Fuse sizing for 48v transformers

[Daley]

I am getting close to having my all in one 48v system up and running but wanted to ask someone that knows about fuse sizing when using transformers.

I am using 48v and have a miniature circuit breaker box which has dc breakers in it, or at least it will have.
I plan to run 48v to the breakers and then on the load side of breakers install my transformers, then run the output to another fuse box for each transformer which is where my circuits will meet the stepped down voltage.

I have 2 x 20 amp 48v - 12v transformers
1 x 10 amp 48v - 24v transformer

So three circuits in total, how do I work out the right size breakers to use-on each one?

 

[krby]

If I understand correctly you're asking this:
You have three fused loads coming out of a breaker box. The loads are all step-down voltage regulators, two 48->12V (20A max at 12V) and one 48->24V (10A at 24V). For each of these, I'm assuming the amp rating is on the output side (10A at 24V, not 10A at 48V) because that's what I've seen on step-down regulators.

So you calculate it like this:

• 20A x 12V = 240W. 240W / 48V = 5A. You need at least a 5A circuit breaker feeding each 12V 20A regulator. The regulator won't be perfectly efficient, so 5A at 48V won't actually give you 240W on the 12V output side. But it will protect the regulator from burning out. You could size up a bit too, like 7.5A or something.
• 10A x 24V = 240W. 240W / 48V = 5A. You need at least a 5A circuit breaker feeding each 12V 20A regulator. The regulator won't be perfectly efficient, so 5A at 48V won't actually give you 240W on the 12V output side. But it will protect the regulator from burning out. You could size up a bit too, like 7.5A or something.

The other way to do this is to find the spec sheet for the step-down regulators. Some will list max input current, and you could size the breaker to that, maybe rounded up a bit. If you know for certain, you'll never hit 240W of power on the output side of the regulators, a 5A breaker would be fine.

 

[Daley]

Thank you very much, exactly what I needed to know!