Carl,
Assuming it's not an autotransformer, then it seems that Sec 250.24 may be
applicable if the one or both of the stepping transformers are "outside," in
which case the serving CT is grounded to an electrode and forms the neutral.
I see no exceptions in the NEC regarding alternate metallic paths on
existing 3-wire systems when isolating transformers are used but in any
event, it would be best to avoid it. Autotransformers have their own set of
grounding/bonding rules under the NEC.
Your exact arrangement would probably need to be researched a bit further to
see what NEC sections and local code apply.
Paul, W9AC
----- Original Message -----
From: "Carl" <km1h@jeremy.mv.com>
To: "Gary Schafer" <garyschafer@comcast.net>; <amps@contesting.com>
Sent: Wednesday, November 04, 2009 9:36 AM
Subject: Re: [Amps] TL922 Power Plugs
> Im running 480V to the shop out back thru a pair of step up/step-down
> 10kva
> transformers. This uses a 3 wire run and has been in place for 20 years to
> run machine tools, compressor, welder, etc.
>
> Is it correct to have a ground rod at the shop end with the neutral
> grounded? If the building is metal should it also be grounded?
>
> This is getting confusing.
>
> Carl
> KM1H
>
>
>
> ----- Original Message -----
> From: "Gary Schafer" <garyschafer@comcast.net>
> To: "'Paul Christensen'" <w9ac@arrl.net>; <amps@contesting.com>
> Sent: Tuesday, November 03, 2009 10:10 PM
> Subject: Re: [Amps] TL922 Power Plugs
>
>
>>
>>
>>> -----Original Message-----
>>> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]
>>> On
>>> Behalf Of Paul Christensen
>>> Sent: Tuesday, November 03, 2009 8:21 PM
>>> To: amps@contesting.com
>>> Subject: Re: [Amps] TL922 Power Plugs
>>>
>>> > My Neutral and Ground is connected at the main panel but also at
>>> > the barn 300 feet away. I am running a 3 wire 100 amp 240v power 4 00
>>> > aluminum line to the barn from a 100 amp 240v breaker located in my
>>> > main
>>> >distribution box. I didn't like the idea of a gnd neutral tie that far
>>> >away from
>>> > the barn, so I tied them together at the barn with a second ground rod
>>> > located
>>> >at the barn. Is that such a bad idea?
>>>
>>> See NEC 250.32(A) and (B). Since your 3-wire system is existing, the 4-
>>> wire
>>> requirement beginning with the 2008 NEC likely does not apply -- but may
>>> under your local code. New detached structure wiring generally requires
>>> 4-wire service whether or not there is a continuous metallic path
>>> between
>>> structures In no event should the neutral and ground come in contact
>>> again
>>> from the feeder path to the barn's sub-panel (as it seems to exists
>>> now).
>>> Being a detached building, the barn service entrance must be
>>> grounded/bonded
>>> to its own grounding electrode or grounding electrode system in
>>> accordance
>>> with Part III of NEC Section 250.
>>>
>>> Paul, W9AC
>>
>> Paul,
>>
>> If I am following what you are saying that is not correct. Under the old
>> rules with a 3 wire feeder to a detached building the neutral was to be
>> connected to the neutral and ground buss in the panel in the remote
>> building. A local ground rod was also to be installed and connected to
>> the
>> neutral/ground buss at the detached building.
>>
>> The neutral and ground would also be connected together at the main
>> panel.
>> They also stipulated that there were not to be any other metallic
>> connections between the two buildings such as water pipes, phone lines
>> etc.
>> that could possibly carry any neutral currents inadvertently.
>> Otherwise 4 wires would be required and the ground/neutral connection in
>> the
>> detached building had to be then kept separate as the new code states. A
>> local ground rod connected to the ground buss is still required at the
>> detached building.
>>
>> 73
>> Gary K4FMX
>>
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>
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