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# Secondary Transformer Circuit Breaker

Discuss Secondary Transformer Circuit Breaker in the Industrial Electrician Talk area at ElectriciansForums.net

#### mjbarcelona

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Hi, I'm just curious about what will happen if I use an MCB lower than the calculated current rating. I have a 1kVA 220v stepdown to 110V transformer, calculated downstream MCB current rating would be 11.3625 if i'm right I multiplied the secondary current to 125%? what will happen if I use a lower MCB current rating?

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#### Megawatt

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Arms
Hi, I'm just curious about what will happen if I use an MCB lower than the calculated current rating. I have a 1kVA 220v stepdown to 110V transformer, calculated downstream MCB current rating would be 11.3625 if i'm right I multiplied the secondary current to 125%? what will happen if I use a lower MCB current rating?
MJ what size breaker do you have the primary is going to pull 4 amps and the secondary will pull 8.3 amps

#### mjbarcelona

MJ what size breaker do you have the primary is going to pull 4 amps and the secondary will pull 8.3 amps
I'm planning to put 6amps on primary whilst on secondary i'll put 10amps? I was thinking that the proper breaker size for the secondary would be 1000/115 = 8.69 x 1.25 = 10.69amps?

#### Megawatt

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Arms
I'm planning to put 6amps on primary whilst on secondary i'll put 10amps? I was thinking that the proper breaker size for the secondary would be 1000/115 = 8.69 x 1.25 = 10.69amps?
MJB where are you getting the 125% from. That transformer is not going to withstand 10 amps. Divide 1000 by 110vac on the secondary and see what that transformer will withstand, it should be on the nameplate.

#### Taylortwocities

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Arms
Esteemed
You don’t “put” the current on the primary. The amount of current that flows through the primary winding will be proportional to the current being drawn by the load in the secondary.

#### Megawatt

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Arms
MJB where are you getting the 125% from. That transformer is not going to withstand 10 amps. Divide 1000 by 110vac on the secondary and see what that transformer will withstand, it should be on the nameplate.
Th
You don’t “put” the current on the primary. The amount of current that flows through the primary winding will be proportional to the current being drawn by the load in the secondary.
You don’t “put” the current on the primary. The amount of current that flows through the primary winding will be proportional to the current being drawn by the load in the secondary.
Tayler it is a 1000 watt transformer it is not going to withstand 10 amps, it will start running hot. I did the math for yall
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MJB where are you getting the 125% from. That transformer is not going to withstand 10 amps. Divide 1000 by 110vac on the secondary and see what that transformer will withstand, it should be on the nameplate.
Th
You don’t “put” the current on the primary. The amount of current that flows through the primary winding will be proportional to the current being drawn by the load in the secondary.
You don’t “put” the current on the primary. The amount of current that flows through the primary winding will be proportional to the current being drawn by the load in the secondary.
Tayler it is a 1000 watt transformer it is not going to withstand 10 amps, it will start running hot. I did the math for yal
Th

Tayler it is a 1000 watt transformer it is not going to withstand 10 amps, it will start running hot. I did the math for yall
You don’t “put” the current on the primary. The amount of current that flows through the primary winding will be proportional to the current being drawn by the load in the secondary.
Taylor no joke do you agree with him

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#### mjbarcelona

MJB where are you getting the 125% from. That transformer is not going to withstand 10 amps. Divide 1000 by 110vac on the secondary and see what that transformer will withstand, it should be on the nameplate.
I measured the output voltage of the secondary and it's 125V, calculating the current 1000/125 = 8Amps, by putting an MCB higher than 8amps, it would make the transformer hot right and burned eventually? To protect the transformer you would rather chose an MCB that's lower than 8amps? You're bloody spot on mate! Cheers on that!

#### davesparks

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Arms
Esteemed
What is the rated output voltage? Did you measure the voltage with a load connected or was the output open circuit?

#### mjbarcelona

What is the rated output voltage? Did you measure the voltage with a load connected or was the output open circuit?
rated output voltage is 115V, when I measured it w/o load(open circuit) it's 125V.

#### Megawatt

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Arms
What is the rated output voltage? Did you measure the voltage with a load connected or was the output open circuit?
I tried now it’s your turn
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I tried now it’s your turn
I tried now it’s your turn
MJ if you put it under a load then test the voltage, it’s going to drop

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B

#### Bobster

All of this is irreverent if the OP want's to use the transformer for a fixed piece of equipment. It may only need 2A.

What is it you are wanting to use it for?

#### mjbarcelona

All of this is irreverent if the OP want's to use the transformer for a fixed piece of equipment. It may only need 2A.

What is it you are wanting to use it for?
I'll use it to power PLC, 24V (10A) power supply, and contactors.

B

#### Bobster

Fit an 8A breaker you'll be fine.

A 10A 24V power supply doesn't need more than 240 watts on the input side, so at 120V 2 amps maximum. PLC will likely require 1 amp maximum. An unless you've dozens of contactors using the 110V and not the 24v then this will be less than 2 amp of load.

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