# Want to ask the questions from another angle ito charging and cabling

South Africa Solar Expert Posts:

**295**✭✭✭ Hi, I am quite confused at the moment, for too many cooks are spoiling the broth.

So I am coming to the experts.

Questions are always from a load perspective, worked back to batts and inverters. I find myself on the other side at the moment.

Picked up a inverter that can generate max 1400w (so I assume 2800w peak for a few seconds) with no cables.

Included in the inverter, is a 20amp charger.

And no manual that I could find.

Question 1: What is the cable thickness I need to install if the inverter is 2 meters from the batteries?

Question 2: How many 100ah batteries (24v) can one charge with a 20a charger, using AC power?

What are the maths?

So I am coming to the experts.

Questions are always from a load perspective, worked back to batts and inverters. I find myself on the other side at the moment.

Picked up a inverter that can generate max 1400w (so I assume 2800w peak for a few seconds) with no cables.

Included in the inverter, is a 20amp charger.

And no manual that I could find.

Question 1: What is the cable thickness I need to install if the inverter is 2 meters from the batteries?

Question 2: How many 100ah batteries (24v) can one charge with a 20a charger, using AC power?

What are the maths?

5kVA Victron Multiplus II, 5.2kW array, 14kWh DIYLifepo4 bank, all grid-tied.

## Comments

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33,545admin- 1,400 Watts * 1/0.85 inverter eff * 1/21.0 volts typical inverter cutoff * 1.25 NEC circuit derating = 98 amp minimum branch circuit rating (wiring+DC breaker/fuse)

If you assume you want a minimum of 23 volts at the battery (~11.5 volts under load for 12 volt battery), and 21 volt inverter cutoff, plus the inverter can supply a maximum of 2,800 Watts for a few seconds:- 23 volts minium battery - 21 volt inverter cutoff = 2.0 maximum wiring drop
- 2,8000 Watts * 1/0.85 inverter eff * 1/21 volts = 157 amps @ maximum load

Use a wiring voltage drop calculator for the length wire run, maximum current, diameter of cable, to give you a maximum of 2.0 volt drop.When using a voltage drop calculator, make sure you use the method they recommend--Some calculators use the "one way wiring run" (assuming 2x actual electrical wire length), and others use 2x one way length.

I am not sure if your location uses mm diameter, mm

^{2}, or AWG (American/other wire guage).-Bill

295✭✭✭Will this be accurate: http://www.bulkwire.com/wireresistance.asp

I get (rounded) 10mm

^{2}, or AWG 7 required for 2m.33,545adminIf you can afford/fit heavier cable (and/or shorter distance), I would suggest. that. The numbers I gave for the calculation would give you the smallest wire that should work.

Add a fuse/circuit breakers, a few connections and such, I would suggest going on the conservative side. Go one or two size larger cable than the minimum diameter to give you a bit of head room (allow for aging, battery bank slowly losing capacity, high temperatures, etc.).

-Bil

33,545admin- 20 amps * 1/0.10 rate of charge = 200 AH battery bank nominal
- 20 amps * 1/0.05 rate of charge = 400 AH battery bank maximum

Also, there is the maximum rated inverter power (loads) output vs battery capacity to supply current... For a 24 volt, 1,400 Watt inverter, the minimum (amp*hour) battery bank sizing would be:- 1,400 Watts * 1/0.85 inverter eff * 1/24 volts nominal * 8 hour discharge rage = 549 AH @ 24 volt "nominal" for 1,400 Watt continuous load
- 1,400 Watts * 1/0.85 inverter eff * 1/24 volts * 5 hour discharge rate = 343 AH @ 24 volt maximum short term power (smaller battery bank)
- 2,800 Watts * 1/0.85 inverter eff * 1/24 volts * 2.5 hour discharge rate = 343 AH @ 24 volt battery bank for starting loads (i.e., well pump)

The above are recommended numbers for a flooded cell battery bank. AGM and GEL can support higher discharge rate.US GEL type batteries are usually rated to a 5% maximum rate of charge.

-Bill

295✭✭✭Using 4m total lenght, 98amps, it says AWG8, I am going for AWG6, and if I cannot, AWG7.

Why, the Brad Harrison / Anderson connector crimping tool is the key. :-)

33,545adminFor off grid folks, that would be a bit on the expensive side.

Also, UPS batteries are usually GEL or AGM type--Which do much better for surge loads (some are rated as high as C*4 discharge rate--I.e., can be taken flat in 15 minutes).

Of course, those UPS systems are usually designed to only run long enough to safely shut down the computer (outage at home) or to get the diesel genset up to speed and stable.

For most off grid folks, they want to run primarily from battery banks for ~10 to 20+ hours at a time (say 5-10 hours a day/evening for ~2 days straight) and have the batteries last 5-8 years (or more). And with limited number of charging hours per day (sun is up only so long)...

Pretty much a different operating environment vs UPS type setups.

-Bill

295✭✭✭80amps or less fuse needed for AWG 6 cable.

295✭✭✭I would have agreed with you Nov last year. Last real load shedding we had was back in 2008.

Then Dec and Jan 2015 came ... suddenly no batteries, no UPs'es and (wow) no solar inverters.

Now people want to run for min 3 hours ... and I believe we can prepare for longer.

So I thought, after losing a deep cycle batt that shorted and a friend of mine walked into a server room where a UPS was busy smoldering, it is time to be wiser.

Most UPS are cheapies imported from elsewhere and most are cheap 2000va upwards with minimum 2 x 100ah batteries, sometimes up to 8 batts.

So idea is to start sorting the cabling and protect the batteries.

Ps. Our grid:

It is reported that IF they start tomorrow morning it may take min 10 years to fix just the grid whilst building more power stations. By 2025 ish, most of the current stations will be end of life.

And the 2 new ones ... still not operational ... 4 years behind with NO end in sight.

33,545admin10 years means I have a job until I retire.

-Bill

295✭✭✭Battery AH: 100ah

System volts: 12v

Inverter efficiency: 0.85

Max Daily discharge: 0.50

= Nominal Daily Discharge: 510w

Am I correct in saying that a 100w load on the inverter, could be powerred for +-5 hours?

33,545adminNote the batteries have less apparent capacity the faster you discharge them.

We use a 20 hour discharge rate for our rules of thumbs.

5 hour discharge to 50% is a 10 hour equivalent discharge rate.

Your 100ah battery may look like a 90ah battery at the 10 hour rate.

-Bill