Using a transformer to get 240/120 circuit

r_marcinko · 11-13-2012, 01:42 PM

I had a thread a few days ago about a dryer connection. I cannot use my current subpanel to wire a proper 4-wire circuit for a dryer. The subpanel feeder contains only three wires - Hot, Hot, and Ground - and is suitable for 240V circuits only.

Would wiring a 240V circuit, from this subpanel, to a transformer allow me to then run a proper 4-wire circuit for a dryer? Like this one: http://www.newark.com/sola-hevi-duty...mer/dp/92F5244

AllanJ · 11-13-2012, 02:08 PM

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.

I would say that the easiest way to do this is to build a portable enclosure for the transformer with a cord and plug that fits the 240 volt only receptacle you already have and has a 4 prong 120/240 volt receptacle for your dryer.

Jim Port · 11-13-2012, 02:10 PM

IIRC that panel was to feed the furnace. I doubt you have excess capacity to add the new load to that panel.

r_marcinko · 11-13-2012, 02:17 PM

Can you please confirm if transformer I linked is suitable for this purpose? Here is the wiring specs for this model, how would circuit wiring look to primary and secondary of transformer (wiring color, breakers, fuses)?

r_marcinko · 11-13-2012, 02:28 PM

Quote:

Originally Posted by Jim Port

IIRC that panel was to feed the furnace. I doubt you have excess capacity to add the new load to that panel.

I remember you saying this before. Yes, this panel also powers the furnace. The 30 amp dryer circuit already exists there, not a new load, no tripped breaker on circuit that feeds this panel in 10+ years. The existing 30 amp circuit is powering an old 240V only dryer. I have enough capacity in panel, it is fed by 125AMP breaker that is located in yet another subpanel, not sure on wire size off hand.

Look, I'm just tryi8ng to figure this out, I don't give up easily. I may be stubborn like that, but I want it to be done properly, hence my questions to you fine folks. Do you have any comment on the transformer idea?

r_marcinko · 11-13-2012, 02:31 PM

Quote:

Originally Posted by AllanJ

I would say that the easiest way to do this is to build a portable enclosure for the transformer with a cord and plug that fits the 240 volt only receptacle you already have and has a 4 prong 120/240 volt receptacle for your dryer.

This is a wonderful idea, thank you!

k_buz · 11-13-2012, 04:24 PM

Before you go and spend $650 for a transformer, post a picture of the subpanel and of the existing wiring at the dryer receptacle.

mpoulton · 11-13-2012, 04:57 PM

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.

dmxtothemax · 11-13-2012, 06:50 PM

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 !

bobelectric · 11-13-2012, 09:34 PM

Pull new wires.

r_marcinko · 11-14-2012, 08:14 AM

Quote:

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 !

My sarcasm meter may be broken, but I will reply anyway. Based on a 240V/30amp circuit the load would be around 6kVA, the transformer I linked above is rated 7.5kVA, giving me a cushion for safetey, do you thing that is enough?

Quote:

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.

This idea is intriguing and is actually originally how I tried to work transformer idea as a solution to my problem. I couldn't wrap my head around how it would be wired to circuit and a single breaker. Would you mind elaborating on this idea?

Quote:

Pull new wires.

Is there a particular reason you are against using a transformer? As I've tried to explain numerous times, this subpanel is not going to get new feeder wires.

Quote:

Before you go and spend $650 for a transformer, post a picture of the subpanel and of the existing wiring at the dryer receptacle.

Unfortunately, I have no means to be able to do that.

rjniles · 11-14-2012, 09:47 AM

An I missing something? Why not rewire the sub panel with a proper 4 wire feed?

r_marcinko · 11-14-2012, 09:58 AM

Quote:

Originally Posted by rjniles

An I missing something? Why not rewire the sub panel with a proper 4 wire feed?

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).

busman · 11-14-2012, 10:28 AM

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

Mark

mpoulton · 11-14-2012, 10:29 AM

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.