OK BigJimmy, I got it. I hadnt used Ohms Law in years, silly me...... Thanks
Now I got one for you.......... When we buy electricity, are we buying watts or amps? I say amps and if we have voltage drop, wont the amps increase? So if the power supplier drops violtage, say 5%, wont that increase billable usage ??
PS. I dont like voltage drop, so I will use #12 for above circuit but a 15A breaker.
Ok, your power company (i.e. often quoted in this site as "POCo" for short) bills you some cost per kilo-watt hour or KWh. Again, this is really just P=IV again. They don't charge you cost/amp because they have not only residential customers but comercial and industrial customers as well. That being said, you are likely connected via a split-phase 3-wire service whereas a non-residential customer may have a 3-phase 480V or higher service, depending on their load. That being said, 5A at 240V is much less instantaneous power consumption than 5A @ 4160V. So, a more standard way of measuring is by watt or kilowatt hours. As an example, you have a 100W bulb that is on for 10 hours. This is simply 1000Whrs or 1 KWh. Remember, the P=IV formula represents the power consumed at any instant in time whereas energy consumption is the power measured over time (and instant. power is generally variable at any given moment depending on what you're using in your house). So, if all service voltages/phases were the same, I guess we could talk about being billed for current consumption but this is not practical as I mentioned above.
Now voltage drop is interesting. In the case of a resistive (or nearly resistive load) like an incandescent bulb, the energy consumed is again based on P=IV. Using ohm's law for a moment, V=IR, if the voltage decreases and the load resistance remains the same, the current will decrease. From the perspective of P=IV, the power consumed will decrease as well. So why don't we just all install variacs at our service entrance and adjust the voltage to say 90V and save a bundle? Well, devices are designed to operate at a nominal voltage which is assumed to be 120VAC. If we turn down the voltage, the lights get dimmer, it takes longer to toast our bread, etc. Then you'd be putting in higher wattage bulbs to increase the light output, re-toasting your bread, etc. In the end, you'd probably be consuming just as much power. However, there is a bad side to low voltage. The most common motor type is induction. They are designed to consume a certain amount of current at their design voltage. When they are subjected to low voltage, they compensate by consuming more current. The problem is, I^2R losses dominate and this leads to increased heating which can shorten their lives and decrease their efficiency (which of course increases the cost to run them).
Most utilities guarantee the end-users' voltage to be some nominal value plus/minus say 10%. To you and I, this means that if I measure the 1ph voltage at a receptacle, it should be 120V +/-10% (108-132V). If you were to install a strip chart recorder in your home, you'd likely see the voltage changes over the course of the year, especially if you live in an area that is subjected to wide temperature fluctuations over the year/seasons. The amount that you'll see can be affected by several things including where you are physically connected to the POCo's distribution system (i.e. are you near or far from a dist. transformer), the degree to which it is loaded, the size of your service, the extent that you are loading the service, etc. Highly inductive loads from industrial customers do a great job of depressing voltages simply due to the nature and size of their loads (but if they are a big consumer, they are typically required to maintain their power factor above some agreed upon value or face fines for failure to do so).
When we concern ourselves with voltage drop at the residential level, most people shoot to keep the terminal voltage at the end of the circuit w/in 5% of the line voltage at the service panel (the NEC does not require us to comply with this. They only recommend). Since the branch circuit conductors have some finite resistance, voltage will always be less (excluding capacitive loads) at the far end of the circuit for non-zero current. Longer conductors and higher currents both lead to more voltage drop. In your case, there is nothing wrong with using #12 wire on a branch circuit protected at 15A and given that it sounds as if you are pretty much expecting that load to be 15A, it's probably not a bad idea. But if your receptacle is 15ft. from your panel, at the end of the day (w/o running a calc), it probably won't make much difference.