Thanks to those of you who helped me discover the "3-pole, 4-wire" 240 VAC
outlet. This is exactly what I was looking for and it is intended for those
applications where a separate ground and neutral are required. I again
went with a twist-lock connector, this time an isolated ground type from
Leviton. I just got back from making my electrical materials purchase.
It's amazing what happens to the cost per foot of electrical cable when you
jump from #10 to #8 AWG.
I suspect that the wiring information contained in the 77Dx/Sx manual dates
back to a time when it was considered "acceptable" to allow the neutral to
function as the equipment ground. It doesn't matter that the neutral and
ground are tied together at the load center; it can very dangerous to rely
on a current carrying neutral to function as a ground. If the neutral is
lost, the equipment chassis can easily be elevated to the high potential
from either phase of the 240VAC supply (Vic, thanks again for pointing this
out as a potential safety concern).
In isolating the neutral from ground in the 77Dx/Sx, one must break the
jumper between terminals 10 and 11 in the Cinch-Jones power connector (P1).
No modification is required within the amp. After removing the jumper,
neutral goes to terminal 11, Ground goes to terminal 10. The power cable
between the amp and wall plug must be upgraded from three conductors to
four. Belden makes a Neoprene-jacketed SO type (#19216) that's perfect for
the job.
Now, if the neutral is lost for any reason, the chassis will not be hot
through the blower winding. If allowed to continue in this condition, I
suspect that the blower would continue to run by finding its neutral return
through the BNC, SO-239, or RCA jacks and potentially complete the path back
to neutral via the ground lead on another piece of equipment. Can anyone
smell smoke? I'm sure that losing the neutral would be a rare occurrence,
but it could happen. Even if this mod is never made on the 77Dx, I would
ensure that the neutral connection point at the house load center receives
periodic preventive maintenance in the form of inspection and hardware
tightening.
-Paul, W9AC
----- Original Message -----
From: "Ed W3NR" <w3nr@vol.com>
To: "measures" <2@vc.net>; "Eric Moore" <emoore@windemullerelectric.com>;
"Paul Christensen" <paulc@mediaone.net>; "AMPS" <amps@contesting.com>
Sent: Tuesday, November 14, 2000 19:31 PM
Subject: Re: [AMPS] 240VAC Amplifier Wiring
> I think what Rich means is 8-3 or 6-3. Either is overkill unless you have
an
> extremely long run. But to be safe you might want to consider 8-3 over
10-3.
> Myself I would go with the 10-3, more than enough. Also remember that your
> 240 ground and the 120 neutral are usually attached to the same grounded
bus
> bar. If you really want to play it safe, make the ground on the 240 a
> dedicated ground, which is attached to its own bus bar that is grounded
and
> should also be attached to its own ground rod.
>
> Ed W3NR
>
> -----Original Message-----
> From: measures <2@vc.net>
> To: Eric Moore <emoore@windemullerelectric.com>; Paul Christensen
> <paulc@mediaone.net>; AMPS <amps@contesting.com>
To: <amps@contesting.com>
> Date: Tuesday, November 14, 2000 6:34 PM
> Subject: Re: [AMPS] 240VAC Amplifier Wiring
>
>
> >
> >>
> >>Hello Paul,
> >>
> >>I am an Electrical Engineer for a large Electrical contractor in
Michigan.
> >>We don't do much Residential, but I think I can help you out. See
> comments
> >>interspersed below:
> >>
> >>>As I begin to add a 240VAC power drop to my radio shack at the new QTH,
I
> >>>began to consider cable sizing, connectorization and grounding issues.
> At
> >>my >previous home, I used #10/3 CU cabling for a total distance of
60-feet
> >>to my >Alpha 77Dx (single tube). I then terminated the drop into a
> Hubbell
> >>30-amp >Twist-Lock receptacle. I then began thinking about various
240VAC
> >>distribution >possibilities:
> >>
> >>The 10/3 is fine if your circuit draws less than 30Amps.
> >
> >10-3 is not fine for capacitor filter supplies that draw an indicated
> >30a. The problem is that peak current is 8 - 10 times higher than
> >indicated current. My advice is to use 10-8 or even 10 - 6 for long
> >runs.
> >- Rich..., 805.386.3734, www.vcnet.com/measures.
> >end
> >
> >
> >--
> >FAQ on WWW: http://www.contesting.com/FAQ/amps
> >Submissions: amps@contesting.com
> >Administrative requests: amps-REQUEST@contesting.com
> >Problems: owner-amps@contesting.com
> >
>
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