Protection for a Power Supply

[tec.batista]

How can I size a circuit breaker for a power supply. For example
this one: http://datasheet.octopart.com/BAE0006-Balluff-datasheet-31971060.pdf.

Can someone show me the math to calculate a circuit breaker for a power supply?

Any help will be very much appreciated!

 

[Bit_Bucket_07]

The rules vary depending upon the device and the type of installation.

Assuming that you will install this PS in a control panel and feed it with a relatively short length of wire that is not in a conduit, you could multiply the device's rated wattage (120W) by 1.25, since it will be a continuous load. Then calculate the amperage. Assuming that you will provide 120VAC to energize the PS, the Amperage will be 150W / 120V = 1.25 Amps.

Use the next largest available circuit breaker, and remember to size your wire to match the circuit breaker size, because the purpose of the circuit breaker is not to provide overload protection for your load, but to protect the wire feeding the load from overheating and causing a fire.

 

[BryanG]

Do you mean to supply it or to protect the output?

If it is on the supply side then you can't really protect it with a circuit breaker or anything else, you can only protect the wires going to it. If the supply fails, it will fail no matter what protection you give it. Switchmode supplies have quite a high inrush, if the data sheet was doing its job it would tell you the size of breaker to use, typically for a 120watt supply you will have to use a 6Amp breaker depending on the tripping characteristics, and wires that can take an overload large enough to trip the breaker. Looking at the drawing the supply has an internal fuse, but I would still have a separate breaker.

If it is on the output side I would expect it to be overload and short circuit protected anyway, again the data sheet doesn't tell you. If it is overload and short circuit protected then the supply will often cut off before any breaker trips.

 

[keithkyll]

Breaker curve rating is important. Automation direct says 16 Amp, curve B for their 24DC, 120 Watt supply. Datasheet.
For some reason, I just don't like that. Forces me to oversize my main disconnect. I use 4 Amp, Curve D. 6 Amp, curve C would be fine too.
As others have said, this is to protect wiring. The supply has an internal, non-user replaceable fuse on the primary side. I expect it to blow first, and if it does power supply will need to be replaced.

The main thing you want to avoid is nuisance tripping. Size it big enough for that.

 

[tec.batista]

Thanks so much guys.

The rules vary depending upon the device and the type of installation.

Where can I learn theses rules?

But I don't understand why multiply the rated wattage by 1.25. Can someone indicate where (books, magazines...) can I find this type of information?

 

[rdrast]

Remember... The breaker does NOT protect the power supply. It protects the wire. There are no issues with a 5 A main breaker and then 12 A breakers in the panel if that is what you want, but breakers do not protect devices.

 

[rupej]

^I agree, but breakers/fuses DO need to be sized to protect transformers, heaters, and motors...

 

[Bit_Bucket_07]

Thanks so much guys.



Where can I learn theses rules?

But I don't understand why multiply the rated wattage by 1.25. Can someone indicate where (books, magazines...) can I find this type of information?

Here in the US, we rely upon the National Electrical Code.

When sizing branch circuits, one multiplies continuous loads (loads that will typically be energized for more than 3 hours at a time) by a factor of 1.25 and non-continuous loads by a factor of 1. The rules vary for certain types of motors, etc. There are many online resources available for finding answers to electrical code questions.