Shunt Resistor
Drop, dissipation, and required rating for a current-sense shunt.
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The engineering
Shunt sizing is a three-way trade: bigger resistance means more signal but more heat and burden. The ×2 derating row is the difference between a stable reading and watching your calibration drift with temperature — I²R heating changes R, which changes your answer.
At 100 A even 1 mΩ dissipates 10 W. Kelvin (4-wire) connections stop lead resistance from joining your measurement.
Where this math comes from
The shunt is how the ammeter survived electrification: pass the current through a known low resistance and measure millivolts instead. Edward Weston's 1893 portable instruments — with his manganin alloy, whose resistance barely moves with temperature — made calibrated current measurement an industrial commodity.
Manganin shunts calibrated a century of switchboards and are still what sits inside your bench meter's current jack.
- 1884Edward WestonManganin — the near-zero-tempco resistance alloy.
- 1893Edward WestonPortable precision instruments make shunt measurement standard practice.
- 1861Lord KelvinFour-terminal (Kelvin) sensing removes lead error.
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