# Mains wiring question: Sizing buck-boost transformer?

Discussion in 'Electronic Equipment' started by Gary Walters, Mar 29, 2013.

1. ### Gary WaltersGuest

In USA.

Source: 208v, 60 hz, 2-wire (2 phases from 3 phase "Y" supply). Load: 240v,
20A.

I presumed that sizing a buck-boost transformer is simple KVA math (source

This PDF document:

<http://www.acmepowerdist.com/pdf/Page_104-109.pdf>

on the last page says:
- - -
"An example of an everyday application is always a good way to explain the
intent of the ³Code.² Example: A 1 kVA transformer Catalog No. T111683 has a
primary of 120 x 240V and a secondary of 12 x 24V. It is to be connected as
an autotransformer at the time of installation to raise 208V to 230V single
phase.

When this 1 kVA unit is connected as an autotransformer for this voltage
combination, its kVA rating is increased to 9.58 kVA (may also be expressed
as 9,580 VA). This is the rating to be used for determining the full load
input amps and the sizing of the overcurrent protect device (fuse or breaker)
on the input.

9,580 Volt Amps / 208 Volts = 46 Amps"
- - -
I'm puzzled by the 10x increase of KVA rating. When and how is this true?

What size B-B transformer do I need?

Thanks.

2. ### tmGuest

Those are common devices, to go from 208 to 230/240 volts. The secondary
current determines the rating in KVA.

It's true because you're only using the transformer to "create" 24 volts at the current you wish
to draw at 230v. This extra 24 volts is added back into the line voltage.

You can switch flip the leads and subtract voltage too, then the transformer is in buck mode.
If you need 20 amps at 230v and start with 208, you need to boost 22volts (208+22=230) x 20 amps
= 480VA transformer. A 24 volt transformer rated over 480VA should be fine.

Autotransformers can be confusing, so pretend it's just DC and some batteries.

Let's say you need 24 volts at 10 amps and have a 12 volt battery that can already output 10
amps.

what size power supply do you need to run in series with this battery to get the 24 volts?

just another 12 volts, at at least 10 amps, or a 120 watt power supply. Those wired in
series (your battery and the new power supply) will provide 240 watts.

If you already had an 18 volt battery, you'd just need a 6 volt, 10 amp or 60 watt power supply.

The less the voltage adjustment, the smaller then buck/boost transformer rating becomes as it's
really not doing all that much work.

4. ### tmGuest

Any electrical supply house will have them in stock. They are not too
expensive either.

tm

5. ### Sylvia ElseGuest

You can look at it this way: All the input current flows through the
primary, and all the output current flows through both the primary and
the secondary. But the output current is in antiphase with the input
current, so most of the current in the primary is cancelled. The primary
has to handle only the difference between the input and output current.

Sylvia.

6. ### Don KellyGuest

--QUOTE
It's true because you're only using the transformer to "create" 24 volts at
the current you wish
to draw at 230v. This extra 24 volts is added back into the line voltage.

You can switch flip the leads and subtract voltage too, then the transformer
is in buck mode.
QUOTE
If you need 20 amps at 230v and start with 208, you need to boost 22volts
(208+22=230) x 20 amps
= 480VA transformer. A 24 volt transformer rated over 480VA should be fine.

Autotransformers can be confusing, so pretend it's just DC and some
batteries.

Let's say you need 24 volts at 10 amps and have a 12 volt battery that can
amps.

what size power supply do you need to run in series with this battery to get
the 24 volts?

just another 12 volts, at at least 10 amps, or a 120 watt power supply.
Those wired in
series (your battery and the new power supply) will provide 240 watts.

If you already had an 18 volt battery, you'd just need a 6 volt, 10 amp or
60 watt power supply.

The less the voltage adjustment, the smaller then buck/boost transformer
rating becomes as it's
really not doing all that much work.
=======================================
lookup "autotransformer".
In this case you have a 2 winding transformer with a 10:1 ratio
With 240V applied the secondary will be 24V with a rated current of
1000/24=41.7A
If this is connected as a boost autotransformer- the total output voltage
would be
240+24 =264V so the output, without exceeding rated output current would be
11KW. only 1 KW (24V*41.7A) is supplied through transformer action and the
rest through a direct connection
The input voltage would be 207V excluding any voltage drops in the
transformer-so 208/230 is close enough.
Autotransformers are great for turns ratios near one as there are size and