Twofold question here: First, can i splice THHN copper to an existing run of aluminum SE cable in order to extend a feeder to a subpanel? I find nothing in the NEC disallowing this, as long as I use an approved copper-to-aluminum device and some No-Alox. Still, it seems weird. Then, if i CAN do this, how would I calculate the voltage drop? would i first calculate the aluminum for just the length of the run that is a SE, and then calculate the copper for just the length that is THHN? Or would i calculate both the copper and the aluminum for the entire length of the run? I appreciate any and all feedback. thanks.
You would need to calculate the resistance for the footage of Aluminum and the footage of copper separately and then add the tow "resistors" together and apply the voltage drop formula that you've chosen to use. Use Table 8 in the back of the NEC for resistance of conductors.
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.
You are correct in regards to the NEC and VD. NEC only mentions VD parameters in fine print which are non-enforceable. However, the equipment manufacturer does specify a VD range and that is why, in my opinion, this type of accuracy is important.
Thanks for your responses. Diver: I'm curious what Vd formula you use that is so confusing. I use this: cmils = (2 x K x amperage x distance) divided by voltage drop. Where: Cmils is the wire size in circular mils. For copper, "approximate K" = 12.6. For aluminum, "approximate K" = 21.2. And my voltage drop in this situation would be 3% x 240v = 7.2v.
If, say,the aluminum portion of my run is 45 feet and the copper portion is 82 feet, my question is whether i calculate the aluminum circ mils by using a distance of 45' or by using the total distance of 127'. And likewise for the copper: would I plug 82' into the equation or 127'? Remember that there is no OCPD between the aluminum and the copper; they are simply spliced so that they are essentially acting as one feeder.
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.
Diver Dan- In all due respect to your obvious education, I don't know vary many electricians out there that have enough background to use your formula. It certianly is proper for the engineers when an exact voltage drop is calculated, but for the average job out there, most electricians get by very well just by using the simple formula found in any electricians handbook and using Table 8 in Chapter 9 of the NEC. They don't even use Table 9 and consider impedance. Most electricians (myself included) use only the minimum amount of math they have to use to get by. They are in trouble when they have to do voltage drop calculations on Sensitive Electronic Equipment Article 647 Section 647.4(D) where voltage drop limits are in place by code, not optional.
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 line-to-line and 60 volts to ground for sensitive electronic equipment.
Bob Badger Construction & Maintenance Electrician Massachusetts
I understand what you are saying and partically agree. Engineers do need to be as percise as possible as they are one of the prime targets for insurance companies in regards to law suits. And, whenever an electrican or contractor performs a "Design / Build" installation, he is now infact the engineer and a potentical target.
Knowing that we all want to do the best job possible and at one point we are only as good as our tools permit us, Volts becomes another tool option. Except, if doing Design / Build projects, it usually pays for itself in the first job.
Also and very important, voltage drop can very much be a safety issue!!!