DC Circuit Cable Sizing 1

[bxny]

I am sizing the feeder cable for a DC circuit from a 125VDC distribution panel to an inverter. The distribution panel is fed from a 125V battery. The inverter normally draws 5A and during a loss of AC power event, it draws 1200A for the first minute and 1A thereafter, for 2 hours ( the duty cycle for the battery). Question is how do I size the cable? What is the basis for the size chosen?
Thanks for the help!

 

[itsmoked]

NEC wise Probably the same 125% but I'll let someone who works in NEC stuff more frequently confirm that.

At the very least you need to provide the required voltage the inverter equipment needs. This means you need the overall resistance to be low enough to prevent excessive voltage drop between the batteries and the inverter.

With massive draw from the batteries you need to know also what they will be putting out (cell voltage) when 1200A is being drawn.

Remember everything comes back from what the inverter needs. You may actually need over-sized cables to minimize the cable drop.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[DanEE]

quoting itsmoked: "Remember everything comes back from what the inverter needs. You may actually need over-sized cables to minimize the cable drop."

ditto..

We also always design our 48 volt DC plant systems wiring based on voltage drop, which means using heavier gauge than what NEC requirements dictate.. Since most PBXs shut down at 42 volts, you can't afford to invest $$$ in battery AH for run time, then lose it in the cabling.

 

[bacon4life]

What is this supplying? I can't quite picture what sort of battery backup would draw less instead of more during a power outage.

itsmoked

Are you suggesting 125% of the 1200 A of momentary current? Wouldn't this be more akin to motor starting? After checking voltage drop, could bxny use thermal modeling to calculate the transient capacity of the wire?

 

[bxny]

To respond to your question, bacon4life, the battery is sized for a 2-hour duty cycle, in an electric generating station. Typically, there will be momentary loads that will energize in the first minute, upon a loss of AC power, and then will 'settle down' after that.
As far as your ampacity question, I was thinking the same thing - treating it like motor starting, using thermal modeling of the wire.
Thanks for your input!