In preparing your shack for a linear, it is critical to make sure that the ac supply is adequate for the job. While this seems straightforward, there are some subtle points that need to be born in mind. In the US, we recommend getting a single-phase 240V line installed. An amplifier putting out the legal limit, 1.5kW will draw somewhat less than 3kW. But let's use this number for an example. The RMS current corresponding to this power and voltage would be expected to be 12.5A. This current is why Alpha amps are shipped with 20A fuses in the line. But this is only part of the story. The above analysis assumes that both the voltage and current are sine-waves. This is not the case. To see why, we have modeled the amplifier using a circuit simulation program known as Spice. We model the important part of the power supply, ignoring most of the components, and referring everything back to the primary side of the transformer for simplicity. This model and the waveforms predicted for it are shown in the accompanying graphic.

The ac line is modeled as a sinusoidal voltage source with a peak amplitude of 415V, corresponding to 240V rms. The power dissipated by the amplifier is represented by the 48 Ohm resistor on the right. The diodes and capacitor are a much-simplified model of the power supply. The green waveform shows C1 being charged at the peak of the ac input, and then discharging as it supplies the current to the load for the rest of each half-cycle. The voltage varies between about 360V and a little over 400V with an average value around 380V. The average current in the 48 Ohm resistor is 7.9A, and the power it is dissipating is close to 3kW. This current is shown by the red trace near the bottom of the screen. So far so good- but look at the blue trace.  This is the current in the ac source, before it goes into the bridge rectifier diodes. It is extremely "peaky", with the tops of the peaks exceeding 150A! And this is the reason why normal voltage-drop calculations for shack wiring are likely to give results that are not in agreement with what you might measure. House wiring is normally either 14 or 12 gauge (American Wire Gauge or AWG). Imagine that you have 200 feet from your breaker panel to your shack. The loop resistance for 14awg is 0.9 Ohms, and for 12awg around 0.64 Ohms. Using the 12.5 Amps calculated using a sine-wave current, the voltage drop at the shack end of the 14awg would be about 11V for the 14awg, and about 8V for the 14awg, both acceptable numbers, and representing less than 5% drop under load. But, using the 150A peak, the 14awg drop would be 135V and the 12awg drop would be 96V!  Now, you would not expect to see quite this much drop for a variety of reasons, such as the inductance of the ac line and so on, which help "smooth" the current peaks a bit- but assuming a peak current of six times the rms would not be unreasonable. The take-home lesson from this is that the wiring for an amplifier should be much heavier gauge than would be calculated using the rms equivalent current. If possible go for 10awg or even 8awg. And, the longer the run to your shack, the thicker the gauge should be. The rule of thumb for AWG is that for three gauge sizes, the resistance changes by a factor of two. So going from 14 to 10 awg, the voltage droop under load would more than cut in half.

A few final notes are in order. The 20A fuses in your amplifier don't blow because they respond to the rms (heating equivalent) current. They have a thermal time-constant longer than the ac waveform, and since they blow based on overheating, they are correctly sized based on the rms current. For those who use their amplifiers off generators, this peaky current can be problematical, and can make it very difficult to get full power even though the generator is apparently rated at sufficient power. The generators' regulator can also have a tough time and not track load variations as well as you would expect. Different generators respond differently, so test your amplifier with the generator before heading off for field day or a dxpedition. Finally, the peaky current is rich in harmonics of the ac line frequency. The current in the conductors produces a magnetic field, which can in turn induce mechanical vibration in the wires and other nearby conductors. That is also why a high quality transformer is used in Alpha amplifiers, as the details of the construction can affect how much "buzz" is audible from the transformer, as the harmonics are much more audible than the fundamental 60Hz. But a buzz may be heard even from wires in a steel conduit! This problem is even worse when running from 110V, and why we recommend using 240V if at all possible. If you must run from 110V, you should probably expect to get more like 1kW. This problem is true for most amplifiers produced today. Even some of the simple switch-mode power supplies that lack active power-factor correction exhibit exactly the same characteristics.