# Circuit breaker, conductor sizing &amp; derating

Solar Expert Posts: 113 ✭✭✭✭
I think I am on the last leap forward in my system designing.
I wouldn't mind a second opinion.
So far I have decided on grid tie/backup.
XW 6048
Trojan 370Ahr @ 48 VDC
2-XW charge controllers.
Unirack Sunframe roofmount rails.
Midnite combiners with breakers.
4800 watts solar(2 arrays of 2400 watts split into 4 strings of 3)
-Suntech STB200-18ud-1 (MC4 connectors on 11AWG = 20 amp max.)
Voc 33.4 volt Vmp 26.2 Volt
Isc 8.12 Amp Imp 7.63 amp

PV watts gives me 43 A STC max. on 4 stings of 3 in series & 122 VDC max @ -40*C

String to Combiner
Each string would supply Isc. X 125% = 10.15 Amps. Next breaker size would be a 15 amp(contained in the combiner box)
Voc X 3 = 100.2 Volts X 125% = 125.25 VOLTS max. So breakers rated for 150 VDC.

Combiner box to Charge controller
Here is where I'm a little unsure. 4 parrallell strings = an Isc of 8.16A X 4 = 32.64 X 125%=40.8 amps. Since my breakers contained in my combiner are 15A X4=60 amps, which number do I use.
Reguardless 6Awg from combiner to charge controller is rated for 60 Amps and protected @ charge controller with a 60 A breaker. Is this acceptable ?
Do I have to derate my 6AWG to 80%(48 amp) rating if I run conduit through the attic above the insulation? Is this acceptable given I have 60 A input from the PV string combiner based on 4-15A breakers.
If a PV array is seen as a continuous load, I assume all Breakers should be derated to 80% anyway, in which case all breakers and wire size should be within limits.

Charge controller to battery bank
I have no issue here. 1 short run of 6 AWG gives me a volt drop of 1.9% MAX @ 44 volts. I used 15 feet in the calculations. I am sure it will be less than 5 upon install.

VOLTAGE DROP CALCULATIONS.
String to combiner: 15' of 12 AWG @8.16 A & 122 VDC=.4% drop
Combiner to charge controller: 40' of 6AWG @ 33A & 122VDC=1.1% drop
Charge controller to battery : 15' of 6AWG @ 60 A & 44VDC=1.9% drop

Sorry for the choppy post and thanks for your insite
My second guessing myself is to be on the safe side. NOT HAVE TO DO IT TWICE.

Thank-you again
Ken

• Solar Expert Posts: 66 ✭✭✭✭
Re: Circuit breaker, conductor sizing & derating

Assuming you looking at getting two MNPV6 combiners ...
Then, each combiner connects 4 parallel strings of 3 panels to one of the two XW SCC units. Every string is connected to a MNEDC15 (15A) breaker in one of the combiners. Next, from each combiner, the POS, NEG, GROUND bus bars are then connected to the charge controller using your #6 wire. In this config, you would have (as you stated) an Isc of about [8.16A X 4 = 32.64 X 125%=40.8 amps] per Combiner. You would need one run of #6 (POS,NEG,GROUND) from each MNPV to each XW SCC.

Total Wire Needed: (This does not include PV wire from Panels to Combiner)
2 x PV rack to combiner (GROUND): 15' x 2 = 30'
6 x Combiner to charge controller: 40' x 6 (2 POS, 2 NEG, 2 GROUND) = 240'
6 x Charge controller to XW bus bars: 15' x 6 (2 POS, 2 NEG, 2 GROUND) = 90'

360' approx. total wire needed for your runs
It would likely be cheaper to get a 500' spool of #6 black for about 2 bills and use red, green, and white electrical tape on the ends to mark them.

You will need a good bit of the remaining #6 to make multiple connections from your XW to your sub-panel (60A), main panel(60A), and external AC disconnect(60A).

You will also need ground wire across each array and from the combiner to a ground rod.

I don't see an XW PDP/Midnite PDP or XW SCP listed.
Are you planning on obtaining them?

The battery bank may be a bit light for your setup.
You may want to consider 400-600ah @ 20hr worth of batteries.

Have you looked into adding Delta AC and DC lightning arrestors to the PV combiner, and XW unit?

David
• Solar Expert Posts: 1,962 ✭✭✭
Re: Circuit breaker, conductor sizing & derating

You need more battery capacity

rule of thumb for the XW is 100ah per 1kw of sell

So for your 4800w pv, for good performance 480ah ( or more ) battery capacity would be advisable
• Solar Expert Posts: 113 ✭✭✭✭
Re: Circuit breaker, conductor sizing & derating

Yes I am planning on:
2 midnite 6 position combiners
1 XW PDP
2 XW SCC
1 XW SCP
1 XW wireless communication device.
1 XW 6048

I am quite comfortable with wiring in general. Wanted to make sure my math was OK with the breaker vs wire sizing.

I am trying to stick with Trojan because of close proximity to dealer and tech support. I will have to check into 2 paralel banks @ 370 ahr each.
• Solar Expert Posts: 8,605 ✭✭✭✭✭
Re: Circuit breaker, conductor sizing & derating
topper wrote: »
Yes I am planning on:

1 XW wireless communication device.

If that's also known as the "gateway" you may want to rethink it's expense, unless you know you will be using it for firmware flashing. See if your installer has one you can borrow as needed.
Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,

• Solar Expert Posts: 66 ✭✭✭✭
Re: Circuit breaker, conductor sizing & derating

"Do I have to derate my 6AWG to 80%(48 amp) rating if I run conduit through the attic above the insulation? Is this acceptable given I have 60 A input from the PV string combiner based on 4-15A breakers."

I do not know if you need can use the Isc @ 40.8A or if the sum of the 4x15a breakers 60A max rating has to be used. (Out of my area)

David
• Solar Expert Posts: 414 ✭✭✭
Re: Circuit breaker, conductor sizing & derating

Here is a site for some Wire calculators

http://www.electrician2.com/calculators/wireocpd_ver_1.html

Do you really get to -40°C. Wire in conduit has to be derated and use wire suitable for wet location. THWN 90°C type.
Looks like with 43 A (-40°C STC) will need up to depending on what is used for attic temperature (61-70°C) #4 AWG in 3/4 inch EMT to (71-80°C)#2 AWG in 1 inch EMT. Your OCPD of 60A is good for either

I am sure someone will chime in with more working knowledge of NEC Tables on this. Things to consider, Attic temp, in conduit and calculated max continous load @ -40°C (43A x 125%), then if over 3 conductors need to go to wire fill derating tables, also on that site.
GT3.8 w/4600W Trina 230W, TX5000 w/5000W ET-250W, XW4024 w/1500W ET-250W, 4 L16, 5500W Gen. (never had to use) Yet!!
• Solar Expert Posts: 113 ✭✭✭✭
Re: Circuit breaker, conductor sizing & derating

Yes -40*C. Coldest day I can recall was in 1994 or 95. -45*C plus windchill. CCCOOOLLLDDD!!!. Most of the time one can depend on -30 to -35*C being the average low +/- 2wks per season.
I have been recalculating the derate factor for combiners to the Charge controllers. The simplest way I can see is to avoid running inside the attic space. I could run 2 conduits -2 conductor with ground of without any derate factor..#6
ground and conductors
It doesn't make any sense to me running a #2 or #4 from the combiners to a splice block for #6 to connect to the charge controllers.(charge controllers only accept UPTO #6.)

Brainstorm with me. think out loud. We'll get it hammered out.
• Solar Expert Posts: 414 ✭✭✭
Re: Circuit breaker, conductor sizing & derating

Rechecked using a 56-60°C attic temperature which is probably more realistic for your location you should not really have problems with #6 AWG 3 condutors (2 plus Gnd) in conduit thru your attic, with a 60A OCPD. Min EMT size would be 3/4". THWN 90°C wire #6 AWG has a derated amp capacity of 53.25 amps in conduit and temp factor's
GT3.8 w/4600W Trina 230W, TX5000 w/5000W ET-250W, XW4024 w/1500W ET-250W, 4 L16, 5500W Gen. (never had to use) Yet!!
• Solar Expert Posts: 113 ✭✭✭✭
Re: Circuit breaker, conductor sizing & derating
See if your installer has one you can borrow as needed.

I will be the installer which is why all the Q & A.

I reran the numbers again and I agree Mr Skidoo.
Isc per string = 8.16 x 125%=10.2 A
15A breaker derated to 80% for constant load=12 A
Isc per array = 10.2 X 4=40.75A
6AWG wire and 60A CC breaker both derate to 80% for constant load=48A
I should be good. Now I have to put it all together and send it in for approval with the utility.

DUMB QUESTION OF THE DAY.
With temp decrease, PV voltage rises and current falls. Does this mean that current increases with temperature as well?
Do PV modules follow Ohm's law? I would assume the relationship is non-linear.
Re: Circuit breaker, conductor sizing & derating

Yes, current does rise as temperature increases... But the effect is very small and is pretty much ignored. The voltage effect is much larger.

Solar panels are really closer to a "Current Source" model than a voltage source model (aka a Battery).

There is not much change in current from Vmp to zero volts. Remembering that:
• Power = Voltage * Current
That is why it is so difficult to hook up loads directly to solar panels and get a "useful" amount of power from them.

With small loads, the panels simply are between Vmp and Voc -- With little power output (because of less than Imp current output).

But, if you actually place a load on the solar panels that is near Imp, and the panel is following with Vmp output--As soon as there is any increase in current draw or decrease in sunlight on the panel, the current output capabilities of the panel are exceeded, and the voltage output collapses.

Many loads are really closer to the constant power model... I.e., they draw a constant power to operate correctly (motor in a refrigerator, florescent lamp, computer, or a off-grid Inverter)... Again, looking at the constant power model where Power=Voltage*Current--If the voltage drops a bit, the device wants to draw more current to maintain the constant power needed for operation. Which when connected to a solar panel, causes the voltage to fall more, forcing more current draw, causing the voltage to collapse.

Batteries work very well at supplying loads because the voltage is (more or less) constant and it can support variable current output without problem (up to the capacity of the battery.

So, you end up with a few loads that work well with solar panels. One is the battery--It is a fixed voltage (very near to Vmp) and accepts any and all current that the panel is capable of outputting--Very near Pmax (because Vmp does not change much with current and over reasonable temperature ranges Vmp only varies by around 20%) and the miss-match from "ideal" is not much.

Or the MPPT type charge controller / GT inverter... Which adjusts it current input levels to keep Vmp close to an optimum value (based on current, available sunlight, and panel voltage). That is why a MPPT type controller can be 10-30% more efficient than simply attaching a solar panel to a battery bank directly.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 11
Re: Circuit breaker, conductor sizing & derating

I am having a flashback from last week....my brother in law is having me go to WI to install a system for him. He wanted to do battery backup in an area that may get one hour a year of interrupted grid electricity. He is a "gadget" type of guy. After explaining the grid tied battery backup is less efficient than a typical grid tie, and batteries are still very expensive, and go bad...especially the first set someone ever has that someone that has grid tie already is in most cases crazy to do a battery backup. unless you have frequent grid failures, get yourself a generator with a grid sensor to turn the generator on when the grid may go down....much cheaper typically, more reliable, and more efficient.

IS there a reason other that it is cool to mess with a complicated system you are looking at battery back-up? I hate to say it, but the old saying K.I.S.S typically holds true...but I always try to tell people my opinion and \$2 will buy you a cup of coffee at any gas station.
Take care,
Jeff