Wire Gauge

[zander1976]

Hello Everybody,

Sorry if this seems like a stupid question but I don't feel like burning my house down. I have no experience at all with wiring so sizes or gauge are concerning me. I have done some looking and really don't know anything about wires. So I will get straight to the questions.

For AC No 14 gauge wire are ok for 15 amp circuits and No 12 gauge cable is good for 20 amp circuits. Does that make sense? The smaller the wire the more current it can handle?

For DC, what gauge should I use for 12, 24 or 48 volt solar setups. I guess its more related to the amps so 3x6 solar cells at 18v produce what 3 amps each? Thats a lot of amps, wait never-mind they are connected in series so the amps will stay at 3amps and the volts go up. I get it. So I need a cable size to handle 3 to 4 amps give or take. Is bigger cable better like should I look for cable that is bigger in cases were I am unsure like my wind generating ( 40 VDC ametec nominal power that I can't see the amps on ).

So the cables coming out of my 18 volt 3 amp solar panel are made to handle 3 amps. When I connect 2 of them in parallel do I need to make sure that it can now handle a 6 amp circuit? Or do I just have my solar panels all leading to a main guide wire that  can handle higher amps. What do you do if you aren't sure how many you are going to have in parallel?

Code: [Select]

Panels           [] [] []
Small wire        |  |  |
Main wire         =======
                        ||

                   Inverter

Do I just put blocking diodes on the end of the small cables so power only runs down the line built to handle large currents.

Thanks,
Ben

[zander1976]

Volts don't effect the cable size needed correct? So I can assume that there must be some rule of thumb, 10 amp or bigger should be X gauge cable and 20 amps or bigger you should use Y gauge?

Thanks again

[joestue]

The wires need to be sized so they don't overheat.
most of the time that means you follow rules of thumb, such as 15 amps for a 14 awg wire, or 20 for a 12 gauge wire.
The max current loading for temperature rise follows the surface area of the conductor, not just the area of the wire, notice that mm^2/amp decreases as the wire gets bigger when you follow the NEC tables.

As the value of the electricity rises, so does the amount of copper you should allocate for the transmission.
how much is up to you, and the cost of the wire.  generally for solar at 4$/watt, for every watt you can reduce in transmission losses, if it costs less than 4$ in copper.. buy more copper.

the math is easy, its just ohms law, and a wire chart off wikipedia, or use one of the thousands of online calculators.

[dave ames]

 hey ben,

 if we can throw a few carrots into the stew.

 it's possible to size a safe system using ampacity tables as our guide and end up with a system that does not function.

 if we size our system with a goal of %2 voltage drop (especially non mppt battery based pv systems) the system will meet ampacity requirements by default.

 this is a problem we see with some "street legal" sparkies..a %10 voltage drop might be a go on some ac systems but will cripple a solar charging system. our "operating window" as far as voltage goes is very narrow with a low voltage system, and as module temps rise the voltage sags lower still..easy to loose a few volts in the summer at the pv so we can't afford to loose much more in the wire run.

your diagram looks good just add a pv combiner and overcurrent protection

Code:
Panels           [] [] []
Small wire        |  |  |
pv combiner box  ** with ocpd (fuses/breakers)
Main wire         =======
ocpd           fuse/breaker
charge controller  CC
ocpd           fuse/breaker
battery bank      BB
ocpd           fuse/breaker
                        ||

                   Inverter

 most charge controllers have the diode/or equivalent built in but you may need them if you are controlling the battery from the back end (in parallel as in dump load)

 my vote is to size the wire for voltage drop!  <-then confirm ampacity.

http://www.affordable-solar.com/wire.charts.htm

 stir the stew!

it's all good fun, dave

[TomW]

Zander;

Wire gauge number to wire size is an inverse relationship.

#0 is bigger than #4, etc.

That seemed to confuse you in your original post by this line:

For AC No 14 gauge wire are ok for 15 amp circuits and No 12 gauge cable is good for 20 amp circuits. Does that make sense? The smaller the wire the more current it can handle?

Tom

[zander1976]

Hey,

Many thanks

I thought gauge and size were different ways of saying the same thing. Obviously I was completely wrong

I check out that chart and I just want to make sure I understand it correctly. My 3 amp panel fits between 2 and 4 amps. So I will go with 4 amps and #6 gauge wire because I want to run at least 50 feet and that is under the 72 feet it states.

The second chart I just now noticed is wire size not gauge. I was going to say something silly like so a #6 wire can handle 65 amps at 75c. But that would be wrong.

My brain has high ohms I guess

[ghurd]

Wow!  I over kill stuff, but #6 for a 50' run with a 50W PV is pretty crazy!

Would be adequate, for most systems, to use #12.  Looses 0.49V.  I can live with that.

#10 looses 0.308V.
#6 looses 0.122V.
#6 is very expensive.  Chasing that last 0.186V at 3A will be a very expensive half a watt, which really will not be a half a watt more into the battery.

Of course, a larger wire now will allow for more PV power later.

Two links that may clear up some of the fog-
http://www.windsun.com/Hardware/Wire_Table.htm
http://www.powerstream.com/Wire_Size.htm
Powerstream has a calculator at the bottom that could help a lot.

G-

[zander1976]

Ohh, lol.

That last link is fun to play with. Yeah, I seemed to have missed something along the way. I just played around with No. 6 Cable on that site and I figured I could have 10 50watt cells on that. I see what you mean by over doing it. Thats kind of like hanging a picture frame using a sledge hammer. Well doing my math i could expanded for years before I would have to replace the main wire

The voltage drop was 1.9 for 30amps over 50' ( 10 3amp solar cells ). Since the panels are actually 18 volts the drop of 2 volts to 16 volts would still be well above the 14 volts required to charge a battery.

Thanks

[joestue]

You don't have 30 amps.
50 watts max at about 18 volts is about 2.77 amps.  (in series...)
one hundred feet of 16 gauge wire is .4 ohms and this will drop 1.1 volts.

[ghurd]

He stated 3A. 
I stated 50W, which typically would operate at about 3A under good conditions.

[joestue]

doesn't really make a difference anyway.
3 amps might even be the short circuit current for all we know.

[zander1976]

Short circuit current? What is open volts and open current?

[zander1976]

I get what your saying (takes a while sometimes)   Thanks for the help.

[ghurd]

Short circuit current is amps while short circuited.  (Isc)
This is at 0V, or it would not be a short circuit.

Open circuit volts is the voltage with no amps flowing.  (Voc)
If there were amps flowing, it would not be open circuit.

There is no open current.
If it is open, current can not flow.

Max Power amps and volts ratings are at the same time.  (Vmp and Imp)

Typically, with a battery charging system (not using an MPPT controller), the panel will operate at a lower voltage and higher current than Max Power ratings.
Often it is good to try to keep the panel operating below about 15.0 or 15.5V, because the amps will be a bit higher.  A decent wire size for a low voltage loss, and sometimes a Schottky for the diode if required, will keep the panel up around the sweet spot.

There is a chart and PV ratings of a fairly typical panel here,
(though the panel's interior construction is a bit unusual)
Notice how the current stays pretty flat until about 15.5V, where it stats to drop at about 16V, then falls like a rock at about 18V.
Meaning they are designed to do well when losing a volt or 2 in wire and diode loss.  If the battery is high enough to put the operating voltage up in the charts knee, then its about charged anyway, or at least taking the charging amps about as well as it can.
http://www.bp.com/liveassets/bp_internet/solar/bp_solar_usa/STAGING/local_assets/downloads_pdfs/pq/BP350J_01-10.pdf

G-

[Ungrounded Lightning Rod]

I thought gauge and size were different ways of saying the same thing. Obviously I was completely wrong

They are.  They just count in different directions.  B-)

I hear the wire gauge system originated as a count of how many dies the copper bar was pulled through on its way to becoming the final wire.  Thinner wire, more dies.  (Of course even if that is true it was quickly standardized for size, leaving the manufacturers free to start with any convenient bar size and use as many dies as they find optimal.)

[zander1976]

Short circuit current is amps while short circuited.  (Isc)
This is at 0V, or it would not be a short circuit.

Open circuit volts is the voltage with no amps flowing.  (Voc)
If there were amps flowing, it would not be open circuit.

There is no open current.
If it is open, current can not flow.

Max Power amps and volts ratings are at the same time.  (Vmp and Imp)

Typically, with a battery charging system (not using an MPPT controller), the panel will operate at a lower voltage and higher current than Max Power ratings.
Often it is good to try to keep the panel operating below about 15.0 or 15.5V, because the amps will be a bit higher.  A decent wire size for a low voltage loss, and sometimes a Schottky for the diode if required, will keep the panel up around the sweet spot.

There is a chart and PV ratings of a fairly typical panel here,
(though the panel's interior construction is a bit unusual)
Notice how the current stays pretty flat until about 15.5V, where it stats to drop at about 16V, then falls like a rock at about 18V.
Meaning they are designed to do well when losing a volt or 2 in wire and diode loss.  If the battery is high enough to put the operating voltage up in the charts knee, then its about charged anyway, or at least taking the charging amps about as well as it can.
http://www.bp.com/liveassets/bp_internet/solar/bp_solar_usa/STAGING/local_assets/downloads_pdfs/pq/BP350J_01-10.pdf

G-

Thats really cool. Thanks for such a detailed explanation. So basically, Voc is the max volts it could produce if it produced 0 amps ( speed but no flow)  and Vmp or Isc is the max amps if it produced no volts ( flow but no speed ). Balancing the 2 gives the best results. Sounds like the max potential volts or max potential current. Can you actually produce open volts or short currents or is it just something you can get close to by doing something foolish like a hammer on 2 posts for Isc. I have no idea how you create open volts.

Thanks,

[TomW]

Think of volts as "pressure" and amps as "flow" for the water analogy. It is good enough for the basics.

Tom