The total amperage of the nichrome wire circuit must not exceed the total amperage of the variable voltage control that is supplying the circuit. As a rule-of-thumb do not exceed 80% of the total amperage rating of the variable voltage control.

Ohms Law:
V = I x R Voltage = Current x Resistance (Ohms)
I = V/R Current = Voltage/Resistance (Ohms)
R = V/I Resistance (Ohms) = Voltage/Current

Example #1:
Calculate the required "hot-wire" circuit resistance that will result in a full scale adjustment (0 to 100%) of a 120 volt, 15 amp, variable voltage control.

Maximum recommended circuit amperage = 15amp x .80 (80%) = 12 amps
Maximum control voltage = 120 volt
Using the resistance calculation given above R=V/I = 120volt / 12amps = 10 Ohms

Example #2:
Using the result from example #1, how much 21 gauge nichrome wire would be needed to meet the requirements?

From the specification table: 21gauge nichrome wire resistance = .831Ohms/ft
Required Nichrome Wire Length = (Required Circuit Resistance) / (Ohms/Ft Of Wire)
Required Nichrome Wire Length = 10ohms / .831ohms/ft = 12.034ft

Example #3:
From example #2, suppose that you only want to use 6ft of nichrome wire. What is the maximum controller scale adjustment that can be made without exceeding the 120 volt variable voltage controller’s 15 amp rating?

Variable voltage controllers have a dial adjustment between 0 and 100% of the maximum control voltage. That is, a 50% setting on a 120 volt control will result in an output of 120 x .50 (50%) = 60 volts. A 30% setting on a 120 volt control will result in an output of 120 x .30 (30%) = 36 volts.

A 6ft piece of 21gauge nichrome wire will have a total resistance of .831ohms/ft x 6ft = 4.986 ohms.

Recall that the maximum recommended amperage is 15amp x .80(80%) = 12 amps
Using the voltage calculation from above V=IxR = 12amps x 4.986ohm = 59.832 volts

What percentage of the dial setting will result in a 59.832 volt output?
% Dial Setting = (Required Voltage / Maximum Control Voltage) x 100
% Dial Setting = (59.832volt/120volt) x 100 = 49.86%

The dial on the controller can be adjusted from 0 to 49.86% without exceeding the controller’s amperage rating.

Example #4:
Based on the examples above what is the maximum estimated temperature that the 21gauge nichrome wire will achieve at 12 amps?

From the specification table:
A 21 gauge nichrome wire at 10.88amps will result in a wire temperature of 1600°F
A 21 gauge nichrome wire at 13.40amps will result in a wire temperature of 1800°F
Therefore, we can expect the wire temperature to fall between 1600°F and 1800°F.
We can estimate the temperature to be approximately 1700°F at 12 amps.