




#95580  09/22/05 01:50 AM
Re: voltage drop

Member
Registered: 03/10/05
Posts: 38
Loc: Sacramento, CA

Depending on the distance of copper run, you might find that the added computation complexity of combining both conductor material types is insignificant. As I remember, Copper usually carries 40% more current per size than aluminum. So, this would make the aluminum run "the point of largest impedance". Although the copper run will have a VD value, by percentage it will be very small compared to the aluminum. I would extend the VD computation an additional "X" feet with aluminum factors to build a nice conservative circuit.
However, with that said, if the copper run distance is larger than the aluminum, then disregard the above. If you post the distances I'll compute the total VD using IEEE exact formula for you as this formula is a bit complex.
Also, don't forget to allow correction factors for ambient temperature in your VD computation. The NEC tables are set at 30ºC(86ºF) which is not practical anywhere in the USA. I personnally use 40ºC(104  105ºF) as a standard ambient with additional temperature rise for attic and roof mount circuits. The Chapter 8 and 9 tables are "borrowed" from IEEE Std 141 and are derived at 75ºC.
Not to sound like a commerical (this is really true), but using software that takes these and other factors into consideration is a lot easier and more accurate for VD computations.
_________________________
Dolphins Software

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#95584  09/23/05 07:49 AM
Re: voltage drop

Member
Registered: 03/10/05
Posts: 38
Loc: Sacramento, CA

Hi tripp,
I use the IEEE Std 141 exact formulae:
Vd = V + IRcos(theta) + IXsin(theta)  sqrt(V^2  (IXcos(theta)  IRsin(theta))^2) where: Vd = Voltage drop (Line to Neutral) V = Voltage (source) I = Current in amperes (A) R = AC Resistance from NEC® Chapter 9 Table 9 X = AC Reactance from NEC® Chapter 9 Table 9 distance (L) is considered from the Resistance & Reactance Tables where Ohms per unit / 1000 * L in same unit = R or X theta = Arccos(device or circuit Power Factor) = angle of phase offset
Line to Line is computed by Line to Neutral VD / Sqrt(3) for 3 phase circuits.
You'll also have to get into some sin wave destruction formulae for power factor averaging when computing the Vd across a panel's bus bar. Total(kW) / Total(kVA) should be close enough for the bus bar PF average.
I forgot to mention that since resistance(impedance) is very sensative to temperature you must multiply the resistance value by the following factor:
R2 = R1[1 + a(T2  75)] where: R2 = Adjusted Conductor Resistance R1 = Table Conductor Resistance @ 75ºC a = Conductor Material Resistivity a(cu) = 0.00323 and a(AL) = 0.00330 T2 = Ambient Temperature in Celsius (TA)
convert temperature in ºF to ºC with: TºC = (TºF  32) / 1.8
Also: You would take the VD across the aluminum first. Then use the lessened voltage from the aluminum run to start the copper VD computation. Most engineers would compute the run as all aluminum to keep conservative.
And: Regardless of the voltage drop formula you use, The fact that you take VD into consideration puts you in the top 1% of electrican's in my opinion!!!
The NEC only references VD in fine print notes. HOWEVER, all electrical devices used in the USA must have a UL label. United Labratories tests these device under "normal" conditions within a VD limit from its nameplate rated voltage. If you, the electrican, install a circuit that is not capable of supplying the correct voltage within the devices voltage range, the following will happen. 1. The UL approval is VOIDED. 2. The conductors will produce added heat. Heat, like raditation, is accumulative (just ask a fireman for verification). This means that a fire will eventually result in a given amount of time from not considering voltage drop. 3. When this happens who does the insurance company go after.......
Personally, as an engineer, I need to be as accurate as possible to not only ensure a problem free installation, but to also cover my butt in case there is a future problem. The IEEE Std 141 Exact Formula is the only VD formula that is recognized as correct both nationally and internationally.
Volts computes VD with all the necessary environmental and device considerations and sizes the conductors to the correct NEC ampacity table, the correct voltage drop and the correct terminal temperature of the device....in less time than you can write a single sentence.
_________________________
Dolphins Software

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#95586  09/23/05 11:53 AM
Re: voltage drop

Moderator
Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA

Diver I have to go with George's post. I fully understand that as an engineer you have to be precise, that is your profession. Most electricians do not need to be that accurate for VD, and regardless of what the books tell us the real world has shown us that equipment is much more tolerant to voltage variations then you would think. Voltage drop is not a safety issue, it is more of an efficiency issue. Just out of curiosity have you ever checked your VD predictions against the real thing after the work is done? George, who the heck uses Article 647? I have never seen that article applied, it does not apply to much. 647.1 Scope. This article covers the installation and wiring of separately derived systems operating at 120 volts linetoline and 60 volts to ground for sensitive electronic equipment. Bob
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Bob Badger Construction & Maintenance Electrician Massachusetts

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