Amperage and Circuit Protection in a system with step down dc to dc converter

Hi All,


I have a 24V battery system and need to add a 12V branch circuit to power some 12V devices.  I have a Victron Energy 24/12-40A DC to DC converter connected to the battery.  I have two questions:
1)  Can I generally assume that a 40A load on the 12V side will put a 20A load an the 24V side

2)  Do I need circuit protection (fuse) on both sides of the DC to DC converter or just the 12V output side.


Thanks in advance for your expert advice!

Dana1)  Can I generally assume that a 40A load on the 12V side will put a 20A load an the 24V side
No. You need to take the converter efficiency into account. With 90% efficiency the 40 A will translate to 20/0.9=22.22A at the input.
2)  Do I need circuit protection (fuse) on both sides of the DC to DC converter or just the 12V output side
It depends what you are trying to protect. Most often we are trying to protect the wiring at the output so we put a breaker there. If you are worried about a short circuit at the input of the converter, then you need a breaker at the battery keeping in mind a battery can supply many,many amps- the regulator is current limited. Local regulations more also play a part in this decision.