Cable size and fuses

Re: Cable size and fuses

Do you have an NEC (National Electric Code book)? If not, it is probably worth getting one or photo copying a few of the tables out of it as there are several issues when deciding minimum wiring sizes and types...

First, if you use the Voltage Calculator in the sticky mentioned, you can determine the "optimal" size of copper wiring (AWG) you would want for your installation... Also, you need to read the manual(s) for any equipment you are connecting

For example:

6x Kyocera 130w panels. (Kyocera KC130TM???)
Max Power Voltage - Vmp 17.6 Vdc
Open Circuit Voltage - Voc 21.9 Vdc
Voltage Temp Coeff - Vtoc -0.0821 V/°C
STC Rating - Pmp 130 Wstcdc
Max Power Current - Imp 7.39 Adc
PTC Rating 115.2 Wptc

Assuming 14F-95F operational range:
Voc=24.75 volts (at 14F)
Vmp=13.9055 (at 95F)

24 volt battery bank (assume about 31 volts max required for equalization)

Assume Current Model MX 60:

140 volts maximum input (running)
150 volts never to exceed
2 volts or more over max battery bank requirement (33 volts minimum)

Need at least 3 Kyocera panels in series (41.7 volts minimum)
Max of 6 Kyocera panels in series (148.75 volts maximum--any more and will fry MX 60))

Given the above specs, you should probably pick 3 series x 2 strings in parallel--but I will also show 6 x 1 string for reasons below (if you are in a warm climate, 6x1 may be OK).

Now, need to figure out wire gauge. 45 feet x 2 (+/- wire runs), 7.39 amps per string:

3x2 panels => 2x7.39amps=14.78 amps -- from default settings on spread sheet, 90 feet and 41.7 volts for panels. 8 awg wire gives a 2.2% (about 1 volt) drop.

For an example, now lets try the 6x1 panel setup (very close to being too high of voltage for your MX 60 controller--will depend on your local weather conditions... 7.39 amps, 83.4 volts: Now you can run 12 AWG wire and get 2.8% drop (about 2.4 volts)--by using much less copper wire.

Now, you have to go to the NEC and find the correct wire (buried, conduit, conduit fill ratio, insulation type/rating, UV exposure (or not), etc.). I don't have an NEC book--so I can't help you here. (remember, that you need to multiply the current by 1.25 as a safety margin when looking for the appropriate size/rated wire for NEC).

Also, you will have to look up the documentation for your Kyocera Panels and find out the series fuse rating--Normally the fuse is not required for a single or 2 parallel strings. But for more strings in parallel, you will have to protect your panels (in case they short circuit) with a fuse/breaker in your combiner box.

Generally, the MX 60 manual should have a good explaination of how to connect all of this together--plus the www.outbackpower.com website has example wiring diagrams for many different situations.

I am sure that I probably missed something and somebody here will correct me--but this is basically what you have to do for every leg of your system.

And remember the fuses/breakers are to protect the wiring--not the devices themselves (but read the manuals to make sure). You have to size the wiring/fuses/breakers for 1.25 the maximum expected load (usually at low battery voltage).

However, you can design it for your needs... For example you may have a 3kW inverter but you will only have to power 1.5 kW of load--so you don't have to wire for 3kW (breaker/wire) if you are not using that load (but remember to divide by the inverter efficiency, multiple by 1.25 NEC factor, and use I=P/V where V=battery bank low voltage--~21 volts in your setup--to calculate maximum current--and watch for starting currents on well pumps and such).

I have to go right now--hope this helps.

-Bill

Re: Cable size and fuses

Glad to be of help...

To answer a few of your questions... Just to be clear, the 2x36 is 2 parallel sets of 3 series modules is the minimum voltage at which the MX 60 can recharge your battery system (24 VDC)... Basically, the voltage of the solar panels has to be 2 volts (or more) higher than the battery bank's maximum voltage (generally during equalization--or "overcharging" to make sure all cells are fully charged--required for flooded cell type batteries--and you have to check the water every month or so as equalization converts water into hydrogen (and oxygen) gas--so it is normal for the batteries to "lose" a bit of water over time).

You can use either 3 panels in series, 4, or 5 and meet all of the MX 60/24 VDC banks requirements. However since you have only 6 panels at the present time, you need to divide them into "even" numbers of panels per string... In this case, 3xseries in 2xparallel strings. If you had 8,9, or 10 (etc.)--you would have to choose other configurations.

Whether 6 panels in series would work or not--it is close. You are on a tropical island, and if you had a light frost and bright early morning sun, you 6x1 panel string could reach ~144 volts DC. The MX 60 won't start over 140 VDC--but where you are building your home it will probably work (assuming never under 32F with the sun up).

Jim brings up the issue that MPPT controllers (Maximum Power Point Tracking charge controllers, like the MX 60) are slightly less efficient the higher the input voltage from the solar cells (you can ignore the rest if the details are not of interest--not real important)...

It is true, but I like to do a second check... Assume that we lose 1.5%
power at 6x1--that is (6x130watts * 0.015) 11 watts or, at $5 a watt for solar panels, a $55 dollar loss in solar output.

But, we need to compare the current bettween a 3x2 set of panels and a 6x1 set. And if you look at my analysis you will see that for your setup that difference is the requirement for 8 AWG wire or 12 AWG wire for the main solar panel run back to the MX 60 controller.

Using Wind-Sun's wire pricing (because it is handy), 2 conductor 8 AWG is $1.60 per foot and 2 conductor 12 AWG is $0.64 per foot. The cost savings is 45'*$0.96=$43.20 for your wiring--pretty much a wash (loss in panel "value" vs loss power in MX 60 controller).

Obviously, the longer the wire runs, the larger the price cost differences will be between a "high voltage" vs a "low voltage" run (you may also need larger conduit and other issues).

Of course, it is always best to save energy everywhere you can in designing/building a solar panel system--once you lose the energy in wasted heat (inefficiencies), that will be lost every day for the rest of the system's life.

Back to question of Breaker vs Fuses--In general, the weak point of any electrical system are the fuses and breakers. Good breakers are expensive, but nice to have if you need to break the circuit to work on something. Fuses (and good fuse holders too) are nice, small, and cheap, for example if you need series protection fuses out at the combiner boxes (fuses will probably never "blow" and you don't have to pull the fuse often to service an individual solar panel.

Sizing of the fuses and breakers? Basically, they are there to protect the wiring. Remember, you have sized your (for example) 15 amp load/source * 1.25 = 18.75 amps... You round up to the next standard circuit size (20 amps in this case). You want to have larger breakers and wire than the actual load for safety and to prevent un-needed trips (breakers/fuses).

For solar, in many cases, the wiring is designed to handle more load because we want low voltage drop (higher efficiency). So, for your 15 amp load, you are using 8 awg wire--which works for well over 15/20 amps--but you would use a 20 amp breaker just because you don't need the extra current (and may help protect the attached devices--MX 60, Panels, etc.).

There is also one other issue with Breakers and Fuses, make sure that they are rated for protecting DC circuits at you rated voltage. DC is much harder for a breaker or fuse to stop in the event of a short circuit (AC is much easier--does not sustain an ARC across contacts like an equivalent DC voltage can).

If you have a good electrician there--they can probably help you with some of the planning of the system wiring and installation.

Hope that was clear.

-Bill

Re: Cable size and fuses

The above by Jim is why I hate code checks and NEC. It was bad enough to do it once when designing a new computer system--but to do it for every solar system install--ugh...

You are a better man than I, Jim.

-Bill

Re: Cable size and fuses

I would question the need for derating of the 75C breaker wire/terminals in a normal solar installation...

I can see 90C rated wires/environment for exposed wires/wires run in conduit/box on a hot roof or exposed wall...

However, if this is for a breaker installed next to a MX 60 inside a building or shaded lean-to, then it would seem that the whole exercise for terminal rating would be 100% at 75C (doubt that the MX-60 is rated for a 90C environment).

How does the NEC address the difference between a 90C and a 75C environment? (don't have a need for an NEC book anymore--trying to avoid buying one for typing on this forum).

Cascading requirements (like Jim's post) was another reason I avoided using breakers/fuses/terminals/gas discharge tubes/etc. all over my design. Made my regulatory engineers/UL/CSA/etc. prove that a protective device was needed (other than as just a handy power off switch for the system).

-Bill

Re: Cable size and fuses

crewzer wrote: »

Bill,
...
Regards,
Jim / crewzer

P.S. I have a spare copy of the 2005 NEC… hint, hint, nudge, nudge…

Jim,

That is very kind and generous of you... Hmmm--Now I have to think if I want to interpret the NEC as a hobby when I "hated" being paid to do that at work (hate is probably too strong of word--but balancing multiple requirements and their impact on the design sure was probably the equivalent of a good chess game--at least this NEC/NRTL/IEC/etc. requirements had a real/physical outcome--something that is missing, for me, in board games).

One of the issues that I had to always go back to was trying to determine, for this example, if an outdoor box was rated at, say 40C ambient and it included the safety factors for temperature rise due to internal power and full sun exposure--or if I had to factor those other influences into the design requirements.

Frequently, I was going back and asking the question if I was getting multiple levels of "safety" when only one level was really required. For example (don't remember if this exact issue was a problem--but I had issues like this): Using 90C wire in my system--I would have my safety engineers derating using those NEC tables, even though I could have used 75C/60C (or whatever lower time wire) rated wire and they would not have pushed the derating (my system was designed for 50C ambient). They where pushing the deratings just because I was using a higher temperature wire (because I could get the gauge/color/flame rating) as a stock item instead of special order the lower temp rated insulation.

Jim, let me think about it. Again, thank you very much for the offer.

Sincerely,
-Bill

Re: Cable size and fuses

Must be DC rated for safety.... Problem is that DC current sustains arcs (sparking across the switch contacts) much better than AC currents (which turn off and switch directions at 120x per second). DC approved breakers are approved at much lower voltages than AC or, for the same voltage ratings, much larger contacts with much more distance between contacts when off.

(large/professional arc welders usually have a DC setting because of DC's several advantages when arc welding).

And not all breakers/switches are rated for DC applications. Using the wrong breaker can result in welded contacts, melted breakers, and failing to trip open when protecting against shorted outputs.

It will be listed on the breaker or with the data sheet the AC and DC ratings for each breaker.

Purchasing breakers and panels from companies (like Wind-Sun) that specialize in Solar/Battery systems (especially if this is your first time) should be useful.

There are a some other manufacturers you can look at (Xantrex, Outback, Midnight Solar) to see what they offer.

Other people here can give you some more definitive links for specific solutions that they use or manufacture.

-Bill