Help with breaker / fusing sizing

Re: Help with breaker / fusing sizing

stereoman405 wrote: »

Just need some help figuring out what size breakers to put in the following locations:

- Combiner box at PV array.

Yes, using the Series Fuse Rating of the solar panel, if more than two parallel strings, then one fuse/breaker per string.

- Just before charger controller.

fuse, breaker, switch, or nothing--your choice. Lots of people like a switch/breaker to turn off power easily for service of the controller. Normally not required unless you have a whole bunch of parallel solar panels and their Isc>Panel Input Current rating...

- Just leaving charge controller, heading into batts.

Actually, the "dangerous current source" is the battery bank. Any wires connected to the battery bank should have their fuses/breakers located close to the battery bank.

My other locations, such as inverters, I'll just be using the manufacturer recommended wire size and fusing.

For specifics I'll have (4) REC 210's in (2) series strings put into parallel at my combiner, then hooked up to a 60A Xantrex controller connected to a 12v bank.

If you only have one or two "parallel" panel strings--then you do not need a series protection fuse/breaker. If you plan on adding more parallel panels later, you can add a combiner box/breakers now.

- REC 210 Specs: 28.1v Vmpp & 7.5A Impp.

Check the specifications for the series fuse requirement--I would bet it will be 15 amps (or one standard size up).

My math led me to a 15A DC breaker for each 'series' at the combiner (56.2V @ 7.5A), and a 30A DC breaker for the 'IN' of the charge controller (56.2v @ 15A), and 60A for the 'OUT' of the charge controller. (I picked 60A because that is the biggest breaker offered through NAWS's store, and I figured the Xantrex can't output more than 60A anyways.)

Again, the 15a and 30a breakers are not required for your current installation plans. No problem if you want to add them now. Remember that the wiring needs to be sized to handle your maximum current (30 amps in this case).

Remember that wiring and circuit breakers when "rated" for 60 amps is the maximum safe limit where fusing/breakers/etc. will begin to trip. For actual 100% rated current, you need the NEC 1.25 factor... So, a 60 amp circuit should never be more than:

• 60 amps / 1.25 = 48 amps

If you plan on running your 60 amp MPPT controller at 60 amps--then you would need fusing/breakers/wiring rated for:

• 60 amps * 1.25 = 75 amps minimum (80 amps next standard rating)

Is there a general rule of thumb for sizing? I thought I've read that you just take the max current from the item and multiply by 1.5. That's basically what I did, then rounded up to get to the closest breaker that NAWS offers.

The NEC safety requirement is 1.25x the maximum current.

For solar installations they want 1.25x1.25=1.56 as their margin of safety for the solar panel Isc current (I think NEC is out of their minds for some of this--but it does not hurt to have heavy wires).... Also, the whole safety multiplication issue really depends on PWM or MPPT type charge controllers. PWM cannot limit heavy currents and can burn out. MPPT does control current/power (switch mode power supply)--and is more rugged in this respect.

If you want to double check my inverter plans, I have an 1800w and 1000w Xantrex mod sine wave inverter. I won't be using them simultaneously, and I won't have a heavy load on either when running. I plan on:

- 1000w Inverter - 2 AWG - 110A fuse
- 1800w Inverter - 4/0 AWG - 200A fuse

Am I in the ball park with all this?

Assuming 10.5 minimum operating voltage (low battery, voltage drop), 80% efficiency, and 1.25x wiring/breaker derating:

• 1,000 watts * 1/10.5 volts * 1/0.80 * 1.25 = 149 amp minimum
• 1,800 watts * 1/10.5 volts * 1/0.80 * 1.25 = 268 amp minimum

So--the actual maximum current depends on you loads--if you never plan on using more than 500 watts on your 1kW inverter--you could step back on the wire/breaker sizes.

Also, check the manuals--I am just using generic numbers to make the above calculations.

-Bill

Re: Help with breaker / fusing sizing

stereoman405 wrote: »

- Forgot to mention my AC to DC backup charger. If I'm using a Trucharge 20, I should put a 25A fuse between it and the batts?

Yep.

I'm curious why I essentially don't need any fusing pre-charge controller. Do the panels themselves already have protection built in, or is the current so low with a smaller setup like this that its just a non issue? Unless of course I'm misunderstanding here.

Solar panels are naturally current limited--basically, at 1,000 Watts per sq.meter they output Isc maximum... A bit more light (reflection from snow, etc.) can increase current proportionally. Hot panels, a bit more current--Hence the use for 1.25*1.25 derating on the solar panels.

PWM are like a switch--Closed, current flows. Switch Open, current stops. No ability to limit current to XX amps maximum.

With MPPT controllers--however, they are both current and power limited by the electronics. So, you can put (at least some) MPPT controllers directly on a 24 volt lead acid battery bank and the controller will not take any more power than it needs/exceeds its specifications.

Switched-mode power supply - Wikipedia,

Sort of like a battery charger or any electronic power supply on a 120 VAC main... They are naturally current/energy limited unless something fails--The reason for Fuses/Breakers to protect the home's wiring.

-Bill

PS: Read about Buck Mode power supply here... Many/most MPPT charge controllers are some version of this type.

Re: Help with breaker / fusing sizing

Or get the 3 Foot MC4 Extender Cable Male/Female


Version... Just cut in 1/2 and splice to your home run.

-Bill