ECN Forum Forums General Topics of the Trade General Discussion Area Here's the Derating Discussion!!! Yee-Haww

I am with Bob on this one, except for the Batman part - Robin was/is the Wuss

Solar
I am not aware of the new type design for laptops - I am going to see what I can find out about that... thanks for the heads up though.

Pierre

OK, I will take a shot at this. First the THD spec is a decoy. THD is not controlled by the distribution or a NEC issue. You could add a 208 delta fed PDU to clean up and prevent harmonics from backing up, but the bottom line is THD is determined by the equipment being used.

Now for the questions:

Is it NEC compliant?
Yes

Will it work?
Yes

Is this a design issue?
Yes, you chose to use a #10 AWG grounded circuit conductor when all you need is a 12 AWG, and you choose to supply an EGC. Good design practice, but not required provided you raceway meets 250.118. In addition you could choose to use an isolation type transformer to clean up the power, but would be a complete waist of money for PC use. You could also use a UPS, but again a waist for PC’s, servers is another story.

Does this example make since?
Yes

Will the Penguin escape again?
Yes, how else could you continue the series?

Thanks to everyone for replying!
So far, this topic has been covered well, and hope it continues to do so.

I'm posting an additional thread starter in this area (General Discussion), which I'll edit this message later on with a link to it.

After doing so, will come back to this thread and sum up all the pending information of the example.

Thanks again, everyone!!!

Scott35

The newly posted item is:

Another Derating scenario, plus Boxfill example, too!

Check it out.

S.E.T.

Returned to edit message for link.

[This message has been edited by Scott35 (edited 09-27-2004).]

Hello all!

Forgot all about this thread, and the other one regarding derating

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&#176;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

Scott good example.
As a constructive suggestion, THD or Total Harmonic Distortion can be current or voltage. In your example, if the Current THD was 10%, that does not directly relate to an increase of the neutral current by 10% x 3.
Harmonic distortion is a Fourier transform analysis of the measured waveform. That method (Fourier transform) results in a breakdown of the waveform into its respective components at different frequencies from nominal (60Hz). THD is a normalized value made up of the square root of the summation of harmonic distorted currents divided by the nominal current at the nominal frequency. (The real formula is easier on the eyes).
So, unfortunately, a 10% current THD could be a very high current at various frequencies other than 60Hz, in which some may cancel if not a triplen, and some are at a high frequency that would effect the grounded conductor somewhat differently than a normal ampacity.

[This message has been edited by Ron (edited 10-27-2004).]

Scott,

I disagree with you on three points.


I don't see how, for the purpose of ampacity derating, voltage drop creates any additional heat that will change the ambient temperature. If you have current flowing in a wire, you will get a certain amount of voltage drop per foot of wire and a certain amount of heat produced per foot of wire. If the wire were 100x longer, you would get 100x the voltage drop, and 100x the heat produced, but 100x the surface area in which to dissipate the heat, so the ampacity stays the same. The only way I could see additional heating with voltage drop is if the load were to draw more current as the supply voltage decreases.


I believe that a #12 conductor in the situation described is _not_ considered overloaded, even if this is an LCL situation. Everything that I have seen that requires a differentiation between short term loads and continuous loads is related to the OCPD. The ampacity of a conductor (as calculated by 310.16 and related) is the _continuous_ ampacity, and in the situation described a #12 could carry 21A. However the OCPD can not be permitted to be loaded at more than 80% of trip rating for an LCL, unless the OCPD is rated for 100% operation, and in addition #12 conductors are required to be protected by a 20A or smaller OCPD. But the neutral doesn't usually have an OCPD, and so should be within ratings up to 21A. Note that if there were a breaker on the neutral, I would agree that it would be overloaded at 19.2A


Actually, harmonic distortion _requires_ interaction between the Electrical Distribution Equipment and the load. If you had a zero impedance distribution system that always provided exactly the proper voltage, then any load non-linearities would result in significant harmonic _current_ flows, but the system voltage would remain undistorted, and other loads on the system would not see harmonic distortion. The harmonic current flows would cause heating in conductors, of course. If the distribution system has high impedance where current flow is limited, then load non-linearities will result in less harmonic current flow, but only because supply voltage distortion will tend to reduce the voltage driving the harmonic current flows. Other loads on the system see distorted supply voltages in this case, but harmonic heating of conductors is reduced.

-Jon

Ron and Jon;

Thank you for posting these points!

To others reading this thread, please take their information into consideration when viewing the data I have posted.

Scott35