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General Category => System Design and Layout => Topic started by: ligwyd on July 02, 2018, 03:02:49 PM

Title: PV Wire Sizing/ Breaker Selection
Post by: ligwyd on July 02, 2018, 03:02:49 PM
Hi there,

I have measured my furthest length of PV wire required, one way, to get to the combiner to be 35 to 40 feet. After the combiner it is all 6 awg and then 4/0 to the batteries.

Panels have 12 awg wire/ connectors installed from factory.

The Isc (short circuit current) on each panel is 8.98 A multiplied by 1.25 = 11.23 A. Each string of 3 panels should use a 12 amp breaker. I get that part.

My concern is that if the PV breaker is a 12 amp breaker should my PV wire be sized to accommodate 12 amps or 8.98 amps over 40 feet? I am guessing size the PV wire to the PV breaker - correct?

I guess my choices is simply between 12 awg and 10 awg wire. Might be smart just to go with 10 awg but just want to be sure and do not want to waist $$$ on wire.

Also, is it common practice to run armored PV wire or does it come down to $$$ and codes? My thought would be better to protect the critters from chewing the wire with conduit/ armored PV wire than to repair/ replace PV wire. Looking for some insight.........

Thank You


Thanks,

John
Title: Re: PV Wire Sizing/ Breaker Selection
Post by: Vic on July 02, 2018, 03:25:07 PM
Hi John,

One needs to size breakers/fuses to protect the cable it is connected to from overheating.

One can,  and often does increase the wire size,   to reduce voltage drops.  SO,  it is fine to use #10,   or even #8 AWG  with a 12A breaker,   for example   ...   as long as the breaker has terminals rated to accept the larger cable,   if the breaker has screw terminals.

Personally,   always run cables from the PVs to Combiners and from the Combiner to the  CC in conduit.   Almost always use metal conduit (usually EMT)  for all exterior,   above ground circuits.

Usually,  make the transition from the cables that attach to the PV modules,   at a water tight AL  junction box,  using "gland nuts"  (cable grips).   The cable run from that junction box to the rest of the system,  uses standard THHN type copper building cable.   If you are in the USA,   that is commonly-available.   In Canada,   there are similar building wire types.

As you know,   the THHN building wire is "compact" and pulls easily through conduit.

The conduit  protects the cable from sun/weather,   rodents,   birds,   etc.

My opinions,   Vic
Title: Re: PV Wire Sizing/ Breaker Selection
Post by: ligwyd on July 02, 2018, 04:43:27 PM
I think 10 awg wire will be perfect. The PV breakers will accept the 10 awg wire no problem. As far a voltage drop goes, I am on the cusp of 12 awg to 10 awg wire. The only reason I need 10 awg is to satisfy the PV breaker that is a 12 amp breaker (rated to trip at 15.6amps) so the wiring will have very little resistance with the 10awg.  According to my Inverter manual I can run 46.4 feet at 12 amps with 10 awg and still have an acceptable voltage drop and I am only running 40 feet max and that is on the longest run of PV wire so we are good there.

I will have all the PV wire in metal raceway/ conduit EMT. There will  be some exposed wire from the back of the panel J boxes down to the metal raceway and where the wire transitions for the raceway to conduit but I am sure I can get creative there and make it happen. Would be nice to find a product that can cover exposed sections of PV wire to protect from critters. Are you aware of any such product or is armored PV wire ($$$) the only thing out there? There has gotta be a product out there.

Re: water tight junction boxes, does not this add another level of complexity to the wiring or is it simply more cost effective? I had never considered this as an option. I figured that PV wire ran right from the panels to the combiner, via mc4 connectors was the way to go? Once an mc4 connector is installed its just "snap" and its connected! Are junction boxes easier than that? Care to briefly expand on this?

Thanks Vic for your support. I am itching to get back to job site once my head stops throbbing :)
John



Title: Re: PV Wire Sizing/ Breaker Selection
Post by: Vic on July 02, 2018, 05:12:50 PM
John,

If your Combiners have MC4 connectors,   then,  by far the best approach is to run cable that is rated for exposure  is the most direct method.

AND,  it is often said that if PV cables are cut,   that the PV's warranty would be void.   There will  almost always be some of the PV's attached cables exposed,   behind the PVs.  This is generally not a problem area.  It is good to try to clip those PV cables to the PV frame,   where you can.   Some might try using Split Loom covering,   but am not certain that would do much good.

Those who install many PV systems probably have some quick,  and inexpensive approaches to protecting the PV end of these cable runs.   I do not install that many systems.

The metal junction boxes need to splice from some form of PV extension cable to a cable that is better suited for pulling into conduit. Any splice  can cause a problem,  if not done well.  That is just the way that I do it for communication facilities,   as the goal is usually to transition to metal conduit with the shortest flying leads from the PVs,   as is reasonable.  This approach is just a way of trying to minimize line-conducted/radiated RF Emissions.   Most systems do not have a need to do this.

It looks like you are a technical person,   and you are doing a thorough system design,   and have  an intuitive approach.   Seems that you should not have very many problems with these systems,  IMO.

FWIW,   Have fun,   Vic
Title: Re: PV Wire Sizing/ Breaker Selection
Post by: ligwyd on July 02, 2018, 05:53:15 PM
Hey Vic,

The MNPV6-Disco I am using on each system does not directly terminate to mc4 connectors, it just accepts the PV wire through your choice of knock outs and directly connect to the PV breakers, as you know of course. Maybe I misunderstood?

I'll have to find a split loom product that can't be chewed through. Maybe PV manufacturers could start making their Panels with armored cable right from the J Box? Minor detail....

Just gotta choose PV wire and hook up the panels. Is tin coated copper the best choice?

Have you ever used any special product in the mc4 connectors or is it not necessary?

https://www.sanchem.com/electrical-contact-lubricant.html

I'll stop bugging you now Vic :)
Thanks
John
Title: Re: PV Wire Sizing/ Breaker Selection
Post by: Vic on July 02, 2018, 07:25:43 PM
Hi John,

Am not certain that split loom covering would do any real good,   unless,   critters find that it tastes horrible,   or similar.  Others may well have ideas.

Tinned Cu cable should do no better than bare copper stranded cable,  unless the connection between connectors and the cable is exposed to moisture.   Think that the MC4 connectors from name-brand suppliers have O-rings,   front and rear.   Have not specifically looked for  Tinned PV cable  that has the correct rating for exposure.   Just avoid finely stranded tinned cable,   as it might not be compatible with MC  connectors.

Most PV cables seem to be 7-strand Cu,   in the sizes that we commonly use.   This should be fine.

If you are not equipped with expensive MC4 Crimpers/Dies,   you can order PV extension cables  with the correct gender MC4 correctly crimped on one cable end,   and nothing on the other. These can be made to the length that you need.

Some of us have used PV extension cables  --  MC4 male on one end,   MC4 female on the other.   These come in various lengths.  Cut them in two, and have each gender needed for each string,   without excessive cable waste,  when you cut them to the exact length that you need  ...   as you probably know.

IMO,   think that there is no need for any addition of conductive lube for the MC4 connectors.   Have not seen any recommendation for this from the connector manufacturers.

Just more opinions,     FWIW,   Vic
Title: Re: PV Wire Sizing/ Breaker Selection
Post by: ligwyd on July 02, 2018, 11:05:40 PM
Hi Vic,

Just got in form the job site. I do have the overpriced crimper tool/set for the mc4 connectors.

Does the mc4 o-ringed male female connections being out doors exposed to rain justify tinned copper?

Unless tinned is all the wholesaler I deal with carries, If it is not necessary, I'll let price help me decide.

Thanks Vic,
John
Title: Re: PV Wire Sizing/ Breaker Selection
Post by: ClassicCrazy on July 02, 2018, 11:12:07 PM
Why doesn't a regular crimper work on MC4 connectors ? I think that is what I used and all was good.
I hate to tell you guys that I have had regular indoor rated wire going to my combiner box just laying on the ground and gets buried in the grass and dirt over time - have had that on for years and no problems. Every once in awhile I pull it up to check and make sure the insulation isn't cracked or rotting away and it has been good. Was just put on for temporary but you know how that goes !   I just tuck the MC4 connectors under the panels and that keeps most of the rain off of them.

So yeah it is fun to see how well indoor wiring lasts exposed - it is surprising durable . I am sure whatever you do to weatherize it will be more than adequate.

Larry
Title: Re: PV Wire Sizing/ Breaker Selection
Post by: ClassicCrazy on July 02, 2018, 11:15:25 PM
I use these at the bottom of my combiner to poke the wires in  and keep everything dry and bug proof . There are different sizes available.

https://www.solar-electric.com/midnite-solar-mnsr-600-strain-relief.html

Might be what Vic was talking about that he uses from panels to his junction box to transition to conduit.

Larry
Title: Re: PV Wire Sizing/ Breaker Selection
Post by: ligwyd on July 02, 2018, 11:37:58 PM
Hi Larry,

Yes I have seen those connectors before and I think I have them somewhere.
I was thinking of running all my 12 - 10 awg PV wires in one EMT conduit and entering in the bottom of the combiner/ disco through one hole/ EMT connector providing that is possible of course.
The last off grid job we did we had all the separate PV wires entering the combiner through the designated knock outs in the bottom and used those strain relief connectors and it worked great as intended, however, I think that one beefy EMT pipe might look really clean and also zero exposure of PV wires for critters to snack on? Care to comment on this approach. Really value your guys input. This is such a great resource.
John
Title: Re: PV Wire Sizing/ Breaker Selection
Post by: Vic on July 03, 2018, 02:15:38 PM
Quote from: ClassicCrazy on July 02, 2018, 11:15:25 PM
I use these at the bottom of my combiner to poke the wires in  and keep everything dry and bug proof . There are different sizes available.

https://www.solar-electric.com/midnite-solar-mnsr-600-strain-relief.html

Might be what Vic was talking about that he uses from panels to his junction box to transition to conduit.
Larry

YES,   Larry,   that is exactly what I was referring to.   Different manufacturers use different names for these.

I sure DO LOVE MidNite.  These grips/strain reliefs with twin holes for individual  wires,   are not commonly available.   At times not available even with a special order,   depending upon the supplier/manufacturer.

MidNite KNOWS  what is needed,   and either  has them made to spec,   or  buys them in quantity and distributes them.   MN   is just about  the only manufacturer  that  tries very hard to source the BOS components.

Thanks MidNite!!    We DO appreciate it,    Vic
Title: Re: PV Wire Sizing/ Breaker Selection
Post by: ligwyd on July 06, 2018, 01:31:27 PM
Another brain buster :)

Regarding PV wire, which is the best choice? I have read that 7 strand copper(non-tinned because I don't think it is necessary) is the most common. I can get 19 strand where I live, is this better? I have read that the current flows on the outside of the wires so the more strands the easier it conducts? So far I am going with 10 awg single conductor black - red + uv resistant not direct burial to keep conduit size to a min and it is not being buried.
Any feed back. I am sure I am overcomplicated it as usual but really want to make the best choices I can. Is ROHS compliant important?
Thank you to all who have some feedback.
John
Title: Re: PV Wire Sizing/ Breaker Selection
Post by: Vic on July 06, 2018, 01:53:05 PM
Quote from: ligwyd on July 06, 2018, 01:31:27 PM
Another brain buster :)

Regarding PV wire, which is the best choice? I have read that 7 strand copper(non-tinned because I don't think it is necessary) is the most common. I can get 19 strand where I live, is this better? I have read that the current flows on the outside of the wires so the more strands the easier it conducts? So far I am going with 10 awg single conductor black - red + uv resistant not direct burial to keep conduit size to a min and it is not being buried.
Any feed back. I am sure I am overcomplicated it as usual but really want to make the best choices I can. Is ROHS compliant important?
Thank you to all who have some feedback.
John

'Morning John,

The statement about current flowing on the outside of conductors,   applies to AC currents.   The higher frequency of these currents,   the more pronounced is this "Skin Effect".

It does not apply to strictly DC  current.   DC voltage drops are affected by the Cross-sectional Area of these conductors.

So,  the stranding of the PV,   battery or other DC conductors  will have no effect.

When pulling cable through conduit 19 strand cable (for the sizes that you are speaking of)  will be easier to pull.   Standard building cable  --  THHN in the USA  --  has a polyester (Mylar(r) type)  coating on the outside of the cable insulation that makes this type of cable  easier to pull,   as well.   If the cable that you are pulling in conduit has NO weather exposure,   then,   that is a great cable type to use.

What is the cable type that you are considering using in conduit.

#10 AWG  cable should be fine for your PV runs,   given the approximate 40-ish (IIRC)  foot max one-way distance you noted before.

Often,   WX resistant has  a heavy jacket (and often 7 strand),   which could be a bit difficult to pull,   and  the jacket thickness  will reduce the permitted number of conductors in any given trade size of conduit.

Also,   would suggest using Red cable for positive,   and White cable for the negative on your cables (PV and CC-to battery).   Battery interconnects do not really matter,  color-wise.   For Inverter-to battery cables,   often  the polarity is identified by colored shrink-fit at the lugs,  on each end  (most of this is familiar to you).

Asking any Inspector-types some of the questions that you may have before buying more parts  can be helpful,   as you have mentioned.

Off to work,   good luck,   Vic
Title: Re: PV Wire Sizing/ Breaker Selection
Post by: ligwyd on July 07, 2018, 12:35:48 PM
I am still in the process off sourcing/ selecting the PV wire.  On the fence about tinned or bare copper. Probably go with 7 strand due to I've read here that it is a little easier to connect. Was thinking Red and Black. What is the reason behind white negative cables. Is a neutral and  not a negative? Does it really matter? Secondary objective, after getting the right PV wire, is keeping it as small of diameter as possible to reduce conduit size. I'll have 12 wires total (maybe a ground - not sure yet)
Thanks everyone for you guidance.
John
Title: Re: PV Wire Sizing/ Breaker Selection
Post by: ClassicCrazy on July 07, 2018, 01:44:43 PM
I use Red for my positive and black for negative just because I am so used to that from DC in most everything else. But I don't have any codes or inspectors. I think the other color suggestions are probably due to avoiding confusion with people who think about AC wiring  where black is hot and white is the neutral . For AC the hot can be any color except white or green.

When you are using something like romex on DC you have to make the choice black for positve or negative - so if you use black for negative then you have to put the white on positive . So if you use white for negative then black for positive it almost makes more sense when looking at it from standard AC wiring scheme . I use red tape on the end of all my positive wires to avoid any confusion .

Larry
Title: Re: PV Wire Sizing/ Breaker Selection
Post by: ligwyd on July 08, 2018, 02:45:28 AM
Thank you Larry.
Title: Re: PV Wire Sizing/ Breaker Selection
Post by: Vic on July 08, 2018, 10:37:28 PM
YES,   Larry is correct.

Some inspectors like to see DC systems use Red,   and White,   rather than Red and Black,  as,   in many AC systems,   Black is also a hot lead,  just like black   ...   as I read it.

But,  it all depends on what any Inspector of a system desires,   IMO.

FWIW,    Vic