Conductor sizing

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Tom G

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Hartand Wisconsin
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Does this work!? New at this by the way!! Iam feeding a 120 /208 3phase panel board for general lighting/ office loads with plenty of future wiggle room. With 4 wire 3ought thhn from 90 degree column after adjustment 180 amp protecting it with 200 amp breaker per next size up rule and a # 6 EGC per 250.122
 
Dennis Alwon said:
Correct, because the conductor must be rated 125% of the continuous load. Hopefully he doesn't have that much in lights
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Around 40 Led lights at 40 watts per light is lighting load 7 work stations and some convenience outlets each work station is a 20 amp circuit for computer
 
infinity said:
#3/0 ampacity at 90° C is 225 amps. The question is why the derating in the first place. :unsure:
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thank you for reply. My first consider was 3/0 in the 75degree column. However I have more than three current carrying conductors I would be at 160 amp after 310.15 B 3 A so my belief was I would have go to table 310.15B3 and apply adjustment and then use 3/0 thhn in the 90 Degree column That gave me 180amps Then as far as breaker size I could use next size up and use a 200 amp breaker!?
 
Yes you can adjust from the 90° C ampacity of 225 amps but the question is why do you need to do so. As I asked in post #2 is more than 50% of the entire load non-linear? If not then you do not need to apply the 80% factor.
 
infinity said:
Yes you can adjust from the 90° C ampacity of 225 amps but the question is why do you need to do so. As I asked in post #2 is more than 50% of the entire load non-linear? If not then you do not need to apply the 80% factor.
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Thank you for clearing that up. Please share code reference for further and future understanding!
 
310.15(B)(3)(a)(5) Also this may help for future reference.

Neutral Conductors:
Here's some examples of when to count and not count the neutral as a current
carrying conductor or CCC:
3Ø- 208Y/120 or 480Y/277 volt system-different circuit types:
A) 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
B) 3 wire circuit w/ 2 ungrounded, 1 neutral = 3 CCC's
C) 4 wire circuit w/ 3 ungrounded, 1 neutral = 3 CCC's*
Notes:
A) A normal 2 wire circuit has equal current flowing in each of the circuit
conductors so they both count as CCC's.
B) In this circuit the neutral current will be nearly equal to the current in the
ungrounded conductors so the neutral counts as a CCC
C) In this circuit the neutral will only carry the imbalance of the current between
the three ungrounded conductors so it is not counted as a CCC, with an exception,
*if the current is more than 50% nonlinear (see below for NEC article 100
definition) then the neutral would count as a CCC.
1Ø- 120/240 volt system-different circuit types:
D) 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
E) 3 wire circuit w/ 2 ungrounded, 1 neutral = 2 CCC's
Notes:
D) A normal 2 wire circuit has equal current flowing in each of the circuit
conductors so they both count as CCC's.
E) In this circuit the neutral will only carry the imbalance between the two
ungrounded conductors so the neutral is not counted as a CCC.
Nonlinear Load. A load where the wave shape of the steady-state current does
not follow the wave shape of the applied voltage.
Informational Note: Electronic equipment, electronic/electric-discharge lighting,
adjustable-speed drive systems, and similar equipment may be nonlinear loads.
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