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. You should have 2
lines (or phases as you have called them) and a grounding conductor. There
is no neutral in this type of an installation. If you have a long run,
voltage drop should be considered. I would be happy to calclate it for you
if you provide the distance and load in ampres.
>1) Some 240VAC circuits employ two phases and a ground, but no neutral.
You will not need a neutral if there is not requirement for 120V. in this
equipment (remember line-neutral = 120V)
>2) Some 240VAC circuits employ two phases and a neutral, but no ground.
There should always be an equipment grounding conductor, I don't know of
what would not use a ground. Maybe if the circuit was installed in conduit,
the conduit could be used for a ground. Quite often distribution is done
with 4 conductors. 2 lines (black and red) , 1 neutral (white), 1 ground
(bare or green) or with 3 conductors and a conduit system, with the conduit
being the ground conductor.
>3) I have not encountered a "4-pole" circuit which provides for two phases,
a
>neutral AND a ground.
There are many 4 Wire devices on the market. Check out Hubble for proper
connectors.
>Although the ground and neutral are tied to a common buss within the home's
>central load center, why is it that a safety ground is not carried out in
addition to
>circuits which employ a neutral? The neutral is allowed to carry AC
current, a
>ground is not. The ground is present only to establish a non-current
reference
>back to the load center. It would seem then that in configuration #2, the
chassis
>of the amp is referenced to a current carrying neutral potential, thereby
negating
>the safety potential to the chassis as would be provided for by a separate
>ground.
Every circuit should have an equipment grounding conductor. The only reason
I can think that yours does not is because it uses a conduit system, which
is allowed by code. Neutral and equipment grounding conductors are bonded
together, usually in the main breaker panel, to create a reference. After
that point the, neutral is used to carry any harmonics, ie from inverters,
and unbalenced current back to the utility.
>I'm no NEC expert, and perhaps there may exist an electrical contractor on
this
>reflector who can better explain the thought process behind 240VAC
distribution
>in a home.
>Interestingly, the Alpha amplifiers (and perhaps others) allow for the use
of
>either configuration #1 OR #2 as referenced above. The manual takes the
owner
>through steps which change various AC input taps to match the wiring
>distribution.
Be sure to use an equipment grounding conductor to keep the cabinet at
ground potential.
>If I were to take a poll, how many of you run your amps with the
configurations
>as stated above? If NEC allows it, have any of you successfully tried
>configuration #3, also assuming there may exist a 4-pole 240VAC
>plug/receptacle?
I can't see any reason for a 4 pole device. unless you need 120V control
voltage is needed, and there is not a tap on the transformer. Use a 3 pole,
twist-loc device, and be sure your receptacle has a grounding conductor to
it, and that you are using it, (ie not connected to a neutral) and you will
be happy, and installed per the 1999 NEC.
If you find out otherwise please feel free to send me any other info back as
there may be something I am overlooking as I am by no means an Amplifier
expert, but you should not use a neutral for a ground, I am sure of that.
73,
Eric
K8CCA
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