cheapest way to drop 480vac to 240vac

Any electricians out there. Need help getting 480VAC (3 phase) to look like 240vac single phase.

The power company here will install a small unit that monitors your AC (208 unit and 240 unit, assume single phase) line and certain times (emergencies) flip off your AC to the air conditioner for 15 minutes with a credit each month.

They said it will only work on 208 or 240vac and we have a 480vac 3 phase AC units. If I can get it to 240 and show them the thermostat wires they will install it for me.

At first I was thinking about a 2:1 step down transformer. They signal the box, so not sure if a transformer will screw the signalling up.

Not sure what power it pulls but I assume from run from my (240vac) power.

Could a couple same sized resistor in series to make a voltage divider work for this. It will be installed by an electrician.

Any other ideas?

Use a single phase.
480VAC is not a 3 phase voltage of 240V

The commonly referred 3 phase voltage of 415V comes from 240 x SQRT(3). It's the voltage difference across each phase of 240V per phase.

All phases are 240V to ground, but 415V between phases.

Forget resistors, they will blow or boil with the power being dissipated in them.

I guess I will have to get a transformer and see if they can talk to the unit still. They send a signal, but not sure if airwave or on the power line. Guess powerline.

You'll need to know how much power the transformer needs to handle. If it's only operating power for their electronics, it shouldn't need more than a few watts.

If all you need is a straight divide by two, then an autoformer would be the simplest. Just find a tx with two 240V windings, wire them in series, and take the output from the center point. Be very careful about the phasing of the two windings, or you'll end up with a short circuit

Since 240/480 volt power line rated transformers don't come very small, you'll probably end up with more transformer capacity than you need.

WTP Pepper said:

Use a single phase.
480VAC is not a 3 phase voltage of 240V

The commonly referred 3 phase voltage of 415V comes from 240 x SQRT(3). It's the voltage difference across each phase of 240V per phase.

Click to expand...

perhaps not in the uk but its pretty common over here for 480vac between the lines and 277 vac between the lines and the neutral or ground.
You can also have 480 delta, which would be 480 volts between the lines, with one of the transformer coils split and grounded in the center, so you get 480 3phase and 240/480 single phase.


In any case, the best solution for OP is to get a 480v to 240vac transformer (10 watts might be all you need?)

Cheapest way = transformer

I will open the box Monday and see what is inside voltage wise. Suspecting 277. From there I will see what transformers are out there.

Thanks for all the input. Hoping it is Johansen's second choice, but betting on the first. I recall seeing 277 on some panels.

OK, got the numbers. It is a ground and 3 hot legs. All legs are 277 to ground and pairs of the legs are 480.

So there is no way I can drop a power resistor on a leg and get from ground to resistor a voltage drop to 240vac and still power the box. Probably 250ma on the box.

Now that I think about it, the power company said transformer only. Maybe I will go look for a 277 to 240vac or 277 to 208v transformer. Anyone know of a cheap one let me know. Ones I saw before were $$$. A 25% drop would get the 277 to 208...

Any ideas

Also if anyone know if a resistor will do the trick, the guy said he will try it just to see it.

is there any reason the ac has to be in phase with something? if not you can get either a 277 to 240 vac transformer, or you can get a 480vac to 240, which might be easier to find. There is a third way, you can connect a 36 vac to 240 vac transformer as an auto transformer and connect that across 277 in such a way that you get ~240 relative to ground.

A neighbor has one. The best I can tell, both the thermostat (24 Vac to turn on AC) and the 240V enter the box. Get an electrican to install one: **broken link removed**

ou need to ask the utility the minimum VA rating that you need. The other piece of useful info would be the wire size range of their lugs.

mramos1 said:

OK, got the numbers. It is a ground and 3 hot legs. All legs are 277 to ground and pairs of the legs are 480.

So there is no way I can drop a power resistor on a leg and get from ground to resistor a voltage drop to 240vac and still power the box. Probably 250ma on the box.

Now that I think about it, the power company said transformer only. Maybe I will go look for a 277 to 240vac or 277 to 208v transformer. Anyone know of a cheap one let me know. Ones I saw before were $$$. A 25% drop would get the 277 to 208...

Any ideas

Also if anyone know if a resistor will do the trick, the guy said he will try it just to see it.

Click to expand...

277 means nothing here. You won't be connecting anything from a phase to ground. You will be going phase to phase, so you need a 480/240 transformer like KISS linked to. I would get that one if I were you, I don't think you're going to find a better deal.

They may be doing it different now due to "smart meter" technology. https://www.boston.com/business/art...lity_option_to_cut_air_conditioner/?page=full The "smart meter" may use Bluetooth or WIFI and cellular combined.

But in any event, their devices are usually installed outside near the disconnect and that seems to be a requirement.

KeepItSimpleStupid said:

https://www.boston.com/business/art...lity_option_to_cut_air_conditioner/?page=full

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...will give customers a way to save $20 annually by allowing the electric utility to cut off their home air conditioners for brief periods on the hottest weekdays of summer. [...]When demand for electricity hits a peak on hot weekday afternoons between June and September, the utility will be able to cut off the air conditioners for 15 minutes out of every 30 minutes.

Click to expand...

That sounds like a terrible deal to me. Sure, great for the planet and whatnot, but if they're going to offer such a feeble incentive as 20$, they might as well offer nothing except an "eco-hero" bumpersticker. 20$ is not worth sitting in a hot swampy house during the hottest part of every day.

I don't know what the credit is here, but the power company does offer it for electric water heaters and AC.

Note that it said that in 1992 it used pager technology.

The smart meter technology could be based on Zigbee. **broken link removed** That would make a lot of sense.

it is zigbee. I read it somewhere after they installed my smartmeter.

strantor said:

277 means nothing here. You won't be connecting anything from a phase to ground. You will be going phase to phase, so you need a 480/240 transformer like KISS linked to. I would get that one if I were you, I don't think you're going to find a better deal.

Click to expand...

oh it certainly means a lot. there is a ground somewhere and what he has is 277/480 WYE because there's only two other options: 480 delta with one line grounded (which means he would have measured 480 or zero volts to ground), or one of the coils is tapped and grounded in the middle, which means he would have measured 240 to ground from two lines and something like 310 iirc from the third one.

it would probably be cheaper if you have to have an electrician wire it, to have the electrician order a 10-50va 480/240 to 120/240 transformer.
ebay doesn't have anything for very cheap, but i only spent 2 minutes looking.
another option is the 36/240 transformer configured as an auto transformer as i suggested earlier.

johansen said:

oh it certainly means a lot. there is a ground somewhere and what he has is 277/480 WYE because there's only two other options: 480 delta with one line grounded (which means he would have measured 480 or zero volts to ground), or one of the coils is tapped and grounded in the middle, which means he would have measured 240 to ground from two lines and something like 310 iirc from the third one.

it would probably be cheaper if you have to have an electrician wire it, to have the electrician order a 10-50va 480/240 to 120/240 transformer.
ebay doesn't have anything for very cheap, but i only spent 2 minutes looking.
another option is the 36/240 transformer configured as an auto transformer as i suggested earlier.

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Johansen, One again, thanks for the help. Correct, it's 277 on all three to ground. Can you explain the 36/240 auto trans)? How to wire. I will go google for info now as well. Confused by the 36.

We have an electrician but not sure he will know either. The one transformer on ebay is a great deal will try for that one (new they are $170US) payback would be 10 months for me at retail. Problem is I need 10 of these. And probably more in our other buildings.

Strantor, thanks for your advise, but these are common area (hallways AC units) so no bumper sticker here, just the $200 a month saving at each building. In the 8 years of the program they have never used them as well and they are only off 15 minutes at a time every hour, 4 hours.

Here is one too, at 100 VA. **broken link removed**

But you would pay a premium price here.

Two 120 to 240 V transformers would work. You can even flip the primaries and secondaries at such low power levels. e.g. 2 - 240 V primaries in series and the 120 120 V in series.

e.g. http://www.federalpacific.com/

AT the number of transformers you need and you will also need some sort of enclosure and fuses, you could consider custom. Custom goes now pretty quick

I've had a transformer designed by these guys: **broken link removed**

it looks like 50A can be had.

I might consider mounting it on a DIN rail clip: http://www.winford.com/products/cat_din.php and making your connections via DIN rail.

Have your electricians mount the box, so that all they need to do is connect 240 VAC. Maybe you could get a template and drill the holes for them too for their box.

If your lucky, the transformer could be mounted inside the AC unit.

Your best bet, is to make a call to some of your electrical supply houses.

Control voltages will be 120 and 24 VAC typically and that's what's making things difficult.

I'll take a finder's fee: http://www.automationdirect.com/adc...nsformers/480x240_VAC_to_240x120_VAC/PH50MQMJ

Here you go, $40. You might even be able to get the integral fuse holders and accessory kits. That was a tough one!!!

I would suggest some sort of preliminary site survey to make sure their devices will communicate.

johansen said:

oh it certainly means a lot. there is a ground somewhere and what he has is 277/480 WYE because there's only two other options: 480 delta with one line grounded (which means he would have measured 480 or zero volts to ground), or one of the coils is tapped and grounded in the middle, which means he would have measured 240 to ground from two lines and something like 310 iirc from the third one.

Click to expand...

How does how it's grounded and the voltage to ground matter? I said the transformer should be connected across 2 phases, as this is how I've always seen it done. I don't think Loads should be connected from phase to ground, as the ground is meant to carry unbalanced load current. So if you're connecting a 480V/240V transformer across 2 phases of 480, why does it matter if it's WYE or DELTA or corner gounded delta or anything? AFAIK if the transformer sees 480 it puts out 240 regardless of phase configuration or voltage to ground.

Right, you can't use ground to carry current. If for some reason you had a neutral, then it makes 277 an option maybe, but who cares.

A control transformer from 480 to 240 is the correct route to go. It MUST be fused. So, "finger safe" and fused sounds like a plan, so you'll need the accessories that go along with the transformer too.

Now the hard parts: There are lots of parts that will be needed besides the transfomer. These can be rrof mounted or ceiling mounted, I don't know. There may be room or may not be room in the controls enclosure. I suspect not. It may need a NEMA 1 or NEMA 4 enclosure, I have no idea. RF might not be able to reach. BIG PROBLEM.

So, it needs 240 VAC power and access to the 24 VAC line that controls the contactor AND the power company has to have a place to mount their gizmo close to a disconnect, Probably 3 feet, but check with codes in your area.