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Old 04-13-2011, 08:01 AM   #16
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Need help with 240V transformer


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Originally Posted by dmxtothemax View Post
What are you concerned about ?
The fact that you are getting a volt reading between hot and ground ?
No.

Quote:
Neutral and ground are usually bonded together some where.
This is normal .
Agreed.

Quote:
Or are you concerned that the reading is 170v,
Which may seem a little high ?
Yep.

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Again normal if load is light or non existant.
What does it measure under a good solid load ?
I'll have to check but it looks like the consensus is that one leg of the secondary should be bonded to ground.

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Old 04-13-2011, 08:44 AM   #17
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Need help with 240V transformer


You designate one side of the transformer secondary as the "neutral" and ground it at just one point for example at that terminal on the transformer or over at its main disconnect (if any, where the wire from the other terminal connects to a switch).

In the NEC there is mention of a ground electrode system, which in this case consists of a non-stop run (home run) of a ground wire of appropriate size from that side of the transformer to a ground rod. One other forum I visited suggested that an additional ground rod is needed near the transformer if the transformer is some distance from any existing ground rod.
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Last edited by AllanJ; 04-13-2011 at 08:56 AM.
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Old 04-13-2011, 02:17 PM   #18
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Need help with 240V transformer


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Originally Posted by freak View Post
This thread seems to argue the same question. I'm not sure that they answered it. I'm not sure we have answered it either.
I honestly don't know if anyone really knows for sure.

http://forums.mikeholt.com/archive/i...p/t-56713.html
That's not the issue being discussed in that thread. The question there is exactly where and how to connect the grounding electrode system and neutral bonding conductor to the secondary center tap on a transformer. There is no question that the secondary MUST be bonded/grounded.

The NEC citation already given answers this question completely. There is no controversy. You must connect one side of the secondary to the system neutral and the grounding electrode system. The only exception is if you were running an intentionally isolated system, which is only permitted in certain industrial applications and requires additional equipment to stabilize the voltage and indicate ground faults.
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Old 04-13-2011, 02:54 PM   #19
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Need help with 240V transformer


They seemed to be arguing over whether or not a transformer was an SDS.
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Old 04-13-2011, 03:58 PM   #20
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Need help with 240V transformer


let's ignore the other website for a minute.

The NEC defines a SDS as:

a premises wiring system whose power is derived from a source of electrical energy or equipment other than a service. Such systems have no direct connected grounded circuit conductor, including a solidly connected grounded circuit conductor, to supply conductors originating in another system.

is your system an SDS per that definition or not?
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Old 04-13-2011, 04:21 PM   #21
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Need help with 240V transformer


I really don't know. Does then fact that the source of electrical energy flows through a transformer make it a source not derived from a service? I honestly don't know the answer to that. To me that would be something like a gas powered generator.
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Old 04-13-2011, 07:06 PM   #22
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Need help with 240V transformer


Transformers are SDS's. They fit the definition for all relevant purposes (even though the energy does, in fact, come from the service) since they have no direct electrical connection between the primary and secondary. In Lincoln, Nebraska and Phoenix, Arizona, the code authorities, electricians, and professors all agree that a transformer is an SDS.

A quick google search for [transformer "separately derived system"] turns up loads of engineering references that clarify this. There is one exception to the "transformers are SDS's" rule, but yours isn't one of those.
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Old 04-13-2011, 08:50 PM   #23
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Need help with 240V transformer


Thank you all very much for the help! I really appreciate your knowledge and will get the problem corrected. Thanks again!!

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