[Sunrescue] 240V wiring

Greg A. Woods woods at weird.com
Sun Aug 27 18:49:25 CDT 2000

[ On Sunday, August 27, 2000 at 12:19:29 (-0700), James Lockwood wrote: ]
> Subject: Re: [Sunrescue] 240V wiring
> I've thought about this, but there is a US standard in place for 220-240V.
> NEMA 6-15R outlets are in regular use where 240 is required (machine
> shops, most frequently).  The plug looks like:
>        _____
>       /     \
>      | -- -- |
>      |   o   |
>       \_____/
> Basically, the same as the traditional US 120V plug but with the contact
> blades horizontal instead of vertical.

Yes, exactly and for a normal 15A(?) circuit this is I believe the only
type of socket that meets Canadian and Ontario electrical code

Different maximum current ratings have different contact layouts,
shapes, sizes, etc. in order to protect the circuit should you try to
plug in a device that ordinarily draws close to, or more, power than the
maximum of the lesser rating.  For example that's usually why electric
stove and electric dryer plugs are different shapes and sizes.  You
don't want to plug a stove into a dryer outlet and then learn after the
fire that the socket itself and/or the wiring back to the breaker panel
was only rated for the dryer load!  :-)

It is safe, so long as you know exactly what you're doing and so long as
you do know how to calculate electrical power loads, to put a plug for a
higher rating on a device so that you can attach it to a higher rated
circuit.  However this is generally a waste -- it's better to get an
appropriate multiple-outlet strip/box/etc. and wire that in so that you
can take full advantage of the higher-rated circuit.

>  240 in the US is usually
> "double-hot" with the neutral at 120V relative to each.

Yes, exactly, as it is in Canada.  I was going to reply to your earlier
message about 240V UPS units where you said you wanted a "unit that also
ran a live neutral so I could run 120-only kit off of the split phases
if I had to" and remind you that a centre-tapped transformer does not
create different "phases", at least not in the same sense as true
two-phase and three-phase systems.  Both sides of the transformer are by
necessity "in phase".  I suppose it's correct to call it a "split phase"
in the context of other true multi-phase circuits, but I wouldn't use
that phrase ordinarily because it has entirely different connotations.

So far as I've seen any general-purpose UPS that has a 220V plug of any
rating will also have at least a pair of standard 120V-15A sockets too.

> I can get 6-15 plugs and outlets at any hardware store, but it gets kind
> of expensive to assemble every power cord with nice removable plugs.

Yes whenever you want to do custom electrical wiring that doesn't match
the same rating as the most common local variety (120V-15A here in North
America) it can get a lot expensive, especially if you buy at "any
hardware store".  Even finding industrial twist-lock plugs for 120V-15A
is often harder to do (I *always* try to use twist-lock plugs for any
production equipment that plugs into sockets anywhere near where a
person might move!).  Generally I scan the bins at local electronics
surplus stores and often I can find the parts I'm looking for at a
fraction of their "new" price, even at big discount stores.

							Greg A. Woods

+1 416 218-0098      VE3TCP      <gwoods at acm.org>      <robohack!woods>
Planix, Inc. <woods at planix.com>; Secrets of the Weird <woods at weird.com>

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