To over Watt, or not

[time2roll]

Myself. I don't trust aluminum. A buddy had a house that was done in it years ago. For a short period of time they were wiring with it. He had a bad outcome. I'm probably paranoid.

Late 1960s early 1970s due to copper shortage during Vietnam. I have an aluminum wired house (1968) and all has been fine. Even with no problems I had the outlets pigtailed just a few years ago when between tenants. Outlets needed replacement regardless.

If pulling wire today it gets copper.

 

[TorC]

Aluminum is common for large feeder circuits where the price is a significant factor and it should only be electricians who know what they're doing with it. Treat it right on install and you'll be fine. The regular circuits in your house it's too much risk and trouble to deal with it for every connection.

 

[Kornbread]

In our area, some new mobile homes were using aluminum into the late 70s. Most of the issues seem to stem from the connection and not the actual wire. IIRC, many changes have been made to the NEC regarding aluminum.

Just wanted to add: While, due to price and ease of install, I have no issues using aluminum for the pv array to home run, I would not want it in my home.

 

[WYtreasure]

Myself. I don't trust aluminum. A buddy had a house that was done in it years ago. For a short period of time they were wiring with it. He had a bad outcome. I'm probably paranoid.

I know there is a reason to be concerned, paranoid if you like, about wiring with aluminum in a house. I remodeled my father's house, built late 60's or early 70's in California, wired with aluminum wire. The master bedroom closet light switch was going to be location for the "fire of known origin" within the near future.

I agree with time2roll, Torc and Kornbread that aluminum wire is fine outside. In a home connections become an issue, especially when made improperly.

 

[TorC]

I agree with time2roll, Torc and Kornbread that aluminum wire is fine outside. In a home connections become an issue, especially when made improperly.

It also goes along with the saying, "familiarity breeds contempt." When aluminum is used only for a few big wire connections you tend to notice and think about what you're doing much more. The mains input must be done right for many reasons. For large numbers of small connections it's much easier to get a little careless and skip one important little step, even if you're trying to do everything right. Practically a guarantee it will happen sooner or later.

 

[Kornbread]

And boom, just like that. The cheap aluminum triplex jumps $.45 per foot. Should have bought it last week instead of going fishing.

 

[Snowynorth]

And boom, just like that. The cheap aluminum triplex jumps $.45 per foot. Should have bought it last week instead of going fishing.

This is the norm with everything now.

 

[Mhoward84]

It sure would be nice if Deye had some dos and don'ts on this subject.

wattmatters says some can be connected together. GXMnow says don't do it. I won't do it unless I saw something in writing from Deye.
And thanks for looking up the 7.6 GXM.

I also received a PM from someone who didn't want to join in on my Q&A I know very little thread.

He/she said the 9880 is the max watts allowed on the 7.6k. I won't need to worry about that. Keep below the 425v mppt range, not the max pv input 500v using the cold factor. If 2p, use a combiner box with surge protection 15a breakers would be fine for each string. Don't worry about going over a bit on the amps. The mppt is amp self limiting. (Who knows? The manual certainly doesn't state anything.)

I also went to rapidtables.com to calculate watts and volts into amps, of the various panels I can get. The only one that's over is the 455 watt longi at 9.13 amps, using rapidtables 18.26 if I use 2p. The others would be under using the calculator. I'm sure longi and Q.cell know there stuff, but I wonder why the discrepancy.

I would call Deye but they wouldn't understand what I'm saying anyway. Lol

It looks like I have to run wire, and tell the wife no kitchen till next year. Ha

Or be very low on panel production.

Why would you need a combiner box and breakers? There should never be a situation where the current would exceed 15A in the input circuit (from panel to combiner). If there are two panels in parallel, A and B. If A shorts, then it will short itself out, but then it would draw the Isc of Panel B through the wire and fuse for Panel A, which would be under its maximum.

And combined together it would be 18.something amps so you wouldn't want that on the output circuit as it would always trip.

Also GSL sent me their manual and it states there are 4 trackers with 12A max on each input. I also attached both the spec sheet and manual I got from them. EDIT: I'm not sure if you got the previous "actual deye" inverter with the 2x MPPTs or the new "alternative" version I listed with the 4x MPPTs.

 

[Snowynorth]

I guess the simple question for me is can I run 6S2P for 21 amps through a combiner box, into a 2 conductor cable, and wire it into one of the 2 inputs of 1 of the 2 mppt in the unit, until I can expand next year? The GSL 7.6k amp input is 18+18, the amps coming in will be 21+- if I wire the panels 6S2P. I hear those units self limit the amps, but is there a limit to how much they self limit?

 

[Snowynorth]

Kinda feel a bit silly. I asked various ways about the max pv impute amps of these, as I could not read anywhere about self limiting pv current. Thanks to a member here, I have the answer. The panels in 6s2p which I want to use would be 21+ amps per mppt, the 7.6k states 18+18a. The spec sheet and the manual state nothing about going over. I guess I should have done what I always tell my kids to do. READ THE LABEL! lol
Max PV Isc 25a. Yes they do self limit. Duh!

 

[Hedges]

"Max PV Isc 25a" in inverter spec would mean "Do not connect panels exceeding this, inverter may be damaged".

"18+18a" would mean "Can process up to 18A Imp per MPPT input"

Sounds like you can do 2p into one input, expand with 2p into the other later.
Looks to me like up to 400W panels x 24 before wattage is limited.

 

[Snowynorth]

"Max PV Isc 25a" in inverter spec would mean "Do not connect panels exceeding this, inverter may be damaged".

"18+18a" would mean "Can process up to 18A Imp per MPPT input"

Sounds like you can do 2p into one input, expand with 2p into the other later.
Looks to me like up to 400W panels x 24 before wattage is limited.

That's what I was thinking too. I thought the manual, and/or the spec sheet would have the same and more info then the label, but not so. Voltage and Max PV Isc amps are good. Now to figure out watts I can buy reasonable 425 and 455 watt panels. The 455w are less money for some reason. 425x24=10,200 watts, 455x24=10,920. The inverter specs state 9880 w. Are the 455w too much?

 

[Hedges]

You can generally get away with over-wattage of array. And it is good to do so to make up for less than ideal sunlight, due to atmosphere and seasons.

Never exceed max open circuit voltage. Assume Voc x 1.2 on a freezing cold day, unless you do the math to get a more accurate multiple.

Usually exceeding Isc wouldn't be a problem. Maybe some inverters draw too much while looking or Imp. Otherwise, the limit on current is a diode that some use to short out the array if PV is connected reverse polarity, and some crowbar the array if over-voltage detected.

You will have 6s2p into each MPPT input. Can you orient one 6s South East, and the other 6s South West? That would reduce peak current and power to about 0.7x as much but maintain it for more hours per day.

 

[Snowynorth]

I am being very careful on voc. I have used 2 calculators just to compare. Temps went lower than -45 here last winter. I will be well under the max voltage. I had to calculate it at -50c here. Is 10,920w to much for a 9880w inverter?

I have one area to build the ground mount. It is wide open to the south with no shading. I have 2 of these 7.6k inverters, and might hook the second up down the road as funds permit. I have enough pipe, and space to build the anchor posts, and cross members wide enough for all 48 72 cell panels. Would you go straight south or put a slight v in the middle? If I v the first set of panels would be side by side from the middle. The second set for the second inverter would be at the outer ends. This is 100% off grid.

 

[Hedges]

10,220W (STC) will almost always not even max out an SCC capable of processing 9880W.
Depending on needs, I would consider over-paneling to 15kW, so long as it wasn't going to over-stress the electronics.
With array split into two orientations about 90 degrees apart, 15kW (STC) would peak at about 10kW (STC)

I'd consider a large V in the middle. Half facing SE (9:00 or 10:00 AM sun depending on daylight savings time), and half SW (3:00 or 4:00 PM).
Tilt would depend on whether you want maximum power for air conditioning in August or battery charging and illumination in January.

Due South produces the most watt-hours from a given PV panel, and can be optimum especially for grid-tie.
For off-grid, multiple angles to produce more early in the day and more late not only flattens production (matching consumption so less likely curtailed due to full battery), but also keeps battery full later in the day. That means a smaller bank or less cycling through the night.

I use a bank which barely holds one night's power for backup. Off-grid you are more likely to have a larger bank, so less of a concern with lithium. With lead-acid, keeping battery full later into the evening means less cycling, longer life.

If a lithium battery is oversized, you'll probably never reach cycle life, so overpaying. With excess PV you can get away with less battery, still produce enough on hazy days. Generator (or greater conservation) for really poor sun days. But some lithium options are looking cheaper. The name brand commercial offerings were so expensive I'd want to undersize and get as many full cycles as possible.

 

[Snowynorth]

15 kw will put my voltage out of whack. Even if I 7s2p the 445 watt panels I'm getting up there in voc the inverter specs write like this
PV input voltage 370v(100v-500v) Unless I'm wrong 370v is the sweet spot per mppt? In cold weather I'm right near that 370v per mppt 6s2p. I don't have the print out in front of me. I'm going to use lithium. Our latitude is 57 degrees. It will have no adjustment. I'm thinking 55-60 degree angle for year around, with the lower rail at about 5ft for shedding snow. I will get some snow reflection in winter. The closest trees are about 50 yards out into the field in front, and they are willows so not very tall.

 

[Hedges]

Lithium, so you need to keep batteries warm and prevent charging below freezing. Ideally charge rate is reduced at very cool temperatures.
Excess PV wattage the idea would be more panels in parallel, but multiple angles so peak current and wattage isn't so high.
For my MPPT (GT PV), 600V max allowed. My arrays are 480Voc, 380Vmp at normal temperatures.

If you were to go with Sunny Island, look for a BMS it can communicate with. REC is one. Some other inverters communicate - see if yours supports that. Many systems are built without that, but the BMS knows state of batteries so ideally it requests charge current and voltage, also communicates regarding SoC so loads can be controlled.

 

[Snowynorth]

I'm building a highly insulated shed for inverter, batteries, with 2 propane wall heaters. One for use one for redundancy. Propane and wire are roughed into it's location. Your in a cold location too, I see.

 

[Hedges]

Nah, balmy San Jose. Once in 10 years we get snow. Once in 50 years, a tornado.
But I looked up record cold, something like -15C, so I use -40C relative to +25C for Voc adjustment. Just to be safe.
For a mobile rig I'm considering, I'll use the much colder Truckee temperatures.

 

[Snowynorth]

Nah, balmy San Jose. Once in 10 years we get snow. Once in 50 years, a tornado.
But I looked up record cold, something like -15C, so I use -40C relative to +25C for Voc adjustment. Just to be safe.
For a mobile rig I'm considering, I'll use the much colder Truckee temperatures.

Last year was a weird one here. +47c to -47c. Maybe I should recalculate to -60 or -70c. Thanks for the help.