208V to 230V buck boost

C

cppoly

Senior Member
Location
New York
If an appliance is 230V and there's only 120/208V available from a panel, would a buck boost transformer would be needed?

For a 2,300 watt, 230V load, this would be a 3 kVA transformer, which is 3,000 / 208 = 14.4 amps * 1.25 = *1.25 = 18 amps = 20 amp OCPD primary?

Then, would a 20 amp disconnect fused at 15 amps be needed on the secondary?
 
I would, and, simply put the boost transformer between the OCPD and the appliance it is serving.

Jap>
 
Thanks for the replies.

retirede said:
You may have already done this, but I’d make sure the load can’t be used on 208 before going the transformer route.
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Thanks for the suggestion, the documentation says it will not work on 208V.

LarryFine said:
Does the load have a neutral?
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Nope

ptonsparky said:
Buck boosts are sized by the voltage Change and the current, IIRC.
22 * 10 = 220 . There is no secondary.
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Could you clarify this? All conductors are primary?
 
cppoly said:
Could you clarify this? All conductors are primary?
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A buck-boost is usually built as an autotransformer. These have 'high voltage' and 'low voltage' sides not primary and secondary and do not create separatly derived sources.
These transformers Work in both directions: they can boost voltage, start low go higher, or buck voltage, start high go lower. The NEC has special rules for autotransformers, although they are very easy to follow for applications like the OP.
 
A 'buck-boost' is slang for a 'autotransformer'.

An autotransformer uses the same coil for both primary and secondary circuit, and only 'transforms' the voltage difference. This means you get away with a much smaller transformer to do the same job. The VA of the actual transformer you need is simply the voltage _difference_ times the load current, or in your case 22V * 10A = 220 VA.

As a practical matter, the transformer is built with separate primary and secondary windings, but these windings are wired electrically in series to create the 'autotransformer' configuration. On top of this, unless you want to spend lots of extra $$ you will use transformers with standard voltage ratings and lower voltage then their rating; this means that the nominal VA will be higher than the bare minimum you need.

But in a way all of the above theory is a tangent to what you need, because the companies that _sell_ autotransformers will ask you for the input voltage, the output voltage, and the load VA, and point you to a table that specifies the part number that will supply it. You don't care if the thing in the box is a 240:32V 250 VA transformer; all you care is that you put 208V in, get close to 230V out, and can safely supply 2300W.

You will go to a page like the below select your input voltage, your output voltage, and load VA, and get a part number. It doesn't matter that the _same_ part number will be used in a different configuration for a different combination of voltages and power.

Autotransformer Selection Guide

temcoindustrial.com temcoindustrial.com
 
A 'buck-boost' is slang for a 'autotransformer'.


Exactly,
And, although they are called Buck/Boost transformers because you can use them to Buck or Boost the voltage, I can always tell the guys that have been around them a lot because they will say, "I need a "Boost" transformer to get my voltage from 208 to 230v", or, they'll say "I need a transformer to "Buck" my voltage from 240 to 220".

The seasoned guys will hardly ever say, "I need a Buck/Boost transformer to change my voltage from 208 to 230v". :)

Jap>
 
winnie said:
A 'buck-boost' is slang for a 'autotransformer'.

An autotransformer uses the same coil for both primary and secondary circuit, and only 'transforms' the voltage difference. This means you get away with a much smaller transformer to do the same job. The VA of the actual transformer you need is simply the voltage _difference_ times the load current, or in your case 22V * 10A = 220 VA.

As a practical matter, the transformer is built with separate primary and secondary windings, but these windings are wired electrically in series to create the 'autotransformer' configuration. On top of this, unless you want to spend lots of extra $$ you will use transformers with standard voltage ratings and lower voltage then their rating; this means that the nominal VA will be higher than the bare minimum you need.

But in a way all of the above theory is a tangent to what you need, because the companies that _sell_ autotransformers will ask you for the input voltage, the output voltage, and the load VA, and point you to a table that specifies the part number that will supply it. You don't care if the thing in the box is a 240:32V 250 VA transformer; all you care is that you put 208V in, get close to 230V out, and can safely supply 2300W.

You will go to a page like the below select your input voltage, your output voltage, and load VA, and get a part number. It doesn't matter that the _same_ part number will be used in a different configuration for a different combination of voltages and power.

Autotransformer Selection Guide

temcoindustrial.com temcoindustrial.com
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Thanks Winnie. So the theory is a 22V *10A = 220 VA autotransformer.

Per the website and following the input / output prompts, it looks like the minimum is 3 kVA transformer 208V primary 230V secondary.
 
cppoly said:
this would be a 3 kVA transformer, which is 3,000 / 208 = 14.4 amps * 1.25 = *1.25 = 18 amps = 20 amp OCPD primary?
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The ocpd for the autotransformer will based on the full-load input current of autotransformer, which depends on the actual transformer used and the configuration.

For example, a 250VA, 240-24V transformer could support your 10A, 230V load. The 250VA transformer connected to give you 208V in and 229V out would have a load rating of 2.39kVA.

The full load input current would then be 11.5A, so the maximum input ocpd size would be 15A.

There's nothing that says you can't use a larger transformer though. You could use a 500VA transformer for instance, and use a larger OCPD.
 
I have commented on this before - the various voltages, 120/204,240 V, have. you guys have for residential. Why? For us in UK and EU it is just 230V. It's just so much simpler.
 
winnie said:
Why, I don't know. But it's Tradition.
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In UK and other countries we did have different voltages but eventually it all got rationalised. Same with pounds and inches. That all went matric (or, more correctly, SI)
 
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david luchini said:
The ocpd for the autotransformer will based on the full-load input current of autotransformer, which depends on the actual transformer used and the configuration.

For example, a 250VA, 240-24V transformer could support your 10A, 230V load. The 250VA transformer connected to give you 208V in and 229V out would have a load rating of 2.39kVA.

The full load input current would then be 11.5A, so the maximum input ocpd size would be 15A.

There's nothing that says you can't use a larger transformer though. You could use a 500VA transformer for instance, and use a larger OCPD.
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David - could you help with your calculations here. 240-24V transformer?

How did you get 208V input / 229V output?

How did you get load rating of 2.39 kVA?

What does the transformer rating , 250 VA or 500VA have to do with the line voltage current? Wouldn't the line be based upon the load power rating (2,300 watts) / 208V (supply voltage)?
 
You might think '208+24 = 232' but remember this is a 240V:24V transformer. When you apply 208V to the 240V coil, you only get 21V on the 24V coil. 208+21=229.

To figure the effective load rating of a transformer reconnected in a boost (or buck) configuration, simply take the rated low voltage current * the resulting voltage. A 250VA 240:24V transformer has a secondary load current rating of 250/24 = 10.4A. The voltage in the boost configuration is 229V, 229*10.4 = 2.38kVA. I think 2.39kVA comes from slightly different rounding of the steps.

When I did my example, I assumed a 240-32V transformer, to get closer to 240V; David was aiming for exactly 230V.

-Jonathan
 
jap said:
A 'buck-boost' is slang for a 'autotransformer'.


Exactly,
And, although they are called Buck/Boost transformers because you can use them to Buck or Boost the voltage, I can always tell the guys that have been around them a lot because they will say, "I need a "Boost" transformer to get my voltage from 208 to 230v", or, they'll say "I need a transformer to "Buck" my voltage from 240 to 220".

The seasoned guys will hardly ever say, "I need a Buck/Boost transformer to change my voltage from 208 to 230v". :)

Jap>
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Huh, I have a full understanding of these type of transformers, and I DO refer to the as Buck/Boost transformers , so I kind of take offense at your characterization. If I wanted to be snooty, then I would say, it is an autotransformer, not either a buck or a boost, because when you buy one it is usually a 12/24 to 120/240 transformer. Just sayin' :cool:
 
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