derating

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Yes, here is a post that Rob/Infinity wrote.

Here's some examples of when to count the neutral as a CCC:

208Y/120 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 one exception, *if the current is more than 50% nonlinear then the neutral would count as a CCC.

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 condcutors so the neutral is not counted as a CCC.
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Ok, I have a 20amp 120/208 3 phase circuit. would I keep my 20 amp receptacle and three pole circuit breaker and just jump my MC to #10 instead of 12? will my connectors and devices accept the #10.
 
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Also I will be having a cord from the ceiling receptacle down to a machine, Is it ok to have 30amp devices with a 20 amp circuit breaker .
 
wireday said:
Ok, I have a 20amp 120/208 3 phase circuit. would I keep my 20 amp receptacle and three pole circuit breaker and just jump my MC to #10 instead of 12? will my connectors and devices accept the #10.
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wireday said:
Also I will be having a cord from the ceiling receptacle down to a machine, Is it ok to have 30amp devices with a 20 amp circuit breaker .
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If you have all three phases and neutral you only have 3 current carrying conductors, no need to apply ampacity correction for number of current carrying conductors. But even if you ran 10, 8, 6 instead of 12 for any reason, you can still make 12 AWG taps to your devices.

You don't need a 30 amp device just because you ran conductors that were increased because of ampacity adjustment. If you have a 20 amp branch circuit you can put a 20 amp receptacle on it no matter what size of conductors were ran.
 
wireday said:
I am confused over the neutral, it will have an unbalanced load from the three hot legs.
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If you have one of each phase per one neutral it will not be imbalanced.

If one phase is carrying nothing then the neutral will carry more but total heat produced in a raceway containing only those four conductors will only create same heat as three conductors, no matter what the balance is.
 
How does the derating work, the #12 THHN 75 degree colum is 25amps. At 86 ambient is it only a .3 volt difference.
 
Looks like from table 310.15(B)(3)(a) at 80% I would be at 20amps anyways. without having to derate for ambient at all.
 
wireday said:
Looks like from table 310.15(B)(3)(a) at 80% I would be at 20amps anyways. without having to derate for ambient at all.
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Yes you would. But if ambient temp is a factor, then having one less current carrying conductor may make a difference in which size conductors are selected.

And you can start deration using 90 deg column values if you have 90 deg conductors. Your final conductor selection can not be any smaller than 60 or 75 deg column size depending on terminal temp rating.
 
kwired said:
If you have one of each phase per one neutral it will not be imbalanced.

If one phase is carrying nothing then the neutral will carry more but total heat produced in a raceway containing only those four conductors will only create same heat as three conductors, no matter what the balance is.
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A subtle point is that if you set up separate neutrals for what was originally an MWBC, you can increase the number of current carrying conductors as well as increasing conduit fill.

Example: If you have a 120/240 split-phase with two hot and a neutral. you have only two CCCs.
But if, without any non-linear loads in the picture, you decide to run a separate neutral for each phase for 120 volt load (for GFCI or whatever), you still end up with four CCCs, not two!
This is the same result you would get if you treat it as two totally separate two wire circuits.
For three-phase supplying 120V wye to cubicles, etc, this makes the big jump from three CCCs to six CCCs as the result of adding two wires.
 
The load is unknown to me as it is, a machine that is under construction. I think I will install #10 mc just for gp.
 
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