My question was simply. If a meter of wire has impedance of 1 uH. What is the impedance of a meter of bus bar?


To make it simpler.

The longer the leads of SPDs (Surge Protector Device) to load, the larger the inductance and larger the let through voltage at the load.

I'm trying to understand (or compute) what would happen when the leads are not just wires but they pass through the bus bar or terminal lugs. Remember in whole house SPD, you install it at one of the breaker load side especially at the top breaker near the main breaker (reason is to make the shunt shorter for lower inductance and lower let thru voltage). What if (for sake of discussion), it was installed far from the top breaker or in the bottom breaker. The path or equivalent lead length would be like longer and would pass through the bus bar cooper. Would this increase the inductance?

Or imagine the whole house SPD unit leads are not made of wires but composed of bus bar cooper (or imagine you just replace the wires with bus bar cooper). How would it behave?

Playing with this online inductance calculator. I tried bigger diameter wire. The inductance actually decrease. Why does it decrease for bigger diameter wire? Does it mean for bus bar, the inductance induced is actually lesser than passing through wire of similar length or even when it pass through terminal lugs?



Consider the following SPD connected to the panel:

enter image description here

Below the main breaker are lead terminals (shown in green) connecting directly to a 240v-120v step down autotransformer powering the Siemens GFCI breakers 120v circuitry directly (I don't have neutral in the building to avail of natural 120v).

If the SPD wires from outside the enclosure at left passes through the top left breakers load side passing through the bus bar. What is the impedance compared to if the wires would be connected to the transformer leads (shown in green) at the terminals directly?

According to the reference below. For high-frequency currents, impedance per unit length of wire is about 1 uH/m. How about when it passes through a bus bar? What is the impedance and how is the length or area determined?

enter image description here


  • 1
    \$\begingroup\$ If the buss bar is very near a RETURN bar, the inductance can be 5:1 smaller. But a buss bar of the same cross-sectional area as a round wire will have about the same inductance. \$\endgroup\$ – analogsystemsrf Feb 25 at 4:05
  • 1
    \$\begingroup\$ We abandoned bus bars due to their cost and weight. We went to a design where the PCB was 90 mils thick and had neutral and ground planes on opposite sides. The MOV modules were on top with direct connections to the phase wires. Bus bars are ok when you have thousands of amps to carry as continuous current. \$\endgroup\$ – Sparky256 Feb 25 at 4:06
  • \$\begingroup\$ @analogsystemsrf and Sparky256 please see above added picture of bus bar and descriptions so you can answer accurately based on the actual implementation above, Thanks. \$\endgroup\$ – Jtl Feb 25 at 4:43
  • \$\begingroup\$ @analogsystemsrf In typical panel like the Siemens PL load center in picture above. Is the bus bar very near a return bar? Where is the return bar? Also how can the bus bar have same cross-sectional area as a round wire? The bus bar always have much larger cross section being wider and flat. I'm still finding the answer after a month. Kindly assist. Thanks. \$\endgroup\$ – Jtl Mar 18 at 23:43

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