Voltage sag limits

[newuser1]

Hi all,

I think I read somewhere on here that a safe voltage sag is around 3% of nominal voltage. I have a 48 volt battery bank and when placing a 2kw+ load on it (microwave for example), it can bend down to 46-47 volts depending on SOC. Is my calculation 48*.03 or 50.8*.03? Thanks!

[Sunking]

You use worse case or nominal of 48 volts. During the peak of the day, sag is not an issue because the panels if your system is properly sized supplies all the power and that will be at 52 to 54 volts. It is after the sun sets sag becomes a problem and is 2-fold.

You have two sources of voltage sag to deal with: Battery and wiring. You have 3% between the two to deal with. Most DIY types blow it when it comes to this point because they have no idea of what they are doing. How many times have we heard here some person with a 12 volt car battery using a 2000 watt Inverter? Way to many times and roughly 90% of folks who come here fall into that category.

All batteries have Internal Resistance and all wire has Resistance. This is the cause of voltage loss. Battery resistance is directly proportional to the battery capacity. Also battery type and construction play a role. Wire resistance is a product of the wire size and length.

For FLA batteries to keep voltage sag to 2%, a good rule of thumb is the highest discharge current is C/8 where C = battery AH capacity at the 20 hour rate. Granted some hybrids like Golf Cart, RV and other market gimmic names can supply C/6 to C/5. So you have a 2 kW Inverter operating at 48 volts. Well 2000 watts / 48 volts = roughly 42 amps and once you take Inverter efficiency into account is 50 amps. That very quickly tells you your battery needs to be at least 400 AH or more.

Next the wire has to be sized correctly to limit voltage loss to 1 to 2 %, and meet minimum safety requirements. Keep in mind 3% is your total budget. So first step is determine the minimum required wire size to meet safety requirement. If you have a 2000 watt Inverter you are likely looking at a 75 amp fuse. If this is a home where NEC applies that is going to be Table 310.17 using 90 degree insulation type is a #8 AWG minimum requirement. Some AHJ may require you to use the 60 degree rating in which case Would be a 6 AWG. Those will work if the 1-way distance is no greater than 6 feet using 8 AWG, or 9 feet using 6 AWG.