit confusing. He just did the 125 to the OCPD and then to the conductor. Then the #4 was just what was required because his needed 66 was just over #6
 

Smart $

Esteemed Member
Location
Ohio
My question is why am I using the 90 column at this point?
There are three aspects to getting the correct minimum size wire: 1) for termination temperature limitations, 2) for ampacity to carry load, and 3) for ampacity [considered] protected by OCPD. The largest of the preceding three items is the minimum size wire you can use.

You use the 90°C column values when using 90°C-rated wire and determining items 2 and 3 above. Note the compare to value for item 2 is the load at 100%. You do not factor the continuous load by 125% for this part of the process.

1) 100% NC + 125% C ≤ 75°C column value
2) NC + C ≤ 90°C × adjustment factor × correction factor
3a) 90°C × adjustment factor × correction factor ≥ OCP rating [>800A]
3b) 90°C × adjustment factor × correction factor > next smaller standard OCP rating [≤800A]
 

jatindall82

Member
Location
troup, texas
Thanks so much for your help guys!! I am struggling with my calculations but because of people like yall I am slowly but surely getting there. Ill be asking more questions lol, so stay with me! lol Again thanks so much for your time. I know everyone's time is precious but this really is a great help!
 
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