Inveterate meddling #103: dedicated mains radial circuits

A separate radial circuit, often mistakenly called 'a spur', is a nice thing to have.

The point is to supply electricity with as little added electrical noise as possible and a minimum of shared series impedance. We do this by connecting a radial circuit unused for any other electrical items back to the distribution point at the main intake (the consumer unit), where the impedance is as low as possible.

the parts

  • Cable of 6mm2 is about optimal for a dedicated radial circuit. This carries a current rating of 30Amps when enclosed. Some people report 10mm2 (45A rating) is worth using , but it's much more difficult to work with, and fitting it into the socket receptacles can be nigh impossible. Whether you install twin-and-earth (Romex), or three separate cables is up to you/your electrician and the route to be installed; separate cables are easier to pull round bends.
  • Sockets: In the UK MK "Logic+" are good, as are the old-fashioned three round-pin 15A sockets. The Logic+ type have copper bus bar for the earth connection and are available switched and unswitched.
  • Plugs: In the UK Crabtree plugs give very consistent results and a tight fit to most sockets; they also have screw terminals inside you can really lean on.

    the method

  • Fit a radial circuit not a ring. We've built both and the radial circuit simply sounds better, regardless of what Russ Andrew says. In fact we installed a 30A dedicated ring, then cut it into two radial circuits after listening to it....
  • How you provide outlets is up to you. A cluster of switched or unswitched sockets at the end of your radial circuit, or one single socket into which to plug a hydra- like powercord connecting all your boxes. Either wy, with a radial circuit you'll be limited by teh upstream breaker so don't agonise over non-existent differences.
  • If you install a cluster of sockets, make a note of the order of connection to the end of the radial circuit. For some the order in which your kit is plugged in appears to make a difference, whether its power amp first, source last (Naim's preference) or the other way around. This is probably down to the number of joins in the cable and adding a little bit of excess contact resistance rather than 'magic'
  • 50% of the benefit is probably in the Earth-equipment 0v bond which is not seeing the superimposed leakage current from other, mains-filtered bits of household equipment (except insofar as they all join-up at the consumer unit or Meter henley Blocks eventually). Conjecture - refer to pages on Earthing and filtering.
  • If a separate earth connection appears unfeasible - for legal or logistical reasons, especially for those of you in apartments - don't worry about it. Run the the radial circuit's earth wire back to the main earth connection on your distribution board and stop worrying. It's still a cleaner earth than the rest of your electrical installation.
  • Buy a couple of different RCD breakers and try them, some report sonic differences between brands. Thi sis in teh realms of Foo.
  • Connecting to the nearest copper pipe (gas/water/ central heating) is NFG as a separate earth. Separate earth means a dedicated earthing stake. The pipework is earthed - 'equipotentially-bonded' more properly - only for significant safety reasons. Using such as a functional earth can compromise this. DONT DO IT. Earthing has its own page and suitable warnings come along for free.
  • Don't believe anyone who tells you a separate earth will drain away all RF interference; the few feet of wire between the boxes and the earth stake has more than sufficient inductance to give the lie to this fallacy. If you believe this kind of thing, I have an AC battery I'd like to sell you.
  • Mains power filters are infra dig., by the all-powerful Law of Inconvenience which holds sway over the Flat Earth. For good reason, too - they all make a mess of the music.

    We'll discuss DIY earthing separately. And UK mains Plugtop fuses, too.

    the disclaimers

  • Play with your electricity supply and you are on your own. Do we need to say this ?
  • From 01 Jan 05 in the UK Electrical works such as this need to be signed-off by a suitably-qualified person or approved by your local Building control inspectorate, following the introduction of the new Approved Document Part P component of the Building Regulations. Part P documents are available as a free download here: Building Regulations
  • Your insurers will take a dim view of homebrew 'modifications' that do not meet prevailing IEE regulations and any other codes in force, especially if a fire or other damage results.
  • Don't be tempted to 'oversize' the radial circuit RCD/fuse. It's there to protect the radial circuit - prevent it overheating in the case of a short - and a small fire will not endear your fringe hobby to your loved ones. In any case the sonic difference is insignificant...
  • Fitting a separate earth requires you understand the potential for electrocution and how equipotential bonding prevents that risk. If you don't know, ask someone who does.
  • If in any doubt, pay a qualified electrician and heed their advice. Compared with the cost of the kit, and the risks, it's cheap; 150-250 is typical for a fully sorted - and legal - installation.

    © the twisted pair 2000

  • 02.06.13 Crap trimmed.
  • 02.01.07 Part P link updated.
  • 02.01.05 Part P update added.
  • 17.12.01 page added.