# Learn a little about Transformers 120Vac to 240Vac

#### Guest3123

Joined Oct 28, 2014
404
How is it done.. I know it's to do with Transformers, and not the movie. I would like to know if the 120v side draws more current from the wall, or what.

So if I wanted to let's say use a 240Vac heater, that's designed for a 240Vac heater, that puts out like 2000 Watts or 3000 watts.. What will it pull from the 120Vac side, coming from the wall, which is 12 gauge romex going to the panel box with a 20 amp breaker.

Let's just say it's wired for 120v, 12ga. 20 amp breaker.. but it's not, it's 14ga. 15 amp. Let's just say..

If I wanted a 2500 watt heater, to use with a 240Vac outlet, or transformer that converts 120Vac to 240Vac..

So the numbers are 240Vac, 2500W, 10.41A, 23.03Ω..

What's happening on the 120Vac side..? Double..? So it's pulling 20.41A on the 120Vac side..? or what..

Here's something.. http://www.voltageconverters.com/itemdesc.asp?ic=VC3000W

Not bad.. only \$119.99

#### RRITESH KAKKAR

Joined Jun 29, 2010
2,829
Power on both side remain the same.
If left side has low voltage that mean more current, on high voltage low current .
P=V*I

#### bertus

Joined Apr 5, 2008
22,217
Hello,

As said the input power and output power will be the same, in an ideal transformer.
There are some losses in the transformer, so you will need some more power on the input.
https://en.wikipedia.org/wiki/Transformer

Bertus

#### R!f@@

Joined Apr 2, 2009
9,881
Power on both side remain the same.
WRONG...

Power drawn from the primary is always more than the secondary load.
Factor is the transformer efficiency or heat loss in the core.
A step up TX draws more current into the primary than the load requires from the secondary.
A step down TX draws less current into the primary than the load requires from the secondary.
Of course the primary current increases as the efficiency decreases

#### Guest3123

Joined Oct 28, 2014
404
WRONG...

Power drawn from the primary is always more than the secondary load.
Factor is the transformer efficiency or heat loss in the core.
A step up TX draws more current into the primary than the load requires from the secondary.
A step down TX draws less current into the primary than the load requires from the secondary.
Of course the primary current increases as the efficiency decreases
Kinda figured.. because after all.. You don't get something for nothing.

Ohms Law says..
240Vac, 2500 watts, 10.41 Amps from secondary..
So that means.. 120Vac, 2500 Watts, 20.83 Amps on the primary.. right?

So what the hell is the sense of using a step up transformer..? My 120Vac outlet would have to wired for 25Amp, 12-3 AWG.. right?

So why do they use these Transformers anyways... I might as well use a 240Vac outlet to start with.. right?

#### RRITESH KAKKAR

Joined Jun 29, 2010
2,829
Hello,

As said the input power and output power will be the same, in an ideal transformer.
There are some losses in the transformer, so you will need some more power on the input.
https://en.wikipedia.org/wiki/Transformer

Bertus
I mean that only.
roughly figure out to understand.

#### BReeves

Joined Nov 24, 2012
410

#### R!f@@

Joined Apr 2, 2009
9,881
Kinda figured.. because after all.. You don't get something for nothing.

Ohms Law says..
240Vac, 2500 watts, 10.41 Amps from secondary..
So that means.. 120Vac, 2500 Watts, 20.83 Amps on the primary.. right?
Ideally , yes but in reallity Power in is bit more for the Tx loss

So what the hell is the sense of using a step up transformer..? My 120Vac outlet would have to wired for 25Amp, 12-3 AWG.. right?

So why do they use these Transformers anyways... I might as well use a 240Vac outlet to start with.. right?
The transformer is used to step up or down depending on your mains rating and load rating.
If you have 120VAC , then for a 120VAC load you don't need a Tx

#### GopherT

Joined Nov 23, 2012
8,009
So that means.. 120Vac, 2500 Watts, 20.83 Amps on the primary.. right?

So what the hell is the sense of using a step up transformer..? My 120Vac outlet would have to wired for 25Amp, 12-3 AWG.. right?
NOTE: 12-3 AWG is only good for up to 20 amps. Use 10 Gauge for the next step up (good to 30 amps). Also, wire should always be rated at a higher ampacity than fuse or circuit breaker.

Also, if you have long runs of wire when 15 - 20 amps are expected on a regular basis, you should step up to the next bigger wire.

A step-up transformer is helpful if you have long runs of wire and the transformer cost and transformer losses are justified by the cost savings of using thinner wire and less transmission losses (because amperage is reduced and I*I*R power losses in the wire are reduced). Note, it is usually only a benefit on mile long transmissions, not a few 100 feet.

Last edited:

#### dl324

Joined Mar 30, 2015
15,827
So why do they use these Transformers anyways... I might as well use a 240Vac outlet to start with.. right?
If you were using an appliance designed for 240V AC and wanted to use it in a country where 120V AC was the standard. Or vice versa if you traveling from the US to, for instance, Europe.

#### #12

Joined Nov 30, 2010
18,224
NOTE: 12-3 AWG is only good for up to 20 amps. Use 10 Gauge for the next step up (good to 30 amps).
Let me clarify. 12 gauge wire is sufficient for a 20 amp circuit breaker and 10 gauge is sufficient for a 30 amp circuit breaker. There are NEC design rules about 80% load limits. That's probably why you can't buy a 2400 watt hair dryer, toaster oven, or microwave oven. 20 amps at 120 V is 2400 watts. The 80% limit calculates to 1920 watts of design load.

Last edited:

#### Kermit2

Joined Feb 5, 2010
4,162
Should be 2400W not 5400...

Just a little nit picked.

#12

#### #12

Joined Nov 30, 2010
18,224
Should be 2400W not 5400...

Just a little nit picked.
Typo!
I typed it right the first time, then slipped a digit or something.
I think the phone rang and I got distracted.

#### dl324

Joined Mar 30, 2015
15,827
#12

#### cmartinez

Joined Jan 17, 2007
8,100
Typo!
I typed it right the first time, then slipped a digit or something.
I think the phone rang and I got distracted.
that was me on the line... deliberately distracting you sou you'd screw up and make a typo

#12