Conductor sizing
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Re: Conductor sizing
The K13 rating tells you the neutral will be a current carrying conductor and included in the count in the raceway and should at least be as large as the phase conductors. I would go with 2/0.
However I have not done the math and 1/0 from the transformer terminals to the disconnect may work. I am sure someone will work the math and give you the absolute minimum.
[ June 09, 2003, 11:05 PM: Message edited by: dereckbc ]
The K13 rating tells you the neutral will be a current carrying conductor and included in the count in the raceway and should at least be as large as the phase conductors. I would go with 2/0.
However I have not done the math and 1/0 from the transformer terminals to the disconnect may work. I am sure someone will work the math and give you the absolute minimum.
[ June 09, 2003, 11:05 PM: Message edited by: dereckbc ]
Re: Conductor sizing
A 1/0 is rated for 150A, but wouldn't I have to derate this conductor 80% based upon Table 310.15(B)(2)(a) since the neutral is considered a current carrying conductor and therefore I would have 4 current carrying conductors in a raceway? Should I be using 5-3/0s instead?
A 1/0 is rated for 150A, but wouldn't I have to derate this conductor 80% based upon Table 310.15(B)(2)(a) since the neutral is considered a current carrying conductor and therefore I would have 4 current carrying conductors in a raceway? Should I be using 5-3/0s instead?
Re: Conductor sizing
Full load current for a 45 kva 120/208 is
125 amps. If you install over current protection
at 125% of full load the OC = 125 x 1.25 = 156 amps. You can use a 150 or 175 amp breaker. Since you have a harmonic load you will need to count the neutral and derate by 80%. 3/0 copper rated at 200 amps x .80 = 160 amps would meet the requirements. Are you sure the transformer has a K13 rating? That is high. What kind of load do you have.
[ June 10, 2003, 03:08 PM: Message edited by: bob ]
Full load current for a 45 kva 120/208 is
125 amps. If you install over current protection
at 125% of full load the OC = 125 x 1.25 = 156 amps. You can use a 150 or 175 amp breaker. Since you have a harmonic load you will need to count the neutral and derate by 80%. 3/0 copper rated at 200 amps x .80 = 160 amps would meet the requirements. Are you sure the transformer has a K13 rating? That is high. What kind of load do you have.
[ June 10, 2003, 03:08 PM: Message edited by: bob ]
Re: Conductor sizing
bwyllie, if you are looking for the minimum conductor size you left out two important specs. What kind of cable insulation? What is the temperature rating of the terminals?
I am going to assume you are using THHN on 75-degree terminals. To figure the cable size start in the 90 degree column of table 310-16 for THHN conductors. Apply ambient temperature correction factor at the bottom of table 310.16 if any (I assume none). I am looking at 2/0 being rated at 195 amps @ 90 degrees.
Now apply adjustment factors from Table 310.15(B)(2)(a) for the number of current carrying conductors (4), which gives you a .8 correction factor. Multiply .8 times 195 equals 156 amps. 156-amp cable is good on 150-amp OCPD. Now check to see if 156 amps exceed the rating of the 75-degree column for 2/0. It does not. Good to go.
2/0 phase and neutral is the smallest size allowed for you application. EGC is sized from 250.122 which is #6 AWG cu. GEC is table 250.66 or #4 AWG cu.
If you wish to use 3/0 AWG for phase, neutral, EGC, and GEC you may certainly do so, but it is not required? Dereck
bwyllie, if you are looking for the minimum conductor size you left out two important specs. What kind of cable insulation? What is the temperature rating of the terminals?
I am going to assume you are using THHN on 75-degree terminals. To figure the cable size start in the 90 degree column of table 310-16 for THHN conductors. Apply ambient temperature correction factor at the bottom of table 310.16 if any (I assume none). I am looking at 2/0 being rated at 195 amps @ 90 degrees.
Now apply adjustment factors from Table 310.15(B)(2)(a) for the number of current carrying conductors (4), which gives you a .8 correction factor. Multiply .8 times 195 equals 156 amps. 156-amp cable is good on 150-amp OCPD. Now check to see if 156 amps exceed the rating of the 75-degree column for 2/0. It does not. Good to go.
2/0 phase and neutral is the smallest size allowed for you application. EGC is sized from 250.122 which is #6 AWG cu. GEC is table 250.66 or #4 AWG cu.
If you wish to use 3/0 AWG for phase, neutral, EGC, and GEC you may certainly do so, but it is not required? Dereck
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Re: Conductor sizing
As pointed out above, its OK to use the 90 c rating for derating but the final ampacity must not exceed the 75 C rating.
This is the advantage of the 90 C rating...
As pointed out above, its OK to use the 90 c rating for derating but the final ampacity must not exceed the 75 C rating.
This is the advantage of the 90 C rating...
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