Wire size math question

Solar Expert Posts: 109 ✭✭✭✭
If P=E x I then can I say P = 12volts x 15A

P = 180 Watts

I used 15A for an AWG #14 max rating wire in this example, can I conclude by saying I can pass up to 180Watts of power through a 14 gauge wire at any given time?

Please correct me if I'm way off or let me know if its good logic.

Re: Wire size math question

The answer, you will love this, is sort of yes and no... :roll:

Yes, in the sense if you use the NEC guide for amperage rating of standard household wire--15 amps max x your working voltage = power

The sort of no--depends on lots of other things too... The correct insulation, the working temperature of the environment, if it is in conduit, the more wire you stuff in conduit, the hotter they run. Also, for NEC, the normal "maximum load" a circuit should be subjected to is 80% of the rated current. So:

80% * 15 amps * 12 volts = 144 watts (or 12 amps maximum)

Circuit breakers and fuses should normally not be operated at 100% rating (where they are expected to eventually open--there are a few CB from Outback and such that are rated at 100% operating load).

There are other issues too... A big one is wire resistance... For example, using the voltage drop calculator (thread here), a 100' (equal to a 50' run for a pair of wires) run of 14 awg wire, hot weather, and 15 amps will drop about 3.8 volts.

For a 120 volt circuit, that would mean 116.2 volts at the load. For a 12 volt circuit, that would mean 8.2 volts at the load.... The 120 VAC load would operate just fine... A 12 volt load will probably not function at all.

For a 12 volt load to operate that far (15 amps, 100' run, with 1 volt drop maximum), you would want an 8awg wire. For a 3% voltage drop (0.36 volts), you would want a 4 or 2 awg wire...

That is why we try to recommend higher voltages for long wire runs. Sending a bunch of 12 volts any distance is difficult/expensive.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 109 ✭✭✭✭
Re: Wire size math question

Thanks.. that makes sense, I guess I was just worried about wire size in my installation as right now everything is AWG 14.. but I have very short distances.. nothing over 30 feet. And I only have 2 lights dc on some awg 18 wire.. using about 600ma.. since today i upgraded my 36AH battery to a 106AH I was concerned about the same wire size.. for my panel its border line as my 80 watt panel will never generate more than 4.67A according to max current output of the manufacturer.. anyways I plan to put bigger wires if needed and add a dc panel with breakers.. i'm growing the system slowly as I learn.
• Solar Expert Posts: 109 ✭✭✭✭
Re: Wire size math question

also.. just thought of something "Crazy worst case scenario".. the wire from my battery to my charge controller is awg 14 cause i have small loads.. but since my battery is 106AH.. if i theoretically hook up a 106 AMP load for 1 hour the wire would melt almost for sure no? how do I protect against such a thing? dc panels with 1-15A breakers? so that the battery would never have to "give" more amps allowed by the breaker?
Re: Wire size math question

You look at the source of the "energy" for your system... In this case it is the battery--and it can supply 100's to 1,000's of amperes into a dead short (and melt your wiring/cause fires/etc.).

So, for your charge controller connection, you should have a fuse/breaker nearest as practicable to the battery that is rated 1/80%=125% of the maximum current of your solar charge controller and/or 100% of the wire's capacity (whichever is smaller).

For your loads, you do the same thing. If you have an 18 awg wire with a 600 mA load--your fuse would be somewhere between 125%*0.6amp=0.75A and 80% of the capacity of the 18AWG wire (somewhere around 80%*6A=4.8A; don't have a NEC table or the rating for your wire, just guessing 18AWG is around 6-8 amps max).

Remember, the fuse/breaker is to protect your wiring--not the device (load or charger). So, for example the battery here has "very high current potential" and needs fusing to protect anything connected. And the fusing is based on the wire's capacity. It is your choice if you choose to have, for example, 2x 18 awg circuits each with 4 amp fuses, or 1x 14 awg with one 15 amp fuse running to your loads.

The charge controller's output is limited--it cannot go any higher than the amount of power supplied by the solar panels... So a "dead" short will not increase the output and cause a fire (--gets a little more involved in that MPPT controllers can output more current than the solar panels because the MPPT controller acts like a stepdown "DC Transformer"--however a solar controller will not turn on if there is a dead short on the battery terminals--so it is not really a problem--plus the controller itself has built in current limits too).

-Bill

PS: Audio guys, and automotive parts places will have distribution fuse holders--where you can attach your heavy wire from the battery bus (keep length short, protected against electrical shorts). And then have your individual fuses for your various loads at that point.
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 109 ✭✭✭✭
Re: Wire size math question

but the main wire comming out of the load terminals of my prostar controller going to my main bus bar/panel would have to be very big then? isnt there a limit to how big of a gauge your screw terminals froim the charge controller can take? and for a 106AH battery what size should I use?
• Solar Expert Posts: 109 ✭✭✭✭
Re: Wire size math question

Would these be good for me?

http://theresourcestore.ca/proddetail.php?prod=MN-BB

Its a small inexpensive panel for DC, I think its what I need. and I could get either 1 15 AMP breaker or 2-3 5A breakers..

what do you think?
Re: Wire size math question

Those are nice breaker panels--A bit of overkill for a small system and probably pretty expensive, comparatively speaking.

I would look at places that supply audio systems for cars and such for a small fuse blocks instead.

Regarding the wiring from the battery... typically the battery cables only need to be heavy enough to manage the maximum load/charge current. And, typically, you run the "heavy wires" from the battery to a bus bar (or use the common connection on the battery for charge/load connection point). If this was NEC, yes, there should be a fuse in the battery wiring to protect against battery wiring shorts... Typically, for cars/boats/etc., the battery wiring is very heavy gauge and can handle the cranking amps of the starter--which is assumed to be near the maximum current capability of the batteries themselves--so people typically don't put a fuse in the main battery harness... For large battery banks, this justification starts to wear a bit thin (and a large fuse per string to protect the harness agasint shorts would be a good idea).

Wind-Sun on their NWAS store site, has a link to Blue Sea Systems' wire gauge calculator--try it out and see if it helps.

NWAS has a page specifically for battery cables here. You can see that 2z 15' of 4/0 cable is \$290... So, you don't want to oversize if you don't need too..

So, for yours, what is the maximum current (charge/load) that you expect?

Also, for wiring, you want the lowest voltage drop possible between the charge controller and the battery... A voltage drop of even 0.1 or 0.2 volts can dramatically undercharge the battery... So, you want short, relatively heavy wire from the controller to the battery (and battery interconnects), and calculate for 0.1 volt or less drop. For small chargers, this is not a big deal--for 80amp units--a bigger deal.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 109 ✭✭✭✭
Re: Wire size math question

i have less than 2 feet of awg 14 wire from the battery to the charge controller and i dont expect more than 15 amp current drawn at any given time .. thats even on the high side right now
• Solar Expert Posts: 109 ✭✭✭✭
Re: Wire size math question

another thing.. being a 3rd year apprentice electrician i have access to alot of old disconnect boxes or old panels we remove from existing installations that i get to keep but this is for AC stuff.. could I use it anyways ? or spend a few bucks and get proper dc stuff.. also i dont like the concept of fuses burning, i'd like to stick to breakers since i replace alot of fuse ac panels to breaker ac panels at work so i dont really like fuses because of the fact they are non reusable
Re: Wire size math question

Look for Square D "QO" type AC breakers... (thread post by Jim/Crewzer here).

You only need 48 VDC maximum of these breakers... The 150 VDC rating of those other breakers are fine too. Just purchase on price/availability for the smaller systems.

I understand and usually prefer breakers myself too... However, for smaller systems, they are sometimes really are difficult to justify... Also, if you have an aircraft junk yard in your area--I have used those types of breakers before too when rewiring an old airplane I used to have in another life.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 109 ✭✭✭✭
Re: Wire size math question

would this work? very basic I know and excuse my drawing abilities
Re: Wire size math question

Not really--Or I do not understand your sketch...

Think of the battery as the center of the universe (or the battery bus bar/common connection). The battery (usually) connects directly to the bus bar (or use the battery + and - posts as the common point for a small/simple system).

Then every wire that leads of the battery should go thorough a fuse. A 14 awg through a 15-20 amp fuse from the + connection to the charge controller. Another 1-4 amp fuse for your 18 awg from the battery to the 600 mA lights. Another 30 amp fuse and 10 awg wire for your 20 amp Battery charger. Etc.

The fuses should be as close to the battery + bus as reasonable. So that as much of the rest of the wiring is protected from shorts as possible.

If you use a breaker/fuse panel... Run a short heavy wire from the battery to the breaker/fuse block and run the smaller stuff from there.

All of the above fuses are in the "+" leads only. The ground / - lead from the battery is the other common bus point, and, if there is a metal ground/chassis/ground rod, a wire from the bus to the ground too. You don't want to put a fuse in both the + and - wires... You only want a fuse in the "+" leads (if there was a negative fuse that opened--it would leave the entire circuit energized and anyone working on, what they think is a "dead" circuit would be very surprised if they contacted the ground with the "+" hot circuit).

You could try to "share" the, for example, 15 amp solar charger circuit with the battery and a 15 amp/14 awg wire load... The reason it is not recommended is because if there was a short at the load end, you would have 15 amps from the solar charger and 15 amps from the battery--adding up to 30 amps available at the load--and possibly frying your wiring.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 109 ✭✭✭✭
Re: Wire size math question

I don't feed off the battery directly since my charge controller has a + and - load terminals to which i connected a bus bar, i thought that was the correct way to use the power, from the charge controller load terminals.
Re: Wire size math question

I believe the load connection in many of the controllers is limited to Xamps of current. I am not sure you need a fuse between the load connection and your load (assuming the wire is large enough to handle the maximum output current available from the load contact).

If you want/need to still put a breaker in there--yes, that is the way you would do it (keeping the wire from the controller to the breaker as short as practical).

The other place you need/want a breaker/fuse, is the connection from the battery to the solar charge controller. You want to protect the 14 awg from the battery to the destination (solar charge controller in this case) from any shorts in the wire run to the controller, shorts in the controller, or shorts on the load circuit.

The controller's LVD is only useful for very light loads--you don't, for example, want to put an inverter on the LVD. The inverters starting surge currents can "Trip" the LVD protection or damage the LVD output.

-Bill

And, the LVD output is not really good/accurate at disconnecting the battery in the case of over discharge--by the time the LVD disconnects, the battery is pretty much near 0% state of charge.
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 109 ✭✭✭✭
Re: Wire size math question

oh.. i have my 200w inverter on the lvd load.. you suggest i put it straight on the battery? do you think i could have damaged my lvd? i only used 13w lights and a sound system as a few tests ..
Re: Wire size math question

Sorry to sound so "wimpy"--it is a generic recommendation for pretty much all of the small LVD equipped controllers.

I don't design, sell, use, or repair them--so I can only pass the warnings, I have read, on (and the warnings make sense from an engineering point of view--I am looking for the original post--but cannot find it right now--so I may be all wet).

Have you damaged your LVD circuit--Don't know--but if everything is still working--then there is probably no problem.

If you are using an inverter--it already has its own internal LVD circuit too (to prevent damaging itself when battery/bus voltage is low). Using the LVD on the solar controller is, more or less, redundant.

I would suggest the XX amp fuse and XX awg based on the loads you are planning on drawing... If the loads don't have a lot of AC surge, and are light, you could use smaller fuses / wiring (say you are only running 12 watts of light on a 300 watt inverter--you can fuse smaller and run lighter wire if you want).

If you plan on 300 watt loads with large starting currents (say a fridge/freezer), then you will want to follow the recommendations of the inverter manufacturer for the proper wiring and fusing to support the inverter's surge capabilities (say 600 watts to start the motor, and less than 300 watts to run--you are now getting close to 60 amps at 12 VDC--So heavy wiring to prevent voltage drops, Low Voltage Disconnecting is needed).

For example, here is a very nice 300 watt true sine wave inverter from Morning Star that can run 600 watts for 10 minutes... From the operator's manual, they recommend a 100 amp fuse and a minimum of 6 awg wire.

If your loads are much less than 300 watts (or whatever), you can certainly downsize the wire/fusing requirements.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 9,583 ✭✭✭✭✭
Re: Wire size math question

Your sketch has no battery in it, so it's difficult to guess if it would work OK.
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• Solar Expert Posts: 109 ✭✭✭✭
Re: Wire size math question

I'm using a prostar 15m charge controller so the battery is connected to the battery + - terminals of the charge controller.