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I'm confused about the sizing of the battery cables for a UPS system, When the battery is discharging the voltage decrease and current increase to a maximum value (which depend on the end of discharge value of the battery) Should I size the cables according to this maximum current value (end of discharge) , or according to the maximum current calculated using the nominal voltage (in my case 415V)

Regards

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  • \$\begingroup\$ Please provide more specifics, and the calculations which you have done already. Its not clear if you are describing exactly what you think you are excplaining. \$\endgroup\$ – Sean Houlihane May 24 '16 at 7:45
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You should size your cables for the worst operating conditions possible. For cables that mean the highest possible current and the highest ambient temperature. If the highest current occurs at the end of the discharge, then you should size your cables for that.

You didn't mention charging currents, so I assume they are smaller that discharge currents.

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You should always size your cables for the maximum expected current, and also use a fuse or circuit breaker to protect the cables for excessive current.

Especially in your case, if you are concerned with higher currents at lower voltages, you would absolutely want the largest cables possible to minimize the voltage drop.

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Another factor not yet explicitly mentioned often plays a crucial role when sizing battery cables for UPS systems. Namely you want to factor in how the voltage drop from the resistance of the cables impacts the maximum runtime of the UPS. The higher the resistance of the cables, the more voltage drop they cause under load, hence the quicker termination voltage is reached (UPS shutdown due to undervoltage protection). If the wrong choice is made then the load may not have enough time to properly shutdown (or the usable life of the batteries may be too short). This is especially important for high-current loads since the voltage drop is proportional to the current \$ V = I\cdot R \$ by Ohm's law.

Choosing the shorter and largest diameter cables minimizes voltage losses due to cable resistance so increases the UPS runtime. It will also increase the usable lifetime of the batteries.

Many UPS manufacturers include formulas and tables that aid in sizing cables and calculating runtime estimates. You may find those very helpful (even if they are for different but similar UPS systems).

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