Hello all!
Forgot all about this thread, and the other one regarding derating
![[Linked Image]](https://www.electrical-contractor.net/ubb/eek.gif)
Here are some comments to sum up this example.
First off, let me say that the replies were great! Everyone had excellent information to add in this thread!
With that said, here is what I was thinking when compiling the scenario:
<OL TYPE=A>
[*] Derating of Conductors per NEC minimums, as to the number of Curent Carrying Conductors in the same Raceway,
[*] Loads' Characteristics,
[*] Designing of Circuitry - on Plans and installation in the Field,
[*] Possible Voltage Drop issues,
[*] Loads on Common Grounded Conductors
</OL>
The Derating portion, as described in the original text, is compliant.
As pointed out by others, there will be a total of 8 Current Carrying Conductors, so the multiplier of 0.7 is used to derate the THHN Conductors per Table 310-16.
This will allow the use of the #12 Conductors on 20 Amp OCPD, and to draw upto 20 Amps maximum for less than 3 Hours, 16 Amps maximum for 3 Hours and up.
The 12 Amp load, in one part, was to figure if at 40 feet, there was any problem with Voltage Drop in the Branch Circuitry; one that would create additional heat, and alter the Ambient Temperature to something above 30°C.
This is Design Issue #1.
The other part of specifying the 12 Amp load, was to equate it with the projected THD levels, according to the equipment of which to be installed. This really should have been a straight 10% THD figure, and applied to each Branch Circuit at the Maximum Load of 12 Amps.
This would equal 1.2 Amps per Line, and result in an additional 3.6 Amperes Load in each Common Grounded Conductor.
So adjusted Amperes would be:
13.2 Amperes per Ungrounded Conductor,
15.6 Amperes per Common Grounded Conductor.
This, along with the use of #10 THHN cu for the Common Grounded Conductors, are Design Issues #2, #3 and #4.
Design Issue #5 comes into play by knowing the designed load per Workstation, and applying this to the Branch Circuitry design.
This scenario targets each Workstation to draw a Load of 240 VA per Line, and each Workstation has outlets connected across Phases "A", "B" and "C". The Loads per Line will be 240 VA on "A", "B" and "C" - or simply 2 Amperes per "Phase".
Total of 6 Workstations per "Area", so each "Area" has a 12 Amp Draw on each Line.
If the Branch Circuitry was calculated using 180 VA per duplex Receptacle, and these Workstations contain 3 Receptacles each (1 on Line A, 1 on Line B, 1 on Line C), the resulting calculations for all 12 Workstations would show 2160 VA per Line - which is lower than the maximum 2400 VA of a 20 Amp Branch Circuit at 120 VAC.
In this case, only one "Full Boat" would be needed to run all 12 Workstations, yet if installed this way, Circuits will become overloaded.
The "THD" thing is kind of a decoy - as it is not a term in the NEC, but an effect that would be a Design Issue. Seeing the "THD" term would make known the Loads are Non-Linear, so the Grounded Conductor would be considered a Current Carrying Conductor.
The "THD" thing would become a real hard issue if it was higher. If THD = 20%, then this would be an additional 2.4 Amperes per Line, and an additional 7.2 Amperes in each Grounded Conductor.
Now we would have 14.4 Amperes per Line, and 19.2 Amperes per each Grounded Conductor.
Figuring this will be an LCL situation (Continuous Load), the Grounded Conductors - if sized to be #12, would be overloaded.
If the THD was figured to be 30%, then each Line would be carrying 15.6 Amperes, and each Grounded Conductor would be carrying 22.8 Amperes.
FYI: as mentioned, Harmonic Distortion is not caused by the Electrical Distribution Equipment, but results from the Load Equipment Connected to the Electrical System.
There are a few other Design Issues included within the scenario. The size of the Raceway (1"), the type of Raceway (EMT), and the use of an additional EGC to Bond the Raceway and equipment.
Hope this was an interesting example scenario!
Let me know if any questions / comments.
Scott35
