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Old 04-12-2011, 10:08 AM   #1
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Need help with 240V transformer


I had a 5KW 240V/120V step down transformer installed on a 240V line. Measuring the voltages shows... Hot/Ground=170V - Neutral/Ground 60V - Hot/Neutral 120V. It is wired correctly as far as the name plate shows. The electrician said it would be ok but it seems strange to me. The transformer is an older Challenger 507-310. Does anyone know about these enough to comment on what I am seeing?

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Old 04-12-2011, 10:54 AM   #2
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Need help with 240V transformer


The measurement from hot to neutral is the one that counts. If you are using a digital meter, be sure you are reading volts and not Mv.

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Old 04-12-2011, 11:12 AM   #3
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The measurement from hot to neutral is the one that counts. If you are using a digital meter, be sure you are reading volts and not Mv.
Yeah I'm reading volts and I agree hot/neutral is what counts but why the strange readings hot/ground - neutral/ground? I've never seen that before on any wiring I have done. Granted I'm not an expert and have not seen everything.
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Old 04-12-2011, 11:13 AM   #4
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Need help with 240V transformer


It sounds like the secondary is not grounded/bonded. You need to bond one end of the secondary to the system neutral.
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Old 04-12-2011, 11:20 AM   #5
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Need help with 240V transformer


Are you saying bond the neutral and ground?
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Old 04-12-2011, 12:38 PM   #6
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Need help with 240V transformer


The strange voltages you are measuring are phantom voltages but the nature of a transformer is such that such phantom voltages can be large enough for a non-digital voltmeter to measure and even large enough to light a low wattage lamp.

An ordinary transformer does not come with either secondary terminal (here, 120 volts) grounded (bonded to the metal frame). There is not (or rather should not) be a metal to metal contact or measurable DC conductiviyt between primary and secondary.

You do not connect either secondary terminal to ground.

All loads (lights, appliances, also receptacles) powered by the transformer secondary do not have neutral connected to neutrals of any other circuits. (This rule also applies to the load terminals of a ground fault circuit interrupter.)

All ground wires are bonded wherever they come together, including the ground wire accompanying the 240 volt feed to the transformer primary and the ground wire accompanying the rest of the circuit powered by the secondary.

Since the ground wire should never be part of a circuit, those strange voltages should never go into lights, appliances etc.
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Last edited by AllanJ; 04-12-2011 at 12:49 PM.
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Old 04-12-2011, 12:48 PM   #7
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Need help with 240V transformer


What you describe is exactly the way it is done.
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Old 04-12-2011, 02:34 PM   #8
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The strange voltages you are measuring are phantom voltages but the nature of a transformer is such that such phantom voltages can be large enough for a non-digital voltmeter to measure and even large enough to light a low wattage lamp.

An ordinary transformer does not come with either secondary terminal (here, 120 volts) grounded (bonded to the metal frame). There is not (or rather should not) be a metal to metal contact or measurable DC conductiviyt between primary and secondary.

You do not connect either secondary terminal to ground.

All loads (lights, appliances, also receptacles) powered by the transformer secondary do not have neutral connected to neutrals of any other circuits. (This rule also applies to the load terminals of a ground fault circuit interrupter.)

All ground wires are bonded wherever they come together, including the ground wire accompanying the 240 volt feed to the transformer primary and the ground wire accompanying the rest of the circuit powered by the secondary.

Since the ground wire should never be part of a circuit, those strange voltages should never go into lights, appliances etc.
BTW - Is there way to overcome this so that the there are no phantom voltages?
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Old 04-12-2011, 04:05 PM   #9
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Need help with 240V transformer


=AllanJ;627952]

You do not connect either secondary terminal to ground.
.[/QUOTE]

what?

It is a separately derived service and must be bonded to ground.

btw; there is no neutral (in the technical sense) with a single winding transformer. You should ground one leg so you will have a grounded (aka: neutral) leg though.

Once you ground on leg of the 120 volt side, you will lose the odd voltage readings. The voltages you have are odd though. Generally when you measure an ungrounded transformer to a ground point such as you have, the addition of the 2 numbers generally comes up to the total voltage. Since you have 230 volts total, it almost sounds like you are measuring the primary legs to ground.

You need to bond your ground and one leg to a grounding electrode system as well since it is a separately derived service.
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Old 04-12-2011, 04:37 PM   #10
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Need help with 240V transformer


Yep...all the transformers I've seen/worked with which have a 120V secondary have one leg bonded to ground at the transformer. My experience is mostly with 120V control circuits in equipment. The green bonding wire is generally connected at the transformer secondary terminal with the white (aka neutral) conductor.
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Old 04-12-2011, 09:35 PM   #11
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I appreciate all the comments. However AllanJ and nap are telling me two different things. Any documentation I could read to settle it?
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Old 04-12-2011, 09:51 PM   #12
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the national electrical code (NEC).
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Old 04-12-2011, 10:03 PM   #13
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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
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Old 04-12-2011, 10:56 PM   #14
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I looked at the NEC more closely and from what I interpret so far I must change my story.

One side of the transformer secondary needs to be grounded. I haven't examined all the details in the NEC but it is likely that the existing ground wire accompanying the (240 volt) primary feed is not sufficient to be the ground for this purpose. That means you probably need to string a separate ground wire.
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Last edited by AllanJ; 04-12-2011 at 11:24 PM.
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Old 04-12-2011, 11:17 PM   #15
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Need help with 240V transformer


What are you concerned about ?
The fact that you are getting a volt reading between hot and ground ?
Neutral and ground are usually bonded together some where.
This is normal .
Or are you concerned that the reading is 170v,
Which may seem a little high ?
Again normal if load is light or non existant.
What does it measure under a good solid load ?

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