FAQ
TL;DR: Current in a series path is constant, even across different wires. Example: #22 Cu ≈ 0.0529 Ω/m; “A wire is really just a resistor.” [Elektroda, Steve Lawson, post #21666727]
Why it matters:** This FAQ helps beginners and hobbyists size wires, predict voltage drop, and avoid overheating in real builds.
Quick Facts
- Series rule: V = I(R1 + R2 + …); individual drops add, current stays the same. [Elektroda, Richard Comerford, post #21666726]
- Typical copper examples: #22 ≈ 0.0529 Ω/m; #18 ≈ 0.0209 Ω/m. [Elektroda, Steve Lawson, post #21666727]
- Example load: 1 V across 73.8 mΩ series wire → ≈13.6 A (risk of damage). [Elektroda, Steve Lawson, post #21666727]
- The load sets current; undersized wire heats to failure. [Elektroda, Steve Spence, post #21666728]
- Perfect junctions add no drop; drops occur along each resistive section. [Elektroda, Frank Bushnell, #21666731
Does current stay the same through wires of different materials and diameters in series?
Yes. In a series path, the same current flows everywhere. Material and diameter change resistance and the voltage drop, not the current. [Elektroda, Frank Bushnell, post #21666731]
Where does the voltage drop happen when the wire changes diameter or material?
Voltage drops along each wire segment according to V = I × R for that segment. At a perfect joint, there is no extra drop. [Elektroda, Frank Bushnell, post #21666731]
How do I calculate total resistance for several wire sections in series?
Add the resistances: R_total = R1 + R2 + …. Then use Ohm’s Law, V = I × R_total, to find current or drop. [Elektroda, Richard Comerford, post #21666726]
Can different wires end up with the same resistance per meter?
Yes. Different materials and diameters can yield the same resistivity per unit length, so their voltage drops can match at a given current. [Elektroda, Richard Comerford, post #21666725]
What’s a real example of current through mixed wire gauges at 1 V?
One meter of #22 Cu (52.9 mΩ) plus one meter of #18 Cu (20.9 mΩ) totals 73.8 mΩ. Current ≈ 1 V / 0.0738 Ω ≈ 13.6 A. [Elektroda, Steve Lawson, post #21666727]
Who decides the current—the source, the load, or the wire size?
The load and source set the current. If the wire is too small, it still tries to carry that current and overheats. [Elektroda, Steve Spence, post #21666728]
What happens if my wire is undersized for the circuit?
It runs hot, the insulation can degrade, and the conductor can melt. That is a heat-related mechanical failure risk. [Elektroda, Steve Spence, post #21666728]
Is a wire basically a resistor?
Yes. “A wire is really just a resistor”—usually a very low-value one—so its resistance still causes voltage drop and heat. [Elektroda, Steve Lawson, post #21666727]
How can I measure and compare wire resistance or current at home?
Use a DMM. 1) Measure resistance with the ohmmeter across a known length. 2) Or put an ammeter in series with a test source. 3) Compare readings between wires. [Elektroda, Richard772 Susan, post #21666735]
Does the junction between two wires add voltage drop?
Assuming a perfect, low-resistance join, treat the junction as a single point. The drop occurs along each segment, not at the ideal joint. [Elektroda, Frank Bushnell, post #21666731]
Why do designers often ignore wire resistance in small-signal circuits?
Connected components usually have much higher impedances than the wire. The wire’s small resistance becomes negligible in those contexts. [Elektroda, Steve Lawson, post #21666727]
How does current density change when wire diameter changes?
Current density increases in the smaller-diameter section. That raises temperature for the same current, increasing drop and heating risk. [Elektroda, Frank Bushnell, post #21666731]
(Of course, you meant "impedances".)
