To make the basic calculations for the design of heating elements, the following formulas, known as Ohm's Law, are essential:

1. Volts = Amperes xOhms E = I x R

2. Watts = Volts xAmperes W = E x I

3. Watts = (Amperes)(Amperes) x Ohms W = I x I x R

USEFUL COIL FORMULAS

Length of one turn of wire = Pi x (Outside coil diameter-wire diameter)

Number of turns of wire per inch of coil = 1/(wire diameter)

Length of wire in 1" of closed coil = [Pi x (Outside Coil Diameter- Wire Diameter)]/(Wire Diameter)

Resistance per inch of closed coil = [Pi x (OD- Wire Diameter) (Resistance /Ft. of Wire)]/12 (Wire Diameter)

Length of coil = [Total Resistance of Coil] / [Resistance per Inch of Closed Coil]

Combining the formulas given above, we get the complete formula for coil calculations.

LENGTH OF COIL =

[Coil Resistance x Wire Diameter] / [.262x (Coil OD - Wire Diameter)*(Resistance/Ft. of Wire Used)]

WIRE RESISTANCE =

[(OHMS-CIRCULAR MILS) / FEET] / [Diameter in Mils squared]

STRIP AND RIBBON RESISTANCE FACTOR =

[OHMS/Sq.Mil Ft.] / [THICKNESS(mils) x WIDTH(mils) x Correction]

If calculations are done for ribbon wire, a correction factor of .94 or .83 must be used depending on the ratio of width to thickness. The higher the width to thickness ratio the lower the correction factor is.

WIRE WEIGHT =

[Pi x r^2 x LENGTH x LBS] / CUBIC INCH

STRIP WEIGHT =

[Thickness x Width x Length x Lbs.] / CUBIC INCH

OHMS/SQ. MIL FT =

OHMS/CIR. MIL FT. x .7854