The following two formulae will help you to calculate the diameter of wire under certain Capacity and Voltage.

Where:

d — Diameter of heating wire, mm
p — Resistivity of heating wire,Ω·mm2/m
P — Power per phase, KiloWatts
U — Voltage,Volts
W — Surface loading,W/cm2
L — Length of heating wire, m

For Example:

1, Supposing the condition is 800W/240V and resistance per meter you required is 8.7 Ω/m, you are going to use Nichrome 80 as the heating wire because of its high and stable performance. Refer to Nichrome Wire Data Table, you will find that dia. 0.4mm is what you need. Here according to technical data of Nichrome 80, p is about 1.09 Ω·mm2/m. So according to the formula 2, L is 8.3 m. To verify the surface loading, W is 7.68 W/cm2,  you can check whether it is ok for you. If your calculated surface loading is too high or too low, please change the grade of alloy.

2, Surface loading is related with heat transfer and element's service life. If you have requirement on surface loading such as W is 5 W/cm2,then you can know d is 0.46mm under the same condition as 1.

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