Connect 120 volt circuits to get 240 volts

It looks mad.

240 volts at a home or business that doesn't have 240 volt outlets is to pi ck off two outlets that are wired to separate circuits on opposite sides of the 240 volt power. There is a commercial product for this and designs on the web. I found this schematic.

ms the issue is that without the input relays when you plug the unit into o ne outlet the path through the output relay coil can energize an exposed pl ug pin. But I don't see the need for two poles. Running the 120 volt conn ection through the input relay contacts on just one side of the output rela y coil will prevent the two inputs from being energized when only one is pl ugged in.

nly advantage I can see is that with the two pole relays a single stuck con tact won't be dangerous. In a one pole approach it can pass a dangerous vo ltage to the input plug pin. But you can hear relays working so a stuck re lay can be detected. Also it has indicators on the inputs so you can tell the relay isn't clicking because of a failure rather than simply no voltage . So this risk seems minimal.

I always appreciate a well thought out criticism.

Rick C.

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Bearing in mind I'm English and don't know your rules (NEC etc), so I'm replying with general principles.

I see that K1 and K2 are ensuring neither of the 240V "hots" are energised until both 120V supplies are present.

Yes you do need K1 and K2 (reasons next):

I don't see the point of K3 though.

a) Normally a 220/240V outlet would be fed by a double pole breaker in the house panel so if either hot is overloaded or trips the GFI, both hots are isolated rather than leaving you with a live by unpowered appliance.

b) (Much worse) - If you put any non trivial load over the 240V load end, and only plug one 120V supply in, the other 120V plug is now hot and in your hand. You have in effect an overly complicated "Jesus Cord".

I wouldn't trust K1 and K2 in this scenario though - if either sticks closed, you have an unsafe and dangerous failure scenario.

My gut reaction is that the whole thing is a sucky hack waiting to go wrong - bit like trying to run a 3 phase machine in the UK by tossing your neighbours a couple of extension leads.

You'd be *much* better off with a simple step up transformer, depending on the VA required. All of my device chargers are 100-250V (or thereabouts) capable, so I assume you are talking about something chunky like an electric car?

Cheers,

Tim

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Connect 120 volt circuits to get 240 volts

It looks crazy.

Are you trying to save wires from main circuit box to the garage, which are usually next to each other? Or are you trying to skip 240V circuit breakers and exposing the 120V breakers to 220V?

Don't forget J1772 are usually 240V 30A. It will brow the 120V 15A immediately.

IMO all designs that use AC line powered relays as some kind of "relay logic" to control their own line currents are mad and disasters waiting to happen.

I remember a number of years ago there was some guy who regularly posted line powered relay logic designs to control stuff in his serial-killer-looking barn at his serial-killer/cult headquarters farm (where wearing yellow aviator sunglasses is a requirement for male heads of household) or something. They were all mad, too

240 volts at a home or business that doesn't have 240 volt outlets is to pi ck off two outlets that are wired to separate circuits on opposite sides of the 240 volt power. There is a commercial product for this and designs on the web. I found this schematic.

ms the issue is that without the input relays when you plug the unit into o ne outlet the path through the output relay coil can energize an exposed pl ug pin. But I don't see the need for two poles. Running the 120 volt conn ection through the input relay contacts on just one side of the output rela y coil will prevent the two inputs from being energized when only one is pl ugged in.

nly advantage I can see is that with the two pole relays a single stuck con tact won't be dangerous. In a one pole approach it can pass a dangerous vo ltage to the input plug pin. But you can hear relays working so a stuck re lay can be detected. Also it has indicators on the inputs so you can tell the relay isn't clicking because of a failure rather than simply no voltage . So this risk seems minimal.

You are starting with a misapprehension. I never said you could or should eliminate K1/K2. I said they can be single pole.

Yes, I get that. K3 is the one that prevents the output from being energiz ed if the two inputs are not providing 240 volts.

So you do understand what K3 is doing???

So you do understand what K1 and K2 are for then?

Ah, you don't understand... K1 and K2 are in series so both have to be plu gged in an active before there is a path from one to the other.

Ok, if you have reached this point, do you understand better? Care to reev aluate?

Rick C.

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Hmmm... auto chargers are not all J1772 and they can all be configured (all that I have seen) for a given current for each situation. Even so, I'm not using a J1772. Care to guess what I'm charging?

Rick C.

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So you completely fail to understand the circuit? Got it.

Rick C.

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Do you understand that relays are not circuit breakers and can fail shorted?

I think you are the person who is misunderstanding here - what he is saying is if either (or any) relays stick closed (contacts welded for example) then you have a dangerous situation.

This would have any competent electrician running out off the building. It is too bloody dangerous and the fact that you found it on a hack web site should be enough information to realize this is not the best idea out there.

I agree!

Step up transformers are safe when used as designed and should work here, otherwise get an electrician in and have hi/her run our proper 220 wiring and outlet for you.

You want to risk your life that is your choice, but I wouldn't use that circuit around anyone I cared about! Much like buying electrical stuff off Amazon - electrical safety is not one of Amazon's concern. And I take it electrical safety is not one of your concerns either.

Pretty sure your home/business insurance wouldn't cover you if something goes wrong with that design.

John :-#(#

No misaprehension - I did skip your query about 2 pole, but I wanted to get my general understanding down first. I haven't thought about the 2 pole aspect in detail, though it dis briefly occur...

So this is an edge case where you plug into 2 120V sockets on the same phase? Given the failure scenario is safe (zero PD) I still can't see much point in it.

Nope - well, nothing very useful as far as I can see.

Well, yes...

No - I understand it well enough to know it's a pretty rubbish setup, with a non zero risk of a wrong side failure.

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The functionality of the circuit is orthogonal to its safety. The functionality is not in much question except with whether 1 or 2 pole relays are needed.

It will function.

It is not safe in my opinion. Several people have told you that.

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My Nissan Leaf, when i visit you? I can't use your non-standard T-charger anyway. Are you planing to down-grade your T-charger to 6KW (15A)?

The only reason i can think of is your builder is totally ignorance of EV and put the garage on far side of the main breaker. In that case, the #10 or #12 wires would not be enough.

My idiot rule to add to the NEC: A garage should be connected to the main via 4 #6 or #8 wires. 440V should be optional. I want 440V in my garage.

Dumb question from a Brit used to a different system:

Are you suggesting a new type of company transformer?

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440V (perhaps 3 phases) is available for certain industrial users in the US. Hope it gets to residential users eventually. 240V is not enough for EV, or aluminum melter. I want to charge up my EV and melt and cast aluminum.

Interesting. Thank you.

Yes, the whole EV thing is going to open a can of worms.

In the UK, we already have 240V (well 230V officially thanks to EU harmonisation, but it's really 240V most of the time) - and many of us have 100A supplies with 60A being the smallest domestic supply I've seen.

Even then, a dedicated EV charging circuit is likely to be 32A (one of our standard breaker ratings) but compared to a supercharger, it would still be slow.

We could get 3 phase supplies (although we'd need a new breaker panel design for that - UK domestic panels are tiny, we'd need to switch to industrial panels more like the US ones in size and layout).

But it's all a bit mute as we're hard on the line for power generation as it is thanks to the greens shutting coal power stations and NIMBYs rejecting new nuclear:

A few years ago in a particularly cold winter, I saw that peak at over

60GW demand for the UK.

Now we only have 50GW generating capacity assuming something doesn't go bang unexpectedly :(

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240v at 200A is 48kW. I struggle to believe that isn't enough to melt ali!

NT

We're stuck with 240v at anything from 40A to 100A incomers. 240v 100A is 24kW, not so good for small flat dwellers with only 40A = 9.6kW. Of course one could only use all of it to charge when there's no other draw.

To upgrade from there would require new underground feeds, 3 phases are generally not available at the incomer. And new incomer, new CU etc. And quite likely a major upgrade in the whole infrastructure.

We can surely have 240v 40A sockets without any new CU or incomer, and probably more. 100A car sockets would require demand management, which isn't hard to retrofit, it's just more equipment.

NT

0 volts at a home or business that doesn't have 240 volt outlets is to pick off two outlets that are wired to separate circuits on opposite sides of t he 240 volt power. There is a commercial product for this and designs on t he web. I found this schematic.

the issue is that without the input relays when you plug the unit into one outlet the path through the output relay coil can energize an exposed plug pin. But I don't see the need for two poles. Running the 120 volt connec tion through the input relay contacts on just one side of the output relay coil will prevent the two inputs from being energized when only one is plug ged in.

y advantage I can see is that with the two pole relays a single stuck conta ct won't be dangerous. In a one pole approach it can pass a dangerous volt age to the input plug pin. But you can hear relays working so a stuck rela y can be detected. Also it has indicators on the inputs so you can tell th e relay isn't clicking because of a failure rather than simply no voltage. So this risk seems minimal.

The purpose of K1, K2 is no mystery. Electrical equipment must only become dangerous when 2 or more component failures occur. Hopefully the relays use the 3mm+ contact spacing required to be acepted as safe isolation. K3 just takes the same principle further.

Whether this setup is sufficiently safe depends mainly on the reliability l evels of K1,2,3. No electrical equipment is free from shock risk after all. Personally I'd not want to bet on its safety, I'm not hugely confident tha t relay contacts wouldn't weld from carrying/switching EV charger currents. But to be fair I don't have the figures that the engineer or hack who desi gned it might have.

One could make it safer by adopting a similar approach that microwave oven interlocks use.

NT

But you need #4 or #6 wires to run 200A, unless you want to melt Al next to the main breaker. My main point is that you don't want to charge EV at more than 50A, or wirings and coolings are real problems.