nancybeaumont avatar image
nancybeaumont asked

Question about bus bar and switch current rating

I have a question about the current rating of bus bars and switches. I have a 12V system, and the bus bar I was planning on using has a rating of 250A. However, it says that this rating is for a 48V system. Does this mean that it has a rating of 1000A for a 12V system? Or does it not work like that - is it only the current value that matters? An explanation of the physics behind this would be greatly appreciated! Thanks

current limitswitchbusbar
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2 Answers
Alexandra avatar image
Alexandra answered ·


The lower the voltage of a system the higher the current draw will be for a given load. The equation is watts = volts x amps

So for example on a 12v system if you are using 1000w, (work out the amps.)

W ÷ V = A

1000w÷12v = 83.33A

And 48v system:

1000w ÷ 48v = 20Amps

So a lower current draw from a higher voltage system.

Bus bars and switches are current rated so you can work out what you need based on your loads and system voltage.

240A bus bar can power at 12v 3000W.

At 48v - 12000W.

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@Alexandra I think I understand but I'm still a little confused, so it would be useful to clarify a couple of bits. So whether you're on 12V or 48V, you would still just add up the total current of your system and compare that to the rating of the bus bar? It's just that on a 48V system, the bus bar would be able to cope with more power?

So it's not correct to look at a 150A bus bar that's rated to 48V, and say that it can cope with 600A for a 12V system?

I've just thought of this analogy, please could you let me know if it's correct? So the bus bar is like a pipe, and the current is like a volume of water. And no mater how fast the water is flowing (the voltage), you can only fit a certain volume of water in the pipe. Is that right? So whether your system is 12V or 48V, the busbar is still rated to 150A (as that's the maximum amount of water that can fit in the pipe).

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Amps are amps. The max of the bus bar in question is 250A max. Whether 12v or max 48v. The thing that will change is the Watts (for example your kettle may be 1500Watts) processed through at the different voltages. A 48v system uses less amps than a 12v system to boil the kettle.

And yes the flow of water in a pipe is sort of how it works. Voltage is like 'water pressure' (higher the voltage the faster the flow); amps are like 'flow rate'.

Battle born's site has a neat explanation of the basics using the water analogy.

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Size the bus bar for your max loads and inverter. So for example, if you are running a 12v 3kva inverter then 2400W (max continuous for a 3kva) ÷ 12v = 240A bus bar minimum.
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seb71 avatar image
seb71 answered ·

In this case you should look at those two values as maximum allowed: a current of no more than 250A (no matter the system voltage) and for systems up to 48V nominal voltage (no matter the current). Both these limits must be followed/respected at the same time.

If you go above the allowed voltage, the electric insulation might no longer insulate (it might became conductive at voltages higher than the limit) and the switches might no longer be able to extinguish the electric arc, so the current will continue to flow even when the switch is in the Off position.

The current (Ampere) limit is dictated by the heating of the conductor.

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