Is it feasible to provide a house wide 24v DC supply that uses mains neutral or ground for the -ve side?

Edit: I removed reference to "NEUTRAL OR ground" because risks, and RCDs, per answers.

Where I am AC is 230v, and and 3-core TPS is normal. If I could get a single 24v at each power outlet, then I can put fairly cheap USB-C Power Delivery points for 5v to 20v DC at each outlet.

But can i use 4-core single phase for this, or do I need to use a separate DC pair? Are there risks beyond the obvious ones (like accidentally bridging the 24v +DC and AC live!)?

  • Mains neutral?? You know, neutral wires are insulated for a reason... – Harper - Reinstate Monica Jul 17 '20 at 18:15
  • any exposed metal with a DC voltage will corrode on the positive end, like a car battery. You also have to worry a lot more about voltage drop at 24v than 230v; losses which could very well exceed the inefficiency of a name-brand AC-DC USB adapter, which these days can be 95% efficient. Unless you have many many points of need and your electric co charges for apparent power (w/ smart meter), point-of-use conversion is best. – dandavis Jul 17 '20 at 19:41
  • @dandavis Yup, I've done voltage loss calculations. The cost for the at-outlet complete 80W USB-C PD module taking 24v is under USD $3, whereas a no-brand full USB-C PD incl power supply is about $15. – Justin Maxwell Jul 17 '20 at 22:05
  • @Monica. Yes, yes I do. Do you? – Justin Maxwell Jul 17 '20 at 22:06

Generally speaking, you can't mix high voltage (120, 240 and above) with low voltage (definition varies, but 24V is definitely low voltage). This means you can't use part of a high voltage cable for low voltage DC, and it also means you can't mix and match low & high voltage individual wires in conduit.

Part of this is the obvious "accidentally bridging the 24v +DC and AC live" safety problem, and there are other reasons as well.

As far as the specific question of neutral or ground as the other 1/2 of the circuit. No, you can't do that. Along with any other safety issues, the wires have fundamentally different uses:

  • "Ground" in DC circuits refers to a return path for the circuit. The "ground wire" in a mains AC circuit (the green or bare wire) is not normally used to carry current, except during a fault situation. Loading it up with 24V DC is therefore not a good idea.
  • Neutral in AC circuits is logically like ground in DC circuits, but it actually carries just as much current and the AC variation thereof. So some of the time that DC current would (theoretically) be opposite the AC current in the neutral (i.e., bringing it closer to 0) and some of the time it would be in the same direction (more current than normal). In any case, it won't work and it isn't allowed.
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    Well said! Hopefully the OP gets the message how badly conceived their query really is. – Michael Karas Jul 17 '20 at 4:05
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    But they asked! That's the key - ask first. Much better than "I tried this and now I wonder if it is OK". – manassehkatz-Moving 2 Codidact Jul 17 '20 at 4:08
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    I know! But you answer provides the strong statement to not even consider the scheme proposed. – Michael Karas Jul 17 '20 at 4:10
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    TL;DR: NO You cannot mix AC and DC on a single wire even if that wire has the same "name" in each of the two, incompatible, electrical systems. – FreeMan Jul 17 '20 at 12:38
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    Neutral is part of the normal-active AC mains loop, so using it as part of the LV loop would be insane. Any glitch in the neutral (these happen all the time) would energize all the LV stuff at 230V. – Harper - Reinstate Monica Jul 17 '20 at 18:18

Conclusion: this is a dumb idea, primarily because 3C+E cable is wayyyy expensive, and it's a lot cheaper to just use 2C+E alongside 2C, which then allows for thicker wires / more current on the 24v circuit.

Thanks to (most of) you all for the informed answers and comments.

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