Combiner Box or not

Solar Expert Posts: 36
I get confused reading through all the good stuff here and now head spinning and need advice. Thanks ahead. So my plan of getting DC to the inverter was to connect together my 3 strings of 255 watt solar world panels. 2 strings of 10 and one of 8. I have two sunnyboy inverters. a 5000 and 4000. I ran the numbers through the SMC website and the 5000 would be happy with 18 panels and the 4000 with 10. As you guys know, the Sunnyboys have 2 DC inputs. I am planning to connect 10 panels to one input of the 5000 and 8 panels to the other input. The other 10 panels would go to the one input on the 4000 and the other a spare. Having said all this, from panels to inverter is 50 feet. I would have 6 current carrying conductors. I would stuff them into 3/4 inch EMT conduit (That I have not yet tried to see if they fit) at the panels, run the conduit to the inverters run the wire through the DC disconnects and then plug into the DC inputs.

Now here is where I am missing something. From what I read, the whole point of the combiner is to get 6 current carrying conductors, down to say 3. Then use cheaper wire from the combiner to the inverter. I would do this if I had say one sunnyboy 8000. Is my plan to just drop wire from the PV modules into conduit then run conduit with wire inside to the inverters? I have to go to the garage and see if I can stuff 6 conductors into 3/4 conduit, but no combiner box needed if I do this. Am I missing something simpler and better? Also, what is skip wiring thing I saw but cant find? I cant remember the specifics, but it appeared to be a way to reduce wiring up a lot of panels like my 28. Mike

• Banned Posts: 17,615 ✭✭✭
Re: Combiner Box or not

The point of using a combiner box is so you can easily connect more than two strings in parallel with appropriate circuit protection. If each of your strings is going to a different inverter input they are not being combined.

Size your wire to be appropriate to 1). carry the current and 2). minimize Voltage drop. Once you have the appropriate wire size you can find out what conduit is needed to carry it. There are rules about how much you can fill conduit, and it all has to do with heat dissipation to maintain a viable current-carrying capacity. It is doubtful that a single 3/4" conduit is going to handle three sets of conductors (6 individual wires).
• Solar Expert Posts: 36
Re: Combiner Box or not

Thanks Cariboocoot. You are always very informative and helpful.......and free.
• Solar Expert Posts: 36
Re: Combiner box or straight wire to inverter
The point of using a combiner box is so you can easily connect more than two strings in parallel with appropriate circuit protection. If each of your strings is going to a different inverter input they are not being combined.

Size your wire to be appropriate to 1). carry the current and 2). minimize Voltage drop. Once you have the appropriate wire size you can find out what conduit is needed to carry it. There are rules about how much you can fill conduit, and it all has to do with heat dissipation to maintain a viable current-carrying capacity. It is doubtful that a single 3/4" conduit is going to handle three sets of conductors (6 individual wires).

Well interestingly my 2014 NEC table c.1 indicates 3/4 EMT can carry 6 conductors of RHW-2. or USE-2. I am thinking the EMT gets hot on a July roof and the conductors inside get hot as well. Probably hard to yank 6 of this type of wire through 40 feet of conduit and not good for them to be hot and packed in there. I didn't know RHW-2 was also USE-2. It isn't that expensive in 50 - 100 ft rolls.
• Solar Expert Posts: 3,208 ✭✭✭✭
Re: Combiner box or straight wire to inverter

AND, am certain that you know that there are current de-ratings for both number of current-carrying conductors in conduit, as well as for ambient temperature. Believe that the 2014 NEC has specific de-ratings for roof-top conduit exposures.

FWIW. Vic
Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
• Solar Expert Posts: 3,208 ✭✭✭✭
Re: Combiner box or straight wire to inverter

RE Skip Wiring PVs,

solarix has his description in post 8 of this Thread:

Good Luck, Vic
Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
• Solar Expert Posts: 36
Re: Combiner box or straight wire to inverter
Vic wrote: »
RE Skip Wiring PVs,

solarix has his description in post 8 of this Thread:

Good Luck, Vic

Thanks VIC, I did indeed find a chart in the NEC for rooftop derating. I missed that so thank you. I find the skip wiring thing a great way of starting and stopping the PV panel wiring in the same place and cutting down on the wire. Thanks for that also. How do you feel about heyco clips for clipping the wiring on the back of the PV panels up? The more I think about my upcoming install as the weather warms up, the more I find out about little things like this.....which could very well short out everything if the wire rubs on something through the insulation. I saw some "trays" that were out there on the web for holding the wiring but expensive. If I can skip wire hopefully there will be very little slack to take up behind the panels. Thanks again for input. Mike
• Solar Expert Posts: 3,208 ✭✭✭✭
Re: Combiner box or straight wire to inverter

Hi michalec,

Well, since this looks to be a GT system, and will be inspected, it is possible that the AHJ Inspector will want to see certain things used to secure cables, etc. Have seen some installers use black cable zip ties UNDER the PVs for this purpose. The quality of these ties does seem to vary, however. If possible, the Inspectors are the best source of info, as they will be inspecting the installation.

Some rack rails to have a channel for PV cables from individual PV modules, but this may not work too well with Skip Wiring ...

Good Luck, Vic
Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
• Solar Expert Posts: 414 ✭✭✭
Re: Combiner box or straight wire to inverter
michaelc wrote: »
Well interestingly my 2014 NEC table c.1 indicates 3/4 EMT can carry 6 conductors of RHW-2. or USE-2. I am thinking the EMT gets hot on a July roof and the conductors inside get hot as well. Probably hard to yank 6 of this type of wire through 40 feet of conduit and not good for them to be hot and packed in there. I didn't know RHW-2 was also USE-2. It isn't that expensive in 50 - 100 ft rolls.
Wouldn't that be if it didn't have it's outer covering, other wise it looks like need to go to 1" EMT to pull 6 conductors.ConductorSize(AWG/kcmil)Trade Size (Metric DesignatorRHH, RHW,RHW-212 — 3 6 9 17 23 38 66 100 131 167 — —10 — 2 5 8 13 18 30 53 81 105 135#10 AWG= 5 in 3/4", 8 in 1" etc with outer coveringAs stated still need to look at temperature deratings etc.
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: 36
Re: Combiner box or straight wire to inverter
SkiDoo55 wrote: »
Wouldn't that be if it didn't have it's outer covering, other wise it looks like need to go to 1" EMT to pull 6 conductors.ConductorSize(AWG/kcmil)Trade Size (Metric DesignatorRHH, RHW,RHW-212 — 3 6 9 17 23 38 66 100 131 167 — —10 — 2 5 8 13 18 30 53 81 105 135#10 AWG= 5 in 3/4", 8 in 1" etc with outer coveringAs stated still need to look at temperature deratings etc.

I assume the chart in the NEC, when identifying RHW or USE as a cable assumes the covering is on as part of the cable identifier. I didn't see anything that broke out non insulated cable separately pertaining to what I intend to use, although 1 inch EMT is what I will purchase and use just because of the derating and physically hard to pull 6 of those things through 3/4 inch. I will check some more NEC charts for info. I will say as a layperson, the NEC is really not user friendly. Even conflicting in some places but lots of good info I didn't know as a homeowner who does upgrades a lot. Mike
• Solar Expert Posts: 3,208 ✭✭✭✭
Re: Combiner box or straight wire to inverter

michaelc,

SkiDoo55 IS correct. Had thought that there WAS a footnote in the NEC Cable-Fill Tables, but did not have then in hand.

Going back a number of years, there is the Footnote,
"RHH, RHW, AND RHW-2 without outer covering." at the end of each table for a given conduit type. The 2014 Code may only have one such footnote for all of the fill tables ... forget.

Some installers transition to THHN / etc other cable at a convenient place on the roof for the home runs. Opinions differ.
Good Luck, Vic
Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
• Solar Expert Posts: 36
Re: Combiner box or straight wire to inverter
Vic wrote: »
michaelc,

SkiDoo55 IS correct. Had thought that there WAS a footnote in the NEC Cable-Fill Tables, but did not have then in hand.

Going back a number of years, there is the Footnote,
"RHH, RHW, AND RHW-2 without outer covering." at the end of each table for a given conduit type. The 2014 Code may only have one such footnote for all of the fill tables ... forget.

Some installers transition to THHN / etc other cable at a convenient place on the roof for the home runs. Opinions differ.
Good Luck, Vic

You guys are correct, on pg 70-793 there is a footnote asterisk for RHW-2 in the 2014 that says "without outer covering". That is the table for EMT. Interesting and why I like chatting here.