Parallel two low amp cables to make one big

stmoloud

I am looking to increase my panel size from 20 amp max to 40 amp max. I have a good amount of 25 amp cable so am wondering if it is OK to parallel another 25 amp cable alongside my existing installation.

Thanks

mcgivor

Yes it is possible to parrallel conductors to gain increase in cross sectional area or gauge, if that is the question, running parallel as separate runs in the same raceway is also acceptable, if using the same insulation rating, a generic answer electrical code restrictions notwithstanding.  

stmoloud

Excellent. So just to be clear. The amps coming from the PV array to the distribution box and then to the two 25 amp-rated cables  going into the controller will evenly split so that for instance 10 amps will not go down one and 30 down the other?  

mcgivor

Correct in parallel circuits with equal resistance  (same length same gauge ) thr current will be divided equally between each leg.

stmoloud

Thanks mcgivor. Making better use out of existing assets is always a priority for me. But not at the expense of safety.

BB.

If you can make a run from array 1 to the combiner box near the solar charge controller, and a second wire run for array 2 to the combiner box, that is a very safe and the correct way to parallel the cable runs. Each current source supplies current to its own dedicated run.

After the combiner you have a single heavier cable run (no parallel current paths) to the charge controller.

Bill

mike95490

mcgivor said:

Correct in parallel circuits with equal resistance  (same length same gauge ) thr current will be divided equally between each leg.

ONLY if all the crimps are high quality, all connections properly torqued, so that both cable paths have the same resistance, then current will be mostly equal.  After it's hooked up, use a clip on DC ampmeter to verify, before a wire melts

BB.

NEC Code only allows larger AWG conductors to be connected in parallel:

http://www.electricallicenserenewal.com/Electrical-Continuing-Education-Courses/NEC-Content.php?sectionID=297.0

 ...conductor in sizes 1/0 AWG and larger. 
....

In order to install conductors in parallel, the paralleled conductors in each phase, polarity, neutral, grounded circuit conductor, equipment grounding conductor, or equipment bonding jumper must comply with all of the following:

(1) Be the same length.

(2) Consist of the same conductor material.

(3) Be the same size in circular mil area.

(4) Have the same insulation type.

(5) Be terminated in the same manner.

The above rules ensure that each set of parallel conductors carries the same ampacity. Where run in separate cables or raceways, the cables or raceways must have the same number of conductors and must have the same “electrical characteristics”.

For National Electric Code (North America), you are not allowed to connect smaller than 1/0 AWG cables in parallel runs.

-Bill

mcgivor

BB. said:

If you can make a run from array 1 to the combiner box near the solar charge controller, and a second wire run for array 2 to the combiner box, that is a very safe and the correct way to parallel the cable runs. Each current source supplies current to its own dedicated run.

After the combiner you have a single heavier cable run (no parallel current paths) to the charge controller.

Bill

This is the best solution as it avoids the code issue, if applicable, regarding minimum gauge allowable for parrallel runs, the end result would be exactly the same, each conductor carrying equal current. Generally parallel runs are used with high current where it's difficult to pull conductors like 500 MCM through conduit, it's like pulling rebar, not to mention bend radius and the cabinet size needed to accommodate the code requirements.

stmoloud

BB. said:

If you can make a run from array 1 to the combiner box near the solar charge controller, and a second wire run for array 2 to the combiner box, that is a very safe and the correct way to parallel the cable runs. Each current source supplies current to its own dedicated run.

After the combiner you have a single heavier cable run (no parallel current paths) to the charge controller.

Bill

Just to clarify a difference in terminology. I am not sure what is referred to here as a combiner box. I am taking all my PV cable (presently two pairs)  and terminating them to 2 x 16 way 100 A brass distribution bars inside the vehicle.

Between the distribution bars and the charge controller are the 25 amp cables 

When I add extra panels to a maximum of 40 amps total and then run those two pairs to the distribution bars, you are suggesting I should replace the 25 amp cables with say, 50 amp cables.

Rather than run another 25 amp in parallel. 

BB.

As background, solar panels are "current mode" devices (more or less). And work very well in parallel current applications (the output current of a solar panel is proportional to the amount of sunlight hitting the panel).

Batteries, on the other hand, are voltage sources... They attempt to hold a voltage, and supply current (or absorb current) to hold the voltage "constant". Paralleling batteries is a pain because relatively minor variations in wiring resistance (and even battery age/temperature/construction/functional failures) can cause wide variation in current flow.

Each array should have its own cables to the "destination" (i.e., Xparallel by Yseries panels down a set of cables, through a fuse/breaker per string, to the common solar bus/solar charge controller).

If you have the common solar bus bar (from above) then run 2 sets x 10 AWG cables in parallel to another bus bar/charge controller interface, that is "iffy". 10 AWG is NEC rated to a maximum of 30 Amps (12 AWG is rated to 20 Amps max).

https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

NEC is quite conservative (compared to marine/SAE wire ratings).

https://www.westmarine.com/WestAdvisor/Marine-Wire-Size-And-Ampacity

Where 10 AWG is rated to a maximum of 60 Amps (open wiring in non-engine compartment?). Note the Marine link in this case is AWG wiring, not SAE gauge. (corrected BB.)

Note that NEC rating is also based on Insulation Rating, ambient temperature, conduit fill, etc.... Usually a bit more thorough than you see with Marine and other specifications (also note that 14 AWG wire is slightly larger diameter vs 14 SAE gauge wire--Not much difference, but is there).

And I even suggest that for solar and battery based power systems, that you even derate further the NEC ratings to NEC "Continuous Current" ratings.... I.e., a 14 AWG cable rated at 15 amps * 0.80 NEC derating = 12 Amperes rated branch circuit and breaker/fuses.

Or, if you want to carry 25 amps, then 25 Amps * 1.25 (1.25 = 1/0.80) = 31.25 Amp rated Branch circuit (round down to 30 Amps or round up to next standard AWG/Breaker).

More or less, in the US/North America, our standard rated breakers/fuses are spec'ed to "open" at 100% of rated current (could be minutes to hours). And not open at 80% of rated current or less.

For several reasons, long parallel wire runs are more "stable" for parallel wiring (and issues with parallel wiring/connection failures) than short runs... The long runs of wiring has most of the resistance "fixed" by wire resistance. Short wire runs, then the connections have a large percentage (50% or more) of the resistance, so the current sharing (I=V/R) is more variable with short runs.

And, to clarify why this is a "big issue"... It turns out (from my experience) that the low resistance wiring path ("best connections", "shortest parallel wire run") is the one that fails first... If the resistance is 1/2 for run A vs run B, then Run A will carry 2x more current.

However, the heating of 2x more current in the 'better current path' is Power=I^2 * R ... So 2x more current is 4x more heating. So, that "better current" run can be the one that fails first.

The "wiring resistance" is a "ballast resistor" (technical electrical term)--That "governs" the shared current flow.

I have worked on equipment that had 5 or more parallel current paths (very expensive electronic test equipment). And I would find the power connectors between circuit boards "unzipping"... One connector (for me, I saw mostly connector failures) overheat and charred. Then another turning black, and a third just starting to turn brown.

And back to solar panels... Generally, you fuse/breaker each "series string" of panels when you have 3 or more parallel connected strings. That way, if you have one shorted string, the other 2+ strings will not overcurrent the one shorted string/panel (pop the fuse/breaker for the shorted panel/string).

Then from the "combiner box" (fuse/breaker box if used), parallel connected together, should be one pair of properly rated cables (I would avoid parallel connections--especially if the run is "short" -- a few feet or less).

Sorry for the word salad... The details behind many electrical design/code rules are based on interlocking issues and hard learned mistakes.

-Bill

Graham Parkinson

Wow - great informative post revealing the reasoning behind the code and issues - Thanks BB!

mike95490

stmoloud said:

.....Just to clarify a difference in terminology. I am not sure what is referred to here as a combiner box. I am taking all my PV cable (presently two pairs)  and terminating them to 2 x 16 way 100 A brass distribution bars inside the vehicle......

A combiner box is a safety requirement, when paralleling more than 2 PV stings.   If a panel goes bad, a good panel in parallel can't easily start a fire.   But if 2 good panels are feeding a bad 3rd panel, things can go badly. The specs for the "series fuse" are printed on the label on the back of nearly every PV panel.
 Combiner boxes are cheap insurance and a handy way to compare string A ,  B, C & D, trouble shoot issues and such.   Breakers are handier than fuses, because most fuses can't be used as a switch, but breakers can  The shiny metal attached to the upper part of the breakers, is the bus and has a large screw terminal for large wire.

BB.

Mike's picture, the bottom is the entry point for multiple solar "strings" with a breaker per parallel string. The upper single connection is for the run of a single pair of cables (generally the + terminal is fuse/breakered.

The details behind 1 or a pair of breakers per string has a backstory regarding floating and isolated power systems--another day.

Also, a good design choice is all exterior cables enter from the bottom of the box. If the cable/wiring enters from the top or side, there should be a drip loop to help avoid water following the water into your electrical box/panel.

-Bill

stmoloud

Thanks all. So to summarize. A 'string' is the same thing as an array. The size of a string is determined by the max capacity that any one particular solar controller can control. Therefore if I had two 20 amp controllers to control 40 amps, that is 2 strings. 

But since my new controller arriving soon in the post is 45 amp I can then say that I have just the 1 string.. So in that case I do not need a combiner box. If I had four panels totalling 40 amp max, I can terminate 4 positives into a positive distribution bar and likewise the 4 negatives into a negative distribution bar

I then replace my current 25 amp wires going into the charge controller with at least 50 amp-rated cables. I should also make sure there is a 50 amp fuse in the positive cable going between the positive distribution bar and the controller.

mcgivor

stmoloud said:

Thanks all. So to summarize. A 'string' is the same thing as an array. The size of a string is determined by the max capacity that any one particular solar controller can control. Therefore if I had two 20 amp controllers to control 40 amps, that is 2 strings. 

But since my new controller arriving soon in the post is 45 amp I can then say that I have just the 1 string.. So in that case I do not need a combiner box. If I had four panels totalling 40 amp max, I can terminate 4 positives into a positive distribution bar and likewise the 4 negatives into a negative distribution bar

I then replace my current 25 amp wires going into the charge controller with at least 50 amp-rated cables. I should also make sure there is a 50 amp fuse in the positive cable going between the positive distribution bar and the controller.

The term "string " is used to describe the topography of an array, how the modules, or panels are arranged to achieve desired voltages for MPPT controllers, I believe you may be confusing yourself, to save time explaining, I've attached a document which dose exactly that.

If however you were using single  modules in parallel with a PWM there are no strings but the point where all the modules connect in parallel is the combiner, the use of circuit breakers is needed to prevent good modules from backfeeding into a module that is in fault, technically a requirement for more than two modules in parallel, but  useful for trouble shooting. Fuses can be used but cannot be disconnected under load so therefore less convenient, the same applies to strings of modules.

Tell us exactly what you have for equipment, along with a decription of intend layout this will reduce the amount of guesswork.
 

BB.

And while I am a big believer in Fuses/Circuit Breakers--A bit of clarification.

When you design the series connected panels (sets the array working voltage), the wiring for that "series string" needs a fuse or breaker to protect the wiring/panel from short circuits (needed if you have 3 or more parallel connected "strings"). Fuses are OK, but circuit breakers can be nicer because you can turn off one or more strings to figure out if you have electrical issues (i.e., if you have a "dead" panel or bad electrical connection--Turn off on breaker at a time and see if the Amperage drops correctly).

You don't really need a fuse between the array and the charge controller. The wiring (and controller) will always safely carry the current. Using a circuit breaker as an on/off switch between array and charge controller can make it nicer to service (turn off power before working on charge controller).

You will need a breaker (or fuse) between the battery bank and the charge controller to protect the + wire from shorts (and controller shorts)--The battery is a source of very high current flow (100's of amps for even a moderate FLA battery bank). The breaker (or fuse) is there to protect the wiring from starting a fire (breaker/fuse is located near the battery bank/+ bus bar/source of the high current).

Again, using breakers is pretty nice. Both protection and On/Off. When you get to larger fuses (and fuse holders), circuit breakers are not that much more money (especially if you need to add a switch too--such as turn off the system for "winter"). And large (100-200+ Amp rated) fuse and fuse holders are not cheap either.

-Bill

stmoloud

Thanks mcgivor the document was exactly what I should have read long ago. I will stick to my present arrangement for now as I cannot think of any other way it can be done. 
Except I won't use two cables to route the amps that a bigger one should be taking anyway. But good to know it could be done in an emergency.
I do want to use breakers but am very limited in space to mount them.  
So inline fuses will have to do for now until I get to my off grid paradise and can arrange something better.