Why does my current drop?

I just installed a new circuit in my house. Being a bit paranoid, I stress tested it; and didn't like the results.
The circuit is 50' long, in #12. The transformer for the house is about
300' away, with #1 aluminum wire. (well, it is #1 from the meter to the box, I don't know what it is from the transformer to the meter; hopefully it is bigger.)
Voltage on the new circuit is 120.6v when nothing is running. When I plug in a shopvac, it draws 6.0a, and the voltage drops to 118.4v When I plug in a toaster, it draws 12.0a, and the voltage drops to 115.6v When I plug both in, they draw 17.5a, and the voltage drops to 113.4v.
So, I have a 7.2v drop on 17.5a. I tried another circuit (one I didn't wire), and got similar results. According to tables, I have a 3.3v drop on the circuit, and a 2.2v drop (or less) on the service entrance.
What could the other 1.7v be? People on alt.home.repair didn't know, and suggested here. One guy said that a shopvac is an inductive load, and doesn't simply add to the toaster; but I vaguely remember from the EE courses I took 30 years ago that impedance tends to sum to less than the straight total; not more. Is that right? Other people suggested that I must have a bad connection, but it would have to be a similar bad connection on two circuits; and I can't see a bad connection giving a consistant 1.7v drop.
Any ideas? thanks
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JS wrote:

This most likely a measurements error - and there is an error in the part about the tables. There's a lot of detail to cover to try to find that "lost" 1.7 volts. I'll try to describe what I think may be wrong and how to proceed.
You said the tables give a 3.3 v drop on the circuit, and 2.2 volts on the service. 50 feet of copper cable in a branch circuit presents .2 ohms resistance per the tables in the NEC. At 17.5 amps, that's 3.5 volts, not 3.3. I'm not sure what you have in mind with the 2.2 volts service drop, or how you got that figure. If you are computing it based on the #1 aluminum, the correct figure is 2.625 volts, assuming a 17.5 amp draw over 600 feet of wire. (Based on .25 ohms per 100 feet for #1 aluminum).
These computations versus yours make a difference. You are trying to find a 1.7 volt difference, but the computations reduce that to a 1.075 volt difference.
Next: when measuring the drop in a branch circuit, you must measure the voltage at the breaker for the circuit at the same time you measure the voltage at the outlet, with two meters that read identically, to be precise. Most of the time, people measure at the panel, then take the meter to the outlet and measure there. But the voltage to the panel can change in the time it takes to move to the outlet.
Best practice is to load a branch circuit to no more than 80% of its rating - which means 16 amps. At 16 amps, the *computed* voltage drop for the circuit is 3.2 volts, which is within the NEC recommendation of 3% or less.
Now, set all that aside for a moment. If you *really* are dropping 7.2 volts in the BRANCH CIRCUIT wiring, something is loose. And if we subtract out what you are calling the service drop (2.2 volts), you still end up with a 5.2 volt drop in the branch wiring, which is still too much. Even if you use the 2.625 figure, the drop in the branch is 7.2 - 2.625 or 4.575, which is too much. If you are *really* dropping more than 3 percent of the line voltage as measured at the circuit breaker, either the wire is too small, the circuit is overloaded, or there is a loose connection. The use of the word "really" in this paragraph means there must be no measurement error.
Here's what you need to do: make your measurement at the breaker for the branch, then immediately go to the outlet and make it again. Write both measurements down. Repeat that process several times at different times of the day. If all, or most of your measurements agree, then you have most likely eliminated the problem of needing simultaneous measurements. Alternatively, if you have two voltmeters that read identically, a single simultaneous measurement with help of an assistant will be fine. That way we'll know the *real* voltage drop, which could very well be the 7.2 volts you posted. Be sure that you measure both at the panel and at the outlet with the circuit loaded - that way you eliminate the service drop from consideration.
If the drop on the branch is less than 3% of the voltage measured at the circuit breaker while the circuit is loaded, you can turn your attention to the service drop. The way to measure that: with all breakers off except the main and the breaker for the branch in question, measure the voltage at the breaker. With the meter still connected, have an assistant turn the load(s) on, and see what the difference is. That is your service drop voltage. Your "lost" 1.7 volts - or whatever the actual figure is - can be due to other devices on other branches pulling current through that 600 feet of #1 while you were doing the measurements.
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