Likely a dumb question but.. here goes: Is it possible to safely split a 240vac input across a pair of 3kva step-down transformers, in order to feed a sub-panel. And in doing so, are you still limited to 3vka or will the realized current capacity now be 6kva?

If I think logically, a 120v device would be limited to 3kva.. but a 240 might have the combined capacity?

 

The Gauge of the Wire used in the Secondary-Winding determines the Output-Current-Capacity.

If You parallel 2-Secondary-Windings that are operating "in-Phase" with each other,
the Output-Current-Capacity will be doubled.

With 2-Secondaries connected in series, the Current-Capacity does not change.

You should specify that You are in the USA, and using a 120V / 120V Split-Supply, and provide a Schematic.

Why do You need to use Transformers to feed a Sub-Panel ????
No Transformers are required.
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You don't say anything about series / parallel / other connections among either the primaries or the secondaries. Please post a schematic of what you are asking about.

ak

 

From what I understand, he needs 110 volts from 220 volts for a range of 110 volts equipment. (or the other way round)
He requires 6 KVA on the load, and has only 3 KVA transformers.
Can he parallel them to get 6 KVA?

Yes. With correct phasing.
1) The transformers must be exactly the same.
2) The power input must be from the same (literally) for both transformers.

If 1 and 2 are not met, there will be circulating currents and will cause overheating and damage.

 

Based on this:

If I think logically, a 120v device would be limited to 3kva.. but a 240 might have the combined capacity?

Here is what I think he is saying:

Given:

240 volt split phase line power
Two 240V to 120V 3KVA transformers

Can I connect them to feed a 240V split phase sub-panel at 6KVA?

The answer is yes, by parallel connection of the primaries and series connection of the secondaries. And yes, a single 120V device could use only 3KV, while a 240V device could use 6KV. Note that each gets the same current (25A.)

But my question would then be Why? You can do the same with no transformers.

Perhaps he has inly a single phase 240V line. Then it would make sense. In fact, that is how the US power is delivered to homes, using a center tapped transformer instead of two in series.

 

Thanks for the input guys!

So as for the why; I have a shop with a single 120v feed at the moment. I regularly weld and my weld is damned near the cheapest arc unit Amazon has. So it's noisy.. very noisy, it also causes the led lights in the house.. specifically the kitchen/living room to be affected. I'm told flickering but I do not know to what degree. Anyway my idea was a big isolation transformer.
And since the shop is ~130' from the main panel in the house, I thought I could run one more 120v line for 240v total.. then step it down using said transformer. I have one 480/240 to 240/120 3kva transformer, that I will try once I get the wire for the second run. But yesterday I had the idea of what about splitting it between two identically sized transformers.
I currently have no devices that require 240/220 so the two 240 feeds would terminate at the sub-panel and only provide 120 out.
Do I require more than 20A? Eh.. not 'really' but it'd be nice to have a bit more cushion.

 

Thank you for explaining what you are trying to do.

From 120V in, you could run the transformer backwards to get 240V out. You could run two in parallel to get 6KVA, but you would need 50 amps in.

Running another 120 line can be done two ways. It will either be the same phase or opposite phase. If you can run a line of the opposite phase, you can get 120 and 240 with no transformers. That is what I would recommend. Each would need to be 25A.

 

Get a Gasless-"Flux-Core" Welder with the option to power it at 240-Volts,
they're cheap, and much more efficient in Power-consumption than a Stick-Welder.

Then You can Weld up to ~5/8-Plate with excellent penetration,
and with no Transformers,
which will make the whole arrangement much more efficient.
It will even noticeably lower your Electric-Bill.

A 120-Volt Stick-Welder is just a bad joke.

Once You get used to using all the versatility that comes with a Flux-Core-Machine,
you'll probably never use a Stick-Welder again.

Stick-Welders have their specific-advantages in certain industrial / commercial settings,
but not so much in a Home-Based-Shop.

For a ~130-foot run, at ~25-Amps,
You will need to run at least #6-Gauge Wiring, and use a 50-Amp Breaker.
This will make your Welding-Arc much more stable and easier to work with,
regardless of the type of Welder that You use.

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You can run the extra wire. Use two breakers that are next to each other and strap them together to make 220 service. (when either brake pops they both turn off) This will reduce the current in the wires and half the load is on each line wire. (half load from house to garage and half from house to power company) Now use a 220 to 120 transformer to make a special 120 at 40A. You can use a 120 to 220 backwards.

Why the flickering. Check the connection of the big wires coming into the main panel. The connection might be loose.

I have some 120 to 120 isolation transformers. I can see wiring the secondaries in parallel and the primaries in series to make 220 to 120.

 

So.. buying a new welder or <insert offending equipment here> isn't a valid suggestion. Aside from the fact that I'm sending a daughter to college so the money is going bye-bye.. I don't weld enough to reinvest in a new one.

I've checked the main panel feeds, everything's tight as a drum. This only happens when I'm welding. Provided this issue is being caused by whatever the welder is doing, isolating it from the rest of the might resolve it yeah?

 

I don't know if this will pass your regs, but for long distances we often send 460V (230V + 230V of opposite phase, no neutral) then use an isolating transformer at the far end to recreate 230V, making a local earth and neutral.

If you only have one cable between the buildings send 120V and out-of-phase 120V down it, with no neutral.
At the far end use the isolating transformer to produce 0-120V (or even 120-0-120V using two transformers). Connect the 0V to local earth (a rod in the ground and any metalwork) and take your neutral from there via a residual current circuit breaker.
That produces the minimum of losses in the long cable.
Our regulations forbid using the earth from the main building. The shed must be wired TT.

 

I don't know if this will pass your regs, but for long distances we often send 460V (230V + 230V of opposite phase, no neutral) then use an isolating transformer at the far end to recreate 230V, making a local earth and neutral.

If you only have one cable between the buildings send 120V and out-of-phase 120V down it, with no neutral.
At the far end use the isolating transformer to produce 0-120V (or even 120-0-120V using two transformers). Connect the 0V to local earth (a rod in the ground and any metalwork) and take your neutral from there via a residual current circuit breaker.
That produces the minimum of losses in the long cable.
Our regulations forbid using the earth from the main building. The shed must be wired TT.

That is very interesting.. is there a term for that kind of setup? Something I could Google perhaps, at least to verify i'd be breaking/not breaking code here?

 

That is very interesting.. is there a term for that kind of setup? Something I could Google perhaps, at least to verify i'd be breaking/not breaking code here?

I'd have to leave that question to the experts on USA wiring. It's perfectly OK here, provided that the neutral is properly earthed at the far end. It has the advantage of being impossible to inject any interference into the neutral and earth at the house.

 

You don't say anything about series / parallel / other connections among either the primaries or the secondaries. Please post a schematic of what you are asking about.

ak

Exactly what I was also going to ask. What does the TS mean by "split"??? I have powered different parts of a single large panel from separate transformers. Separate circuits, not in parallel.

 

I'd have to leave that question to the experts on USA wiring. It's perfectly OK here, provided that the neutral is properly earthed at the far end. It has the advantage of being impossible to inject any interference into the neutral and earth at the house.

So, just to be sure: Using black and white wired both to line but out of phase at the source. Then at the destination, use both hots as line input on two transformers. And the neutrals to earth, then from transformers to circuit panel.. as 220/240, yes?

 

So, just to be sure: Using black and white wired both to line but out of phase at the source. Then at the destination, use both hots as line input on two transformers. And the neutrals to earth, then from transformers to circuit panel.. as 220/240, yes?

Don't ask me about black and white wires! Ours are brown and blue. I could draw you a diagram to show how I would wire it according to UK regulations, and it is probably the same where you are, because the principle is the same, but it just might not be.

 

Are you stuck with the wiring you have, or could you replace it? If you can, why not just use a 3 conductor cable to bring 2 lives and neutral, and eliminate the transformers?

 

So.. buying a new welder or <insert offending equipment here> isn't a valid suggestion. Aside from the fact that I'm sending a daughter to college so the money is going bye-bye.. I don't weld enough to reinvest in a new one.

I've checked the main panel feeds, everything's tight as a drum. This only happens when I'm welding. Provided this issue is being caused by whatever the welder is doing, isolating it from the rest of the might resolve it yeah?

Umm, so how would you afford wire to to run a second leg to the shop plus a second 3kva transformer?

I propose that your welder is switching on the current during only part of the phase. This causes the LEDs in your house to flicker because your wiring is too small for the amount of current that you're drawing. I mean a combination of the in-house wiring and the "from telephone-pole transformer to main circuit panel" wiring.

But to answer your question (and I'm not an electrician, I just know what some electricians do in cases like this):

A: Get 4 transformers. Put two transformers in parallel on the side in the house as close as you can to the circuit panel to step up the voltage to the shop AND ONLY THE SHOP to 240v. Then put the other transformers into the shop to step the voltage back down to 120v. It will half the current going to the shed with the goal of helping the welder to draw current more evenly.

B: Get a permit and run larger gauge wiring. Hey, if you can run 1 new leg and get a transformer, you could run new wire and solve the problem, right? This might solve both problems you mention at the same time.
OTOH: After learning about wire codes, I found out that our house's incoming wiring (telephone pole to main 200A circuit breaker), is about 75' too long for the wire gauge used (2AWG aluminum IIRC). How this got past inspection I don't know. It's not that old of a house either.

C: Generator.

D: Mod the welder so it has an active PFC circuit. Don't ask me how. I'm not up to that level yet.

I hope it helps.

EDIT: To be clear, these are in addition to your first idea.

 

You are going to have a bunch of problems if you try to satisfy code. Which is often to everyone’s benefit. Most electricians struggle with code, outside of their immediate exposure. Google won’t solve your problem. I second the comment that stick welding on 120, isn’t really welding. Migs and tigs can be reasonably priced used. Trench or overhead an appropriate service to a local breaker panel. You need lights, utility outlets, a compressor, and your welder. Or pull a local circuit at your main and weld up by the house.