Point at or tap any part to see its name. Tap a lug or the neutral bar to move a meter probe there.
This panel runs on 208Y/120 volts, a nominal system voltage listed in 120.5(A). A real panel reads a little above or below these numbers.
The other common system is 480Y/277. It works the same way. 277 times 1.732 is about 480.
Black, red, and blue for A, B, and C is common trade practice on 208Y/120. The code does not set these colors.
The meter shows the RMS value, 120 or 208. The waves show the voltage at each moment, which peaks near 170 volts.
Each pointer is drawn 120 units long to match the meter. The yellow line joins the two points the meter touches. Its length is the meter reading.
Two pointers and the line between their tips make a triangle with a 120 degree corner. That line is 1.732 times as long as a pointer. 1.732 is the square root of 3.
The bus bars run A, B, C from left to right, as 408.10(E)(1) requires. The drawing spreads them apart and stops each one at its breaker. In a real panel they run the full length, stacked, and the breakers clip on in turn.
Amber dots show which way current flows and how much. They do not show electron speed. Real electrons drift less than a millimeter a second.
Each row is four 300 watt lamps, 1200 watts in all. That is 10 amps on a 20 amp breaker. The picture treats each row as a plain 12 ohm resistance.
The meter is on live parts. Testing in a live panel is a trained task. It takes gloves, eye protection, and a meter rated for the job.
The panel cover is off so you can see inside. On the job it goes back on before the power does.
This panel has main lugs only. The feeder comes from a main breaker that is not drawn. A building transformer sits upstream of that breaker. Neutral and ground are joined upstream, not in this panel.
Each row is fed through EMT, which is thin steel tubing. The EMT is the equipment ground for the row, as 250.118(A)(4) allows.
The feeder neutral carries zero amps only when the loads are balanced. Each row's own neutral still carries 10 amps back to the panel. Treat every neutral as live.