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Given that I don't have neutral, sounds like, even for just running a simple irrigation controller, that I don't want to use the ground wire for my neutral and simply pull 120v off the split phase 240V since a potential short would energize the ground wire.

It seems like running a small transformer is the way to go. However often recommendations I have found mention adding a sub panel breaker after the transformer, which seems like overkill for my need. Would adding a 5A fuse to new transformed 120V be fine instead of a whole breaker box?

I'm surprised how much transformers are, and most of them are for transforming single phase Euro 220v to US 110v, anyone have a good source?

As an aside, if this existing ground to the 240V did connect directly to ground at the main panel instead of neutral (haven't looked yet, in the neighbor's panel) then to me it would be ok to just pull L1 and ground off the 240V and skip the transformer right? These are very large wires running a 20A load to a well pump, might 1000' away, so I think this is the only load on that breaker.

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    What are you trying to power? It may well be easier to modify the device to accept 240V power than to try and get 120V in the building.
    – Nate S.
    Mar 29, 2019 at 16:18
  • Also there's nothing wrong with using a transformer meant for travel within the US. This one, for example, might do nicely for you: amazon.com/Power-Bright-VC1500W-Transformer-converter/dp/…
    – Nate S.
    Mar 29, 2019 at 16:25
  • @NateStrickland right on about changing the load! The problem with those autotransformers is they will provide an approximate center-tap but it's not a bona-fide neutral, so both wires will be hot.
    – Harper - Reinstate Monica
    Mar 29, 2019 at 16:52
  • What is the load? Will it be plugged into a receptacle or hard wired?
    – batsplatsterson
    Mar 29, 2019 at 16:54
  • if it is just for an irrigation controller, then maybe all you need is an appropriate 240VAC transformer ..... probably one with a 24VAC output ..... something like this .... ebay.com/itm/… ........... or buy a controller designed for 240Vac ..... ebay.com/itm/…
    – jsotola
    Mar 29, 2019 at 17:17

3 Answers 3

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At 0.15A x 120V, this thing is about 18 VA (similar to watts). That is significantly less than 40 VA.

As it happens, 40VA thermostat transformers are cheap and common as dirt, typically around $13. They make them with both 240V and 120V primaries. Get one of each.

You connect the two 24V secondaries to each other. The 240V primary goes to the 240V main via a 1 amp fuse. The 120V primary feeds your controller. This is double-isolated from 240V, so it won't be terribly dangerous.

If you really want to, you can call it a separately derived service and add a neutral-ground equipotential bond to whichever leg of the 120V you want to call "neutral", but given that it is going straight into another transformer that will also isolate it, that seems like much ado about nothing.

I recommend to snip the plug off the cord and hardwire this thing, to avoid the temptation of someone plugging a circular saw into the outlet.

Wow, that was easy and cheap.

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  • This is the best answer if it's not practical to reconfigure the controller to take 240V directly.
    – Nate S.
    Mar 29, 2019 at 18:43
  • Thanks for the reply, I'm a little unclear on what you mean. Although may be getting it. So connect them in series but reverse polarity? Kind of genius. so 240v>24V>24V>120V? So polarity will not matter to my controller? Thanks also for catching the 0.15A max input, I saw 0.5A max load, but that was the output at 24V DC.
    – farmtech
    Mar 29, 2019 at 19:08
  • Looks like two of these will work? amazon.com/Packard-Control-Transformer-Class-Mount/dp/…
    – farmtech
    Mar 29, 2019 at 19:23
  • @farmtech, yes, that will work. On the 240V input transformer, you'd connect the white and orange wires to the supply, connect the two transformers' red and green secondary wires, and on the output to your device you'd use the white and black primary wires. Transformers can step up or down just as easily, so you're just stepping down from 240 to 24, then stepping back up from 24 to 120.
    – Nate S.
    Mar 29, 2019 at 19:27
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    @Dotes -- this is a "back to back" configuration with only inches between the two transformers
    – ThreePhaseEel
    Mar 29, 2019 at 23:00
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For the specific irrigation controller you linked, there's almost certainly a way to configure it to accept 240V directly.

From the international manual on their website:

Rated input:

Australia—240V AC 50/60Hz 0.15A

Europe—230V AC 50/60Hz 0.15A

USA—120V AC 60Hz 0.15A

I'd recommend contacting them to see if you can plug the one you have into 240V directly, or if there's some internal wires or jumpers that need to be reconfigured first.

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  • This would be best of course, thanks I missed that! I will call.
    – farmtech
    Mar 29, 2019 at 19:00
  • I called any they couldn't me with anything internal of course. But they did say I could buy from the UK, just no warranty support in the US then. I bet I could reconfigure, but gambling on that one for sure.
    – farmtech
    Mar 29, 2019 at 19:10
  • It is almost triple the cost from the UK.....
    – farmtech
    Mar 29, 2019 at 19:14
  • If you want to try reconfiguring it yourself, and can post pictures of the inside, we may be able to help. But if you'd rather not bother and/or you'd like to keep your warranty intact, Harper's answer will be the way to go.
    – Nate S.
    Mar 29, 2019 at 19:23
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    yes turns out it is a pretty normal power brick inside the case, although luckily it is openable so I can just swap out the transformer. Inside is a 120v to 26VAC 0.75A transformer, so my 240 to 24V transformer I bought will work fine! That probably helps my voltage loss issue too by transforming at the timer (550' away on 16 ga wire) instead of at the well house. I do need to run a seperate ground at the timer since I only ran 16/2 cord.
    – farmtech
    Apr 3, 2019 at 15:04
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Something similar to this comes up all the time in industrial applications. For example there may be a three phase 480V supply to a motor, but you need lower voltage for the controls.

The usual solution is a control transformer. These are modestly priced, especially for tiny loads. For example one brand, Hammond, makes control transformers as small as 50VA for under $50. (USD)

Take a look at this document from Hammond for wiring diagrams for various primary / secondary configurations:

https://www.hammondpowersolutions.com/files/HPS_Instruction_Sheet_Control_Transformer_WiringDiagram_PHWD1.pdf

With these products, there are fuse holders built in, and you can also buy enclosures ready made for the device for a finished install with minimal fuss, and there are enclosed versions that are built in an enclosure - lots of options.

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