The 1st DC breakers encountered inside the PSDC

Discussion about the Power System DC Disconnect (PSDC)

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DennisM
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The 1st DC breakers encountered inside the PSDC

Post by DennisM » Fri Jan 04, 2008 4:17 pm

In bringing in the current from my combiner box, I will have 2 pos leads each carrying a maximum of Isc 32A and/or Ipm 28A. Is it standard to use the OBB-175-125VDC-PNL breaker at this point? Since the amps are low enough, I was thinking of using the OBB-80-150VDC-PNL. Any problems?

Thanks, Dennis

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Post by crewzer » Fri Jan 04, 2008 6:13 pm

Dennis,

Assuming the DC breakers are rated for 100% continuous duty, I believe you'll need two breakers, one for each positive cable, and each rated for 32 A Isc x 125% = 40 A.

HTH,
Jim / crewzer
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Post by DennisM » Fri Jan 04, 2008 9:22 pm

Jim,

I do intend to use two breakers on the way in and had already bumped the Isc the additional 25%. At this point I'm looking at the 18 of the Sharp 216w in series of 3 combined at the PSPV through 6 each, 15-125vdc breakers, into 2 runs down to the PSDC. The 216's seem to be available where as some of the lower wattages 208's are in short supply. Availiblity at purchase time will determine the ultimate size, may as well size for the larger.

On the breakers though, I was wondering if it was standard practice to install the 175-125vdc, or possibly a NEC requirement?

Dennis

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Post by crewzer » Sat Jan 05, 2008 5:16 am

Dennis,

NEC 690.7 (A) requires that the circuit breaker voltage spec be rated to meet the PV source circuit Voc spec multiplied by the NEC Table 690-7 temperature correction factor for your location. For Sparks, NV, the TCF is 125%.

If you use series strings of three Sharp ND-216U2 modules (STC Voc = 36.3 V), the temp corrected voltage spec will be 36.3 V x 3 x 125% = 136.25 V. YouÔÇÖll need to use circuit breakers rated for 150 VDC or better for this application.

The OutBack OBPV breakers are rated for 150 VDC when used with the PSPV combiner, so that shouldnÔÇÖt be an issue. PV CBÔÇÖs in the DC panel should also be rated for 150 VDC. A 40 A model for each sub-array would appear to be indicated.

With an STC Isc spec of 8.35 A, a 100% continuous duty breaker specÔÇÖd at 10.44 A is nominally required for each PV string. The next standard size up available from OutBack is 15 A. However, a 12 A model is available from Midnite Solar.

Additionally, wire specs, temperature deratings, fill factors, and final wire sizes may influence the final CB spec. John Wiles has published a handy guide for sizing PV circuit wire and breakers. IÔÇÖm not in complete agreement with his process, but it appears to be widely accepted. See: http://www.nmsu.edu/~tdi/pdf-resources/ ... IX%20I.pdf

WilesÔÇÖ complete document (3.5 MB) is available here: Photovoltaic Power Systems and the National Electrical Code: Suggested Practices

HTH,
Jim / crewzer
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Post by mvrck » Sat Jan 05, 2008 9:26 am

On the breakers though, I was wondering if it was standard practice to install the 175-125vdc, or possibly a NEC requirement?
That big 175A breaker is normally used for the inverter (48V is the typical voltage when using the 175A breaker).

The smaller DC breaker slots are for the other items in the PSDC, like charge controller input and output. Based on Crewzer's calcs, you would use at least two OBB-40-150VDC-PNL.

What voltage inverter will you use?
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Post by DennisM » Sat Jan 05, 2008 9:46 am

Jim,
Again thanks for your re-direction in the details. In the series run of 3ea 216's to the PSPV, each inbound lead would have an STC of 36.3v x 3 x 125% = ~136v and the amps remain at 8.35. Correct? Wouldn't a breaker at this point rated at a safe amperage, say an OBB-15-150VDC be ok at this point in the PSPV?

My next in line and down-stream elec connection would be to join 3 of the above breakers with a FW-CBUS and then make a wire run to breaker #1 in the FW1000-DC rated @ 50-150VDC-DIN ( the 11/07 Outback catalog pg 27 bottom set OBB-50-150VDC-DIN). My math at this point would be, on the Isc side, 8.35 x 3 = ~25A. Though I think I could go with a 30A, but I'll choose the larger 50A. This arrangment would be duplicated in lead #2 into the FW1000-DC. This would complete the wiring into the FW1000-DC, less the ground wire, of course.

Thanks for the links, I'll be sure to down load & read them.

Dennis

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Post by DennisM » Sat Jan 05, 2008 9:51 am

Maverick,
The system is to be a VFX3648 x two.

Dennis

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Post by tallgirl » Sat Jan 05, 2008 2:34 pm

As an aside, you can combine the strings earlier and then use a single larger feeder from the array back to the charger controller. In that case you'd have to fuse each string prior to connecting it to the positive bus bar in the combiner box. Your negatives needn't be fused. From the positive and negative bus bars you'd then bring two conductors (plus ground) from the array to a DC disconnect prior to entering the cabinet where you have your equipment located. That disco allows you to make the DC inputs from the PV array dead so those conductors can be serviced.

The fuses you'd use would have to have the correct voltage and current rating (and I didn't check your Isc or Voc values, so I can't say anything) for each of the strings and their associated conductors. Since the fuse blocks, bus bars, and a 12x12 access box are probably cheaper than a mess of copper and associated DC breakers, this might be a way to save you money and tidy up your installation.
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Post by crewzer » Sun Jan 06, 2008 5:55 am

Dennis,
In the series run of 3ea 216's to the PSPV, each inbound lead would have an STC of 36.3v x 3 x 125% = ~136v and the amps remain at 8.35. Correct?
No. The NEC voltage calculation is correct, but the not the current calculation. The NEC "maximum circuit current" calculation is 8.35 A Isc x 125% = 10.44 A.
Wouldn't a breaker at this point rated at a safe amperage, say an OBB-15-150VDC be ok at this point in the PSPV?
Maybe. NEC 690.8(A) would define the source circuit current as 8.35 A Isc x 125% = 10.44 A. For a DC circuit breaker rated for 100% continuous duty [690.8(B)(1)(Exception)], the next size ÔÇ£standardÔÇØ size breaker is 11 A. The next readily available standard size breaker that will work in the PSPV is Midnite SolarÔÇÖs 12 A model.

Your PSPVÔÇÖs location may expose the circuit breakers to temperatures above 40 C (104 F), The CBI breakers used by OutBack and MidNite Solar are rated for use in environments of use to 60 C, although their behavioral characteristics still vary a bit with temperature. However, SharpÔÇÖs series fuse rating for the 216 is 15 A, and Wiles makes the following comment:
In PV source circuits, the value should be less than or equal to the value of the maximum series protective fuse marked on the back of the module. If desired (for unforeseeable reasons), this selected value could be increased to the size of the maximum protective fuse found on the back of the module. However, this will impact conductor sizing and other overcurrent device requirements.

In addition to voltage and current, youÔÇÖll also need to consider fill-factors and temperature derating. With few exceptions, I believe that youÔÇÖll need to end up with a breaker with a condition-of-use ampacity rating less than that of the conductor itÔÇÖs to protect. So, if you decide to go with a 15 A breaker, youÔÇÖll need to make sure that the fill-factor- and temperature-derated ampacity of the conductors is higher than the rating of the circuit breaker.
My next in line and down-stream elec connection would be to join 3 of the above breakers with a FW-CBUS and then make a wire run to breaker #1 in the FW1000-DC rated @ 50-150VDC-DIN ( the 11/07 Outback catalog pg 27 bottom set OBB-50-150VDC-DIN). My math at this point would be, on the Isc side, 8.35 x 3 = ~25A. Though I think I could go with a 30A, but I'll choose the larger 50A. This arrangment would be duplicated in lead #2 into the FW1000-DC. This would complete the wiring into the FW1000-DC, less the ground wire, of course.
I think the math should be 8.35 A Isc x 3 x 135% = 31.3 A. The next standard size up is 40 A. ItÔÇÖs my view that the temperature-derated ampacity of the connected conductors would have to be 40 A or better.

I donÔÇÖt understand why you would want to use a 50 A breaker if a 40 A model would be appropriate. Additionally, itÔÇÖs not clear to that the OBB-50-150VDC-DIN breaker is actually rated for 150 VDC applications if itÔÇÖs not used in the PSPV. You might need get that clarified by OutBack. Accordingly, the OBB-40-150VDC120VAC-PNL breaker may be the appropriate breaker for mounting in the DC side of the FW-1000.

HTH,
Jim / crewzer
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Why not combine all six strings in combiner?

Post by Fred@Simlicitysolar.com » Sun Jan 06, 2008 7:21 am

Am curious what is gained by splitting the array into two subarrays and running two sets of conductors to DC disconnects. Are you using two MX60s? Is distance an issue, and you are not wanting to use huge wire? What is gained by this configuration rather than as July pointed out above? At 8.5 A x6 strings, one MX60 would appear to be sufficient.

Trying to understand this array plan.

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Post by sparky » Sun Jan 06, 2008 7:50 am

Fred,

The one thing you gain by using 2 separate feeds over distance is smaller gage wire. Another gain is that it is far easier to work with. In your location imagine trying to get the wire into conduit this time of year! It is pretty cold at my place now so anything that helps make the job faster and does not sacrifice quality is always a plus.

I have not been following this thread but I think the MX voltage max is too close and two MX-60's are required.

Good question!

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Post by mvrck » Sun Jan 06, 2008 8:34 am

I agree. Two runs might be more economical, especially if it works out you can buy a whole spool of wire to do the job. For instance if the PSPV is 125 feet away from the PSDC, then 5 wires of 4ga is cheaper than 3 wires of 2ga.

And remember the power loss formula is I┬▓*r. So reducing the current has a better effect than reducing the resistance (even though the resistance is going up with the smaller wire).

Ok... Too much math this early!
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Post by crewzer » Sun Jan 06, 2008 8:38 am

Gents,

Part of the issue is that Dennis is planning on using 18 each 216 W STC modules to build an array rated at 3,888 W STC. This value is greater than a single MX60ÔÇÖs official power handling capacity (3,000 W or 3,200 W, depending on the document), and it may well be beyond its practical limit.

8.35 A Isc / string x 6 strings is 50.1 A Isc. If one accepts that the MX60ÔÇÖs input current limit is 48 A Isc (allows for the 690.8 mulitplier of 125%), then an array rated at 50.1 A Isc is too large. On a related note, the MX60 manual (Rev 7.0, page 9) contains the following comment:
The largest PV array that can connect to an MX60 should have a rated short-circuit current of 48 amps STC (Standard Test Conditions)
Dennis and I have been discussing his array Voc off-line for quite a while. I think it looks OK. The Sharp 216ÔÇÖs STC Voc is 36.3 V, and the record low temp in his area are down to -19 F. The NEC 690-7 temperature correction factor for this environment is 125%, so that puts the temp-corrected array Voc at 136.125 V, which is below the MX60ÔÇÖs operational cut-off of ~141 V.

The 216ÔÇÖs cut-sheet doesnÔÇÖt include a temperature coefficient specification, so IÔÇÖm not able to run the specific calculation.

HTH,
Jim / crewzer
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Post by mvrck » Sun Jan 06, 2008 8:48 am

It is shame the PVs will only put out that value one or two days a year. Meanwhile, you've built a system to handle 50A that will only see 30A on average... (WAG, but still)
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Post by DennisM » Sun Jan 06, 2008 11:51 am

MAN, what a conversation to wake up to, I need some more coffee !!

OK, I see it's important to make sure the fuse/breaker does it's job instead of the wire becoming the fuse.

A little more disclosure on my system.

After a 12 mile dirt/mud/snowed-in access road, our new home site is 3/4 mile to the nearest power pole, and that's just to the property line. Then probably another 1000 feet to the home site. Figure that out @ $40/ linear ft + poles + tranformer + required minimum usage of >$250 / mo. Our local power got involved with ENRON so our kW charge sits @$0.12 where as 5 years ago it was 1/2 that. So IF I spend even $40k on my system I'll still be money ahead WITH no increases.

My system. 18 Panels of Sharp 208's or 216's depending on availability. Mounted on my barn roof (roof = 3 in 12 pitch, raised seam sheet metal 33' x 32' wide), mounting on UniRac structure and approppriatly tilted directly south & for my latitude. Panels arranged in 3 courses of 5 and one of 3.

I'm 53 now (NOV '54), so in 10 years I don't want to have to climb on the roof, possibly with snow & ice to open my PSPB to figure out which string of 3 panels has a fault. So I will place the PSPB where I can get to it, probably in the main bay of the barn (to keep it & some of the wiring out of the sun preventing degradation from UV and heat). This will make it easy to shut down any string of 3 or replace any of the 6 PSPB breakers. So the maximum run from any panel to the PSPB might be 45'. From the PSPB to the PSDC might be 15' max. The PSDC and remainder of the elec syst will be through a wall and into the shop area (12'x30') which is insulated R19 walls & R38 ceiling. To help keep the AGM batteries warm in the winter, there's also a wood stove and I'll pobably locate the radiator from the genset in the shop too, cooled by a 12v fan powered by the alt on the genset. FYI, the genset I'm looking at is a diesel 10kW 120/240v 3cyl Isuzu (could burn veg oil if the time's come to that).
I want 2 runs from the PSPB to 2 breakers in the PSDC so that a problem would possibly only take down 1/2 of the array production.

I plan on using 2 of the FM80's when they become available for 2 reasons, continuous MPPT, and again duplicity. If I were to have only 1 MX60 or FM80, and it goes down, I'd be forced to use the genset continuously for a couple of weeks. Though my monthly kW usage stands at 300kW or less I don't want to build a marginal system. Though I probably could "get by" with one VFX3648, I'll have 2 wired in Outback stacking with an X-240 transformer, again over-built and I have no 240v loads, excepting my 175 welder that I'd directly plug into the genset. I'm going to mount it all on the FW1000 MP to insure expansion space and ease of wiring space in the larger PSDC & PSAC cabinets. I'll also employ a HUB10 and the new FM-DC and 2 of the new FW-SP-ACA.

Since there is no house yet, and the price of lead (Pb) is making battery price 50% more than 18 months ago, I'm going to wait on buying the 1200ah 48v AGM bank.

Now, I need some more coffee,
Dennis

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Post by tallgirl » Sun Jan 06, 2008 2:11 pm

crewzer wrote:In addition to voltage and current, youÔÇÖll also need to consider fill-factors and temperature derating. With few exceptions, I believe that youÔÇÖll need to end up with a breaker with a condition-of-use ampacity rating less than that of the conductor itÔÇÖs to protect. So, if you decide to go with a 15 A breaker, youÔÇÖll need to make sure that the fill-factor- and temperature-derated ampacity of the conductors is higher than the rating of the circuit breaker.
That's correct -- with few exceptions, none of which apply here, the ampacity of the circuit breaker must always be less than or equal to the ampacity of the conductors.
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Post by DennisM » Sun Jan 06, 2008 5:02 pm

So it looks like the standard #10 MC wire rated THW or THHW will be more than large enough to get me to my PSPV, even from my farthest string of 3 Sharp 216's, <50ft. From the PSPV to the PSDC (<20 ft), the wire size to accomodate ~136v @ ~32amps (3 strings of 3) should be no smaller than the same 10ga (remembering we're insidoors now) though I'd probably go to 8ga as long as that wire keeps me over current of the OBB-40-150VDC120VAC-PNL, for it's length, to the breaker inside the PSDC.

Jim, I'll be sure to use the 12a breakers from Midnite in the PSPV .

Dennis

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Post by sparky » Sun Jan 06, 2008 5:32 pm

Dennis,
Since the house is not built yet why wait to have the panels installed on a roof. You said it yourself that you don't want to go up there. Who is going to come down that X mile road and clean the panels?

Consider a tracker or at least a fixed mount on a pole where you can get at the panels with a pole and automotive car wash brush. There are too many reasons and my time is short tonight but being off-grid without a tracker in your location is always going to be second best. A split of one tracked array and a fixed mount is very nice with a big system like yours.
Have some coffee and mull it!

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Post by Kent Osterberg » Sun Jan 06, 2008 6:01 pm

Dennis,

I don't think #10 is sufficient for the run between the PV combiner and the PSDC!

Let me summarize your system design to make sure that I'm analyzing the same system that you are talking about. Eighteen Sharp 216 watt PV modules. Six strings of three series modules. The six strings are combined into two identical circuits in a combiner box. Both circuits (four conductors and one ground wire) carried in one conduit from the PV combiner to the PSDC.

NEC 690.8(B)(1) says the circuit conductors and overcurrent devices shall be sized to carry not less than 125 percent of the maximum currents as calculated in 690.8(A).

NEC 690.8(A)(1) says that the maximum current shall be the sum of the parallel module rated short circuit currents multiplied by 125 percent.

For your modules, the wire from the PV combiner to the PSDC must be rated to carry 3 x 8.35 x 1.25 x 1.25 = 39 amps. It would have to be at least #8 wire as crewzer has pointed out. Here's why. The ampacity of #8 THHW with three conductors in a conduit with an ambient temperature of 86┬░F is 55 amps. You'll have four conductors, that's a 0.8 derating factor, and in the summer the outside temperature in Sparks exceeds 96┬░F so there is a temperature derating of 0.91. For the conditions stated, the ampacity of a #8 copper wire with THHW insulation is 55 x .8 x .91 = 40 amps. So #8 wire is barely adequate. If the conduit to the PV combiner was on the roof, it would be hotter yet, the temperature derating would be more severe, and #6 THHW wire would probably be needed.

The equipment grounding wire is sized differently, it only has to be sized for 125% of the short circuit current of the circuit. That means it must be able to handle 31 amps. Considering temperature factors, a #10 wire probably isn't sufficient for the equipment grounding conductor either.

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Post by The Electron » Mon Jan 07, 2008 9:19 am

Great ideas and calculations.
I always like to use bigger wire than needed. Who knows what kind of technology we will see someday that will give much higher output from any given surface area?..... I just hate line loss. It's all too common (years later sometimes) to come behind someone and say ...'I wish they would have ran bigger wire!".......

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Post by crewzer » Tue Jan 08, 2008 5:02 am

Dennis,

KentÔÇÖs made some excellent points about deratings and wire gauge. There might be another to consider. Specifically, the circuit breaker terminal temperature rating may well influence your final wire spec.

If the circuit breaker terminal specs are for 60 C, youÔÇÖll need to compute wire ampacity and temperature derating based on 60 C conductor temperature rating, even if you use wire rated at 75 C or 90 C.

To handle 39.14 A (8.35 A Isc x 3 x 125% x 125%), a four-conductor adjustment factor (0.8), and a 0.82 temperature correction factor for 60 C wire in a 96 F environment, the calculation may look like this: 39.14 A / (0.8 x .82) = 59.66 A required ampacity. Per NEC Table 310.16, this will require #4 AWG wire.

If the circuit breaker terminal specs are for 75 C, youÔÇÖll need to compute wire ampacity and temperature derating based on 75 C conductor temperature rating, even if you use wire rated at 90 C.

To handle 39.14 A (8.35 A Isc x 3 x 125% x 125%), a four-conductor adjustment factor (0.8), and a 0.88 temperature correction factor for 75 C wire in a 96 F environment, the calculation may look like this: 39.14 A / (0.8 x .88) = 55.6 A required ampacity. Per NEC Table 310.16, this will require #6 AWG wire.

So, youÔÇÖll need to check the applicable specs for the breakers. Unfortunately, I donÔÇÖt have any of the medium-sized DC breakers lying around. My main battery breaker is rated at 85 C, so I believe that 75 C wire specs and derating are warranted for that breaker.

HTH,
Jim / crewzer
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