Another Electrical Question

[DaveBMusik]

I'm about to wire my solar panels and most people recommend a switch between the MPPT and battery.
I have two 100W panels (5.8 Amps), each feeding a Blue Sky 1524ix MPPT. Blue Sky recommends a 20/25 amp fuse between the battery.
I was hoping to combine the positives of the controllers to a single 25Amp surface mount breaker/switch. An alternative would be a 50 Amp breaker with a 20 Amp fuse between the breaker and each MPPT.
Any thoughts or other ideas from my electrical expert friends?

Thanks, Dave

[KWKloeber]

 Dave,

If you're speaking of installing overcurrent protection at the controller, on the  charge feed to the batteries, That would be absolutely useless as posted previously.  You can't protect anything and there's no value in placing placing a fuse or breaker there.

The  location for overcurrent protection is within 7 inches of the battery on the charger wire to them.

Ken

[DaveBMusik]

Dave,

If you're speaking of installing overcurrent protection at the controller, on the  charge feed to the batteries, That would be absolutely useless as posted previously.  You can't protect anything and there's no value in placing placing a fuse or breaker there.

The  location for overcurrent protection is within 7 inches of the battery on the charger wire to them.

Ken

This is from the manual:

[KWKloeber]

Dave


Yup,  The protection is at the battery (within 7 inches of the post) so that if anything happens to the charge cable  your battery doesn't cause an electrical fire. I.e., the fuse is actually on the battery, not on the controller.


The reason there's no fuse at the controller end of the cable is this:
The cable would be sized to at least handle the maximum output of the controller/panels. Therefore, if you put a fuse that was rated slightly above the maximum output of the controller, it would never do any good as it would never blow. If you put a fuse less than the maximum output of the controller it would blow, but serve no purpose because the charge cable is plenty heavy enough to handle the load.  It's  what's referred to as a current limited source, i.e. you can't put more current through the charge cable than what the panels will produce, and you don't want to fuse it at less than that amount because then theoretically it would blow at some point while charging.


Ken

[Stu Jackson]

From Electrical Systems 101:

Wire Sizing Charts from West Marine  http://www.westmarine.com/WestAdvisor/Marine-Wire-Size-And-Ampacity   www.bluesea.com also has good material on this and fusing, but it's a PDF download and doesn't link.  Here's the link to the page, scroll down for the PDF download of the Fuse & Wire Sizing Tables:  https://www.bluesea.com/support/reference

Think of the battery bank as a possible HUGE explosive device.  If it goes "wrong" and "blows" the power is going to expand outward form the SOURCE, which is, bingo, the battery itself.

That is the reason for the 7" Rule.

You fuse AT THE BATTERY BANK for the WIRE size.

You size the wire based on the load and the distance of the wire run.

You size equipment fuses at the equipment for the load of the equipment.

[DaveBMusik]

I understand the 7 rule.

I'll try to re-phrase my question. Can I combine the outputs of the two chargers into one cable running to a 25 Amp breaker/switch near the battery. I'm assuming this would satisfy Blue Sky's guidance but what about current going the other way or doesn't a breaker work in both directions (sorry for my ignorance) ? Each panel is rated for 5.81 Amps but I believe the chargers boost the output current.  http://www.blueskyenergyinc.com/products/details/solar_boost_1524ix

[J_Sail]

Yes, you can parallel the two controllers feeding a single 20 or 25 amp fuse near the battery.

The actual point where you tie the two controllers to a single wire can be chosen for your routing convenience. Just make sure that at all points you use heavy enough wire to both keep within the safe ampacity of the wire (easy) and to also keep the voltage drop on both the positive and negative wires to a total of well below 3%. The lower the better, both to minimize losses in the wire and so that the controller's sensing of its output voltage accurately reflects the true voltage at the battery. Even relatively small voltage drops across the wiring from the controller to the battery result in less than optimum charging profiles. Don't go nuts over it, but increasing one size from whatever a wiring chart recommends for your length and current can sometimes be a worthwhile investment.

Also, my experience is that wiring runs are always longer than they look. So either measure or over-estimate.

If the wire run is 15 feet one-way, then using #10 wire (@1.0 ohms / 1k ft) for each panel and tying together near the battery, the total drop at 7 amps (peak output on a super sunny day) would be 0.105v on the positive and 0.105v on the negative, for a total drop of 0.21v, which would be great. If you tie the controllers together and use #10 for the two together then the drop would double to 0.42v (14 amps passing through the wire's resistance). That's slightly over 3% of 13.5v (a typical voltage while charging).

If you use a #8 (.6282 ohms/1k ft) for the two combined, though, your drop is only 0.26v (2%). 

BUT if your one-way distance is much more than 15 feet (which it could well be), you need to adjust the calculation.

Disclosure - I just did this calc quickly without much checking, so others on the forum should feel free to double check my work. You can also use an online calc, such as:
http://www.calculator.net/voltage-drop-calculator.html
Also I don't own a C34 so don't have a good gut sense for the wire length involved.

I used 7 amps as the max output from each controller based on a theoretical peak of 100w into a 14v load (battery during charging at twice that current due to two panels). The reality is that it is likely to be more like 5 amps (or less) per controller in real-life conditions. So if the wire gauge becomes a cost or other challenge, you can redo the calcs at the lower current.