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Old 11-14-2012, 10:32 AM   #16
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Using a transformer to get 240/120 circuit


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Originally Posted by busman View Post
Installing that transformer would constitute a Separately Derived System (SDS) and require you to establish a Grounding Electrode System on the secondary side.

Mark
Only if it's hard-wired in. A portable transformer connected by cord and plug would not need bonding to the GES since it's not part of the premises wiring. Bonding the secondary neutral to ground would be adequate. However, avoiding this problem is another advantage of using an autotransformer.

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Old 11-14-2012, 10:33 AM   #17
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Using a transformer to get 240/120 circuit


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Originally Posted by mpoulton

Only if it's hard-wired in. A portable transformer connected by cord and plug would not need bonding to the GES since it's not part of the premises wiring. Bonding the secondary neutral to ground would be adequate. However, avoiding this problem is another advantage of using an autotransformer.
that is tru
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Old 11-14-2012, 10:37 AM   #18
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Using a transformer to get 240/120 circuit


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Originally Posted by r_marcinko View Post
Simple, cost. It would be much more money to redo w/ new 4 wire feeder to this panel. The 3 wire feeder I currently have is sufficiently sized and already there, what I'm proposing makes the most sense, IMO. That is why I am angling for the transformer as a solution. Even if I have to spend $500 on the transformer, the savings vs. rewiring the subpanel is significant. I realize the use of a transformer in a residential application is uncommon and many electricians may not be familiar with their use(outside of electricians familiar w/ industrial applications where this is common).
I can buy a lot of wire for $500+
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Old 11-14-2012, 11:14 AM   #19
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Using a transformer to get 240/120 circuit


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Originally Posted by mpoulton View Post
Most of the power the dryer uses is 240V power for the heating elements. It only needs a neutral connection to run the motor and controls on 120V. So the 240V load is already provided for directly by your circuit. Your transformer only needs to produce a few amps at 120V to run the motor and controls. One way to do this would be with a transformer that has a 240V primary and a 120V secondary. You would connect the primary across the two hot lines, and one end of the secondary to one of the hot lines (it has to be the correct end - out of phase). The other end of the secondary would be your neutral connection, because it would be 120V with respect to each of the two hot lines. Just make sure you don't have the wires reversed, or you get 120V to line A, and 360V to line B!

The even simpler alternative is an autotransformer. This combines the primary and secondary into one winding with a center tap connection. This is essentially JUST the secondary of a 120/240V transformer, with no primary. You connect the incoming hot lines to the two ends of the winding, and the center tap becomes the neutral connection. Each half of the winding induces an equal voltage in the other half, but out of phase so they cancel each other out. It effectively takes the two hot lines and generates a usable neutral for you. You can use a dedicated autotransformer for this, or you could just use any transformer that has a 120/240V winding, and simply cap off the wires on the other windings. The transformer needs to be rated for at least the 120V component of the dryer's load, which will likely be a few amps. You should probably measure it first.

Pretty much any transformer with a 500VA rating or higher and a 120/240V winding should work fine as an autotransformer for this application. Something like this:
http://www.automationdirect.com/adc/...12_VAC/PH500PG

You will need to use fuses to protect the transformer from excessive current, since it is rated less than the 30A of the dryer circuit. An appropriately sized fuse in each of the hot lines to the transformer will do the job. Perhaps 2A fuses for a 500VA transformer, giving 4A max through the neutral and 480VA.
This post right here has so much awesome I don't know where to start, thanks!

Couple of questions off hand. Is that transformer something that gets mounted inside of existing subpanel or do I need a separate enclosure for it to mount near the panel? Same questions for the fuses, is that something that can be mounted in the existing subpanel?
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Old 11-14-2012, 11:36 AM   #20
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Using a transformer to get 240/120 circuit


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Originally Posted by r_marcinko View Post
This post right here has so much awesome I don't know where to start, thanks!

Couple of questions off hand. Is that transformer something that gets mounted inside of existing subpanel or do I need a separate enclosure for it to mount near the panel? Same questions for the fuses, is that something that can be mounted in the existing subpanel?
That transformer (PH500PG) is roughly 5x6x4.5 inches, so I doubt if it could be mounted inside the subpanel.
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Old 11-14-2012, 02:02 PM   #21
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Using a transformer to get 240/120 circuit


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Pretty much any transformer with a 500VA rating or higher and a 120/240V winding should work fine as an autotransformer for this application. Something like this:
http://www.automationdirect.com/adc/...12_VAC/PH500PG

You will need to use fuses to protect the transformer from excessive current, since it is rated less than the 30A of the dryer circuit. An appropriately sized fuse in each of the hot lines to the transformer will do the job. Perhaps 2A fuses for a 500VA transformer, giving 4A max through the neutral and 480VA.
Okay, I'm going to give this a go. Here's what I'm getting, plus an enclosure of some kind to mount next to and connect to subpanel. Sound about right along lines of what you suggested? Thanks again!
Transformer
Fuses
Fuse Holder
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Old 11-14-2012, 08:58 PM   #22
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Using a transformer to get 240/120 circuit


What a novel idea! Creating a neutral using a control transformer?

Have you done this type of thing with loads like a dryer before?
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Last edited by kbsparky; 11-14-2012 at 09:20 PM.
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Old 11-15-2012, 06:15 AM   #23
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Using a transformer to get 240/120 circuit


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Originally Posted by mpoulton View Post
Only if it's hard-wired in. A portable transformer connected by cord and plug would not need bonding to the GES since it's not part of the premises wiring. Bonding the secondary neutral to ground would be adequate. However, avoiding this problem is another advantage of using an autotransformer.
If you check Article 400, I don't think you'll find this installation to be a permitted use of a cord.

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Old 11-15-2012, 08:30 AM   #24
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Using a transformer to get 240/120 circuit


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What a novel idea! Creating a neutral using a control transformer?

Have you done this type of thing with loads like a dryer before?
I'm not certain if this is directed at me or not, but, no, I have never done this before. Your post is very confusing, I can't tell if you are being sarcastic about the fact this should have been thought up naturally and I'm silly for asking the question or if your sarcasm is leaning the other way and you are trying to say this won't work?
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Old 11-15-2012, 09:45 AM   #25
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Using a transformer to get 240/120 circuit


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Originally Posted by r_marcinko View Post
I'm not certain if this is directed at me or not, but, no, I have never done this before. Your post is very confusing, I can't tell if you are being sarcastic about the fact this should have been thought up naturally and I'm silly for asking the question or if your sarcasm is leaning the other way and you are trying to say this won't work?
I think KB is saying that no licensed electrician would do it and no inspector would approve it. Imagine the person buying your house and finding this as the dryer connection.
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Old 11-15-2012, 10:08 AM   #26
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Using a transformer to get 240/120 circuit


Okay, I haven't done anything yet. I WANT it done to code and safe while using my existing subpanel. Let me recap:
Quote:
Originally Posted by AllanJ View Post
Yes it is okay to install a transformer (of adequate ampacity) with a 240 volt primary and a center tapped 240 volt secondary to obtain 120/240 volt power for the dryer.
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Originally Posted by mpoulton View Post
You don't really need this kind of transformer. You only need an autotransformer, and it only needs to be rated for the 120V load (perhaps 5A) not the full load of the dryer. That would be much cheaper and easier.
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YES !
Using a transformer will work !
But make sure the tranny has enough current capacity !
And the breaker for the circuit must be correct also !
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Originally Posted by busman View Post
Installing that transformer would constitute a Separately Derived System (SDS) and require you to establish a Grounding Electrode System on the secondary side.
Quote:
Originally Posted by asman_haftum View Post
that is tru
All of the above either imply or outright say that this is an acceptable way to do this given my current 3 wire subpanel. I also know that the use of transformers in this way for industrial applications is common (via an electrical engineer whom I trust).
So, why do you say this:
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I think KB is saying that no licensed electrician would do it and no inspector would approve it. Imagine the person buying your house and finding this as the dryer connection.
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Old 11-15-2012, 10:11 AM   #27
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Using a transformer to get 240/120 circuit


It's not that unusual in commercial and industrial for electricians to "create a neutral" with a transformer. It's done all the time with Delta-Wye transformers, but as I've said, it is an SDS and requires a connection to a GES and cannot be cord connected unless it meets one of the conditions in Article 400.

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Old 11-15-2012, 01:07 PM   #28
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Using a transformer to get 240/120 circuit


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Originally Posted by busman View Post
It's not that unusual in commercial and industrial for electricians to "create a neutral" with a transformer. It's done all the time with Delta-Wye transformers, but as I've said, it is an SDS and requires a connection to a GES and cannot be cord connected unless it meets one of the conditions in Article 400.

Mark
Thanks, busman. Your point is well taken, I understand that a new ground must be used if the larger transformer is used in my application. However, can you clarify what you mean by not using a cord? Are you saying that if the transformer was properly installed and grounded and a new circuit was run to dryer receptacle, that a corded dryer would not be able to be plugged into the receptacle? If that is the case, would it be acceptable to hardwire a dryer to the circuit w/ junction box?

Do you have any opinion on the use of an "autotransformer" in the application that mpoultan suggests?

Last edited by r_marcinko; 11-15-2012 at 01:12 PM.
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Old 11-15-2012, 05:02 PM   #29
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Using a transformer to get 240/120 circuit


Quote:
Originally Posted by r_marcinko View Post
I'm not certain if this is directed at me or not, but, no, I have never done this before. Your post is very confusing, I can't tell if you are being sarcastic about the fact this should have been thought up naturally and I'm silly for asking the question or if your sarcasm is leaning the other way and you are trying to say this won't work?
Actually my statement was directed at mpoulton:

Quote:
Originally Posted by mpoulton View Post
You don't really need this kind of transformer. You only need an autotransformer, and it only needs to be rated for the 120V load (perhaps 5A) not the full load of the dryer. That would be much cheaper and easier.
I was not being sarcastic, but was genuinely praising the novel idea. I would be one of the first to try such a scheme. Just be sure that the control transformer would be sufficient to handle the motor/controls load.

As for the "cord and plug" issue, I think they were referring to the transformer being so connected. I would hard-wire it in a separate enclosure, and then hard-wire the normal dryer outlet from there. Plug in the dryer as normal.

Should not be a problem.

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