How to get 120V from 240V

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  #1  
Old 03-27-15, 03:25 PM
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How to get 120V from 240V

I have a pump house about 50 feet from anyplace. This is at least 16 years old, predating my purchase of the property. It has a Square D HOM612L100F Homeline™ Fixed Main Lug Load Center. It serves (2) 2HP water pumps for irrigation at 240V. The power comes in via a 2 wire + ground cable. The power tests at 240V. I need to add a 120V outlet at the site. I was expecting 2 hot wires but there is only one black. So... What the heck does it take to get 120V or is it even possible? Thanks for your continuing wisdom.
 
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Old 03-27-15, 04:06 PM
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I was expecting 2 hot wires but there is only one black.
Bot wires are 240 volt.
What the heck does it take to get 120V
You need a neutral. What was the wiring method, buried cable.

Tech: A 240 volt feed such as you have consists of the two legs of the 240 feed to your house. I'm guessing from what you wrote they used cable and did not remark the white of the cable black or red to indicate it was a hot. 120 volts is a derived voltage from either leg of the 240 volt supply to your house and the neutral.
 
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Old 03-27-15, 05:06 PM
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It sounds like, and I seriously doubt there is a neutral. You sound like you have straight 240 and a ground, but no neutral.
 
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Old 03-27-15, 06:54 PM
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IF you do not want to replace the existing wiring then you will have to use a transformer at the pump house. The transformer will have a 240 volt primary and either a 240/120 volt secondary or a straight 120 volt secondary, the latter is preferred. You need to size the transformer for the expected (and unexpected) 120 volt load. The primary will connect to the existing load center with a new two-pole circuit breaker and the secondary will need to be fused on one leg with the other leg grounded. There should be a ground rod at the building already.
 
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Old 03-28-15, 05:11 AM
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You should think about what you are going to plug in down at the pump house and how much 120 volt power you will need before buying the transformer. The existing power feed might be barely large enough for the two pumps.

This is another example of where an interim solution (the transformer) does not advance you, cost wise, towards a long range solution (new hot-hot-neutral-ground power feed).

I would say, don't even think of converting the entire pump house, pumps and all, to 120 volts. You will almost certainly need to run a new power feed anyway. The existing feed is probably unable to carry enough amperer at 120 volts to run the pumps let alone the small tools or appliances you want to plug in down there..
 

Last edited by AllanJ; 03-28-15 at 05:32 AM.
  #6  
Old 03-28-15, 11:51 AM
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Thanks for the help guys. I thought I was going nuts for a while. I don't get how it can work without neutral, but I guess I don't have to know everything, just the people who already know that stuff. I need an outlet for a little wall wart transformer for a underground dog fence, and maybe hang a light in there. On Amazon I found a "Siemens MT0050A Industrial Power Transformer, Domestic, 240 X 480, 230 X 460, 220 X 440 Primary Volts 50/60Hz, 120/115/110 Secondary Volts, 50VA Rating" I'm not sure how to wire that.
the secondary will need to be fused on one leg with the other leg grounded.
So i'm guessing from the secondary, black wire to outlet, and white back to ... ground? There is no ground rod, but I have rebar and a big hammer if I need one. Who ever said a free puppy was going to be cheap?
 
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Old 03-28-15, 12:26 PM
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I don't get how it can work without neutral
A neutral is only for deriving 120 volts from the 240 volts your house is supplied with.
So i'm guessing from the secondary, black wire to outlet, and white back to ... ground?
No. The two two legs of the secondary go to the receptacle.* That transformer sounds way over complicated for what you need.

Some example of what you need: 1000 Watt Transformer: Multipurpose
Batteries & Power | eBay


*Just for your information if you use just a transformer and not a converter here are the connection. An auxiliary control transformer rated 240/120 may work depending on output amperage. (need about 10 amps).

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Last edited by ray2047; 03-28-15 at 01:08 PM.
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Old 03-28-15, 02:10 PM
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Ray has given you a good drawing of how to connect a transformer although he omitted the green equipment ground from the transformer to the receptacle. The transformer secondary is grounded at the transformer and this becomes the combination neutral and equipment ground and also is where the grounding electrode conductor to the ground rod originates. This is called a "separately-derived system" and has to be wired in this exact method.

The transformer you originally referenced can be used except the rating of 50 VA is only about a 1/2 ampere output. You really should have about a 500 VA output minimum and 1000 (1 kVA) or more is better to allow more to be used from the 120 receptacle. The 15 ampere two-pole circuit breaker in the panel feeding the transformer will be hugely oversized so you might want to also use supplemental fuses or circuit breakers with a more appropriate rating.

For the secondary fuse you can use something like this and use the appropriate screw-in fuse.

[ATTACH=CONFIG]48615[/ATTACH]
(Image courtesy of Amazon.com)

They also make these in two-fuse models which would work for the supplemental primary fusing.
 
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Old 03-28-15, 05:42 PM
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It might be easier/cheaper to just buy a 240 volt power supply for the dog fence.
 
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Old 03-28-15, 09:30 PM
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It might be easier/cheaper to just buy a 240 volt power supply for the dog fence.
Probably true but that doesn't do anything for the light he is also thinking of adding or any other possible usage.
 

Last edited by Furd; 03-28-15 at 11:23 PM. Reason: fix misspelling
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Old 03-28-15, 11:12 PM
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There is no ground rod, but I have rebar
No you shouldn't use rebar. You need to use a real ground rod (½"x8') and acorn clamp.

Furd, what size wire for the GEC in a separately derived system like this? Would you still go with #6 to so it didn't have to be protected?

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  #12  
Old 03-29-15, 10:41 AM
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The original question was, is this even possible. Well it is possible, with exotic workarounds and expensive devices, but in the end, the best solution for me is, as Astuff suggested,
It might be easier/cheaper to just buy a 240 volt power supply for the dog fence.
My old HotRodder brain tells me the simplest solution is usually the best. It turns out I fried my power supply before I figured out I had 240V, and needed a new one anyway. I could always setup a 12V bus for security devices or whatever. But a 60 pound dog, and a flashlight is likely all I really need... ... Thanks for all of your well chosen words, and technical illustrations. I hope others find this thread useful as well.
 
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Old 03-29-15, 12:18 PM
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Installing lights on a 240v only circuit is not that difficult. Simplest is a $3.00 keyless light fixture and a 240 volt medium base (Edison bulb) (probably best to fuse it but that is easy). For safety use a 2-pole switch.

Example: Satco S4077 60 Watt 570 Lumens A19 E27 230 Volt Incandescent Light Bulb, Soft White - - Amazon.com

http://www.electronicsurplus.com/sat...er-20-amp-250v

Best to use two of the fuse holders referenced above, one for each leg. Of course me I might be temped to jerry rig using two keyless fixtures and Edison fuses.
 

Last edited by ray2047; 03-29-15 at 08:01 PM.
  #14  
Old 03-29-15, 07:08 PM
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Now I can do all kinds of cool stuff... 240V fixtures and 12V DC 5A ...I can rule the world! but probably wont.
 
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Old 03-29-15, 07:16 PM
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Ray... Do you have a "derived voltage" topic or sticky? This may get some interest, and prove helpful to other clods like me....

[Advertising link deleted. Forum rule violation.]
 

Last edited by ray2047; 03-29-15 at 08:00 PM.
  #16  
Old 03-29-15, 08:01 PM
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Normally we recomend running a new feed.
 
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