DC Cable Calculation

Small.

Simple.

Welcome.

As lyn said it has to be a small cable.

The maximum current permissible- for long time- it is usually 10% of capacity that means 10 A. If it could be considered 1 hour fast charging-if the UPS could withstand it- then you have also 100 A for an hour.

Let's say the cable is XLPE insulated copper conductor.

If the cable runs on a rack you may consider it as" in free air". Let's take 40oC the ambient and following NEC Table 310.15(B)(20) you'll get 8 awg [as a minimum] for 57 A rated.

Following IEEE 242/2001 recommendations -see IEEE 242/2001 ch.9.5.2.4 Development of intermediate characteristics formula:

(IE/IN)%=SQRT{[(TE-To)/(TN-To)-(Io/IN)^2*EXP(-ts*k)]*(230+TN)/(230+TE)/[1-EXP(-ts*k)]}*100

[-it is translated in Excel language except { and [ has to be ( of course]

where:

IE is emergency operating current rating,[100 A]

IN is normal current rating,[57 A]

Io is operating current prior to emergency[10 A],

TE is conductor emergency operating temperature,[130oC for xlpe]

TN is conductor normal operating temperature,[75oC]

To is ambient temperature,[40oC]

K is a constant, dependent on cable size and installation type (see Table 9-5),[0.33]

ts overload time in hours[1h]

you'll get 284% -that means the cable could withstand very good 2.84*57=161A in an hour.

It's a simple enough calculation for an Electrician, Mildred. Why don't you hire one?

What I don't understand is why you are feeding a battery from an UPS when there are batteries inside the UPS and the external battery is also an UPS. If that's your design then you really shouldn't be playing with the electricity.

Again, British Standard 7671 gives full protocols <sigh>.