Prof. Trace's lab

Prof. Trace works late. The lab is lit by the moon, an oscilloscope and a Tesla coil that should probably be switched off, and the chalkboard holds the one equation everybody remembers. The Electrical department's calculators are on the bench, the shelf, the fuse board and the floor.

Most electrical sizing comes back to Ohm's law and to heat. Current through a resistance drops voltage and turns power into heat in proportion to the square of the current, which is why cables are sized for both voltage drop and how hot they run, and why transformers are rated by apparent power. The calculators here cover power in single- and three-phase circuits, voltage drop along a run, cable sizing and transformer sizing.

The poster on the wall is the water analogy: voltage as pressure, current as flow, resistance as a narrow pipe. It is a good place to start, and the red question mark is where it stops working.

“Double the current in a wire and it makes four times the heat.” — Prof. Trace

In this office

• Ohm's Law: Basic electrical calculations (V=IR)
• Transformer Sizing: Choose a standard single-phase transformer rating for a load, and find the primary and secondary full-load currents
• Cable Sizing: Smallest copper or aluminium conductor, in AWG/kcmil or metric mm² sizes, that keeps voltage drop within an allowed percentage over a single- or three-phase run (voltage drop only, not ampacity)
• Power Calculation: Real, reactive and apparent power of a single-phase or balanced three-phase load from voltage, current and power factor
• Voltage Drop: Calculate voltage drop in conductors

Project guides

• Size a Home Backup Generator: Work out how many watts a backup generator needs to run and to start your loads, convert that to amps, then check the wiring between the generator and your panel.
• Wire a Subpanel: Work out the current an individual load draws, then size the feeder that runs from the main panel out to a garage or workshop subpanel so the far end still gets full voltage.

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