24v system wiring diagram

thentro · July 2012

This is a followup to a voltage question I had a few weeks ago. The short version is this is a remote cabin with low power requirements, minus a few minutes of high current from a septic effluent pump a week with even higher starting current requirements from the motor. Thus the 24v system and large inverter.

Since people were so helpful before, I wanted to double check my plans before I buy and install things. I am particularly interested in what people have to say about grounding, since I know it is important. What gauge wire is appropriate for grounding a system like this? My inverter calls for # 8 AWG.

The two 5 amp breakers will be in a mini box, and the large 150 amp breaker is a marine breaker switch. Thanks in advance!

Attachment not found.

Cariboocoot · July 2012

Re: 24v system wiring diagram

Actually grounding is not essential to operation; it is a safety thing. But if it is used it's important to do it right because doing it wrong can mess up the operation or make the system unsafe. Bit of a trap there.

You're diagram is pretty much correct: ground lines from each device brought to one central grounding point.
It gets a bit tricky in the details, such as how far the panels are from everything else (if it's distant you may want/need to use two grounding points and tie/not tie them together). See how easy it is to get confused on this "simple" thing?

The other issue is the matter of the inverter's AC out ground and whether it is tied to the same ground point and bonded to the AC neutral. That will depend on the particular inverter involved.

BB. · July 2012

Re: 24v system wiring diagram

The 120 AH battery bank would need 5% to 13% charging current for proper recharging (10% is a "good" number).

120 AH * 0.05 rate of charge * 29 volts charging * 1/0.77 panel+controller derating = 226 watt solar array "minimum"
120 AH * 0.10 rate of charge * 29 volts charging * 1/0.77 panel+controller derating = 452 watt solar array "nominal"
120 AH * 0.13 rate of charge * 29 volts charging * 1/0.77 panel+controller derating = 588 watt solar array "max cost effective"

That is the "killer" with large battery banks and very short/high power loads... It requires a good sized power source to recharge the battery bank.

The 85 watt array is closer to being a float charger rather than a true source of power for your system. You will need a genset+AC battery charger for normal operation (my guess/suggestion anyway).

-Bill

thentro · July 2012

Re: 24v system wiring diagram

From what I understand of my inverter, a Samlex 1500w, I will be essentially grounding the body via a grounding bolt. The AC and DC connections are not bonded to the chassis. As far as the distance, the panel will be about 25ft away from the ground, and the rest will be more like 10 feet away. The system will be unattended for long periods of time so safety is important. But perhaps floating it would be more fool proof?

As far as the 85w panel, I probably should go up a few notches. I am relying on the fact that this will be summer use only in a northern climate so will be getting over 12hrs of sunlight hours pr day when not cloudy. It will also be used for a few weeks on, a few weeks off so even at .02 rate of charge it should get there eventually right? Or am I missing something.

Cariboocoot · July 2012

Re: 24v system wiring diagram

thentro wrote: »

From what I understand of my inverter, a Samlex 1500w, I will be essentially grounding the body via a grounding bolt. The AC and DC connections are not bonded to the chassis. As far as the distance, the panel will be about 25ft away from the ground, and the rest will be more like 10 feet away. The system will be unattended for long periods of time so safety is important. But perhaps floating it would be more fool proof?

Ground it. That will work out better. It's a sine wave inverter so there should be no problem with the neutral-ground bond on the AC side (check the manual carefully to see if this is already done within the inverter). 25 feet to the panels is no trouble at all.

As far as the 85w panel, I probably should go up a few notches. I am relying on the fact that this will be summer use only in a northern climate so will be getting over 12hrs of sunlight hours pr day when not cloudy. It will also be used for a few weeks on, a few weeks off so even at .02 rate of charge it should get there eventually right? Or am I missing something.

No, this will not work. 2% charge rate will barely keep ahead of self-discharge.
There was a time when many solar installs were like this: panels were expensive and batteries were cheap. So you ended up replacing the batteries very often. You really want more panel for that 120 Amp hours. In this application you could try for the 5% minimum. That's 6 Amps @ 24 Volts, factor in panel efficiency and you get 187 to 200 Watts minimum. Two 140 Watt Kyoceras for example would be an inexpensive solution.

BB. · July 2012

Re: 24v system wiring diagram

You really need to satisfy two different requirements with your solar panels... There is the standard:

YYY Watt*Hours of load * 1/0.85 inverter efficiency * 1/hours of sun per day = ZZZ minimum size of array

And you should only plan on a minimum of 66% to 75% of the "average power per day" to allow for bad weather and that loads slowly build over time.

Using PV Watts for Saint Cloud Minnesota (as an example), with the array tilted to 30 degrees from horizontal (favor summer, fixed array):

Month    Solar Radiation (kWh/m 2/day)
1      3.11     
2      4.39     
3      5.23     
4      5.31     
5      5.91     
6      6.12     
7      6.20     
8      5.76     
9      4.89     
10      3.92     
11      2.92     
12      2.24     
Year      4.67

Say you get 2x 140 watt Kyocera or similar panels and assume 5.31 hours minimum sun for "spring/summer" occupation:

280 watts of solar panels * 0.52 system derating * 5.31 hours of sun minimum = 773 Watt*Hours per day (of 120 VAC power)

The other is the rate of charge for the battery bank (5% to 13% * 20 Hour battery AH rating) * 1/0.77 for panel+controller derating for the battery bank.

Below 5%, self discharge and "insufficient" electro/chemical activity, plus a lot of days to bring battery back above 90% state of charge, all hurt overall battery life.

Above 13% rate of charge--batteries will recharge quickly and "waste" hours of sun--And if you were deep cycling and recharging for many hours (such as a forklift operation where you discharge the battery to ~20% state of charge and have to recharge over the next 8 hours), the batteries can overheat if not monitored/actively cooled... For solar systems, you can probably get away with ~25% maximum rate of charge and a remote battery temperature sensor for your charge controller(s) and still be OK.

-Bill

24v system wiring diagram

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