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Apparently the US segment of the ISS runs on 124v DC, while the Russian segment is on 28v DC. Why the difference? I understand it is often safer to have several different voltages rather than one, but is one better than the other? Is the Russian segment lower because it is older, or because it is more efficient?

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    $\begingroup$ just fyi there are several ISS power-related links in the question Why was the ISS' DC vacuum cleaner replaced with an AC version? Are there “wall sockets” throughout the ISS where it's plugged in? as well as in answer(s) there. $\endgroup$ – uhoh Mar 20 '19 at 5:06
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    $\begingroup$ My guess regarding the 28V: because that was used on Mir and ultimately, because that was/is used in aircrafts. So they could reuse systems designed for this voltage. $\endgroup$ – DarkDust Mar 20 '19 at 7:51
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    $\begingroup$ Related: Why is 28 V a common nominal spacecraft bus voltage? $\endgroup$ – DarkDust Mar 20 '19 at 7:53
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    $\begingroup$ Generally the trade-off is this: a lower voltage is less efficient (more heat loss in wires), but it is also safer. I can imagine that older segments are built to be safer, while newer segments are built to be more efficient (having built up experience) and more compliant (able to plug-in commercial electronics). The reason behind choices of voltage standards could be as simple as this, without having changed ever since, because the real advantages/disadvantage over different voltage are marginal, whereas implementing a change is cumbersome. Bear in mind, this is just my speculation. $\endgroup$ – JJM Driessen Mar 20 '19 at 9:34
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    $\begingroup$ Because the ISS project deliberately did everything different from the Shuttle project. $\endgroup$ – Organic Marble Mar 20 '19 at 13:40
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This answer is a near-exact copy of answer, by a NASA employee, given on Quora to the question "What voltage is used on the International Space Station" ?

The primary power at the solar arrays is at 160 V dc. That is transported across the ISS and then dropped down to 124 V dc at which point it is called secondary power. Most of the US equipment uses that 124 V dc. 124 V dc decreases the current and lowers the cost, weight, and power loss. The voltage is stepped down to 28 V dc for use by most Russian equipment and some visiting vehicles.

[My addition] The first-launched parts of the ISS, Unity and Destiny, were of original American design, so the choice of voltage could be made by NASA and its subcontractors. Coupling with Russian equipment came later. This answer to Soyuz MS spacecraft battery voltage? tells you that 28V DC is a common voltage in Russian vehicle, specifically Soyuz.

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    $\begingroup$ This answer contains a number of factual errors. The first-launched piece of the ISS was Russian, the FGB. The first US space station mission, STS-88, carried Node 1 and linked it with the FGB, so coupling with the Russian equipment didn't "come later". Note that this very early proto-ISS didn't have US solar panels, the US side was powered by the Russian side. The US Lab (Destiny) wasn't launched for almost 3 years, and came after the Russian Service Module was added to the station. Also note that the US Space Shuttle used 28V DC. $\endgroup$ – Organic Marble Aug 24 '19 at 12:47

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