36 Volt Wire Chart

Joe J

12,24,48 Easy to find........Why not 36 volt?? I'll have 16.1 amps running my panels at 36 volt. That's about 12 gauge wire a/c. Running about 25 to 30 feet max. Why is there no chart? Something against 36 volts?

BB.

Re: 36 Volt Wire Chart

Engineering wise--Factor of 2 changes are "significant"... Changes less than 2 are almost "don't care"/not worth paying attention to.

(another engineering rule of thumb is the 10x difference--typically you can ignore the "thing" that is 1/10th as big/heavy/bright/etc. in your calculations).

So, the standard 6v/12v/24v/48v (and 120/240/480 VAC) progressions are all a factor of 2 larger than the previous size (and AWG wiring is ~3 awg size for every doubling of wire cross-sectional area--I.e., 8 awg wire is ~4x more cross-section vs 14 awg wire).

There are other voltages in history--8 volts and 32 volts used to be very common for off grid power (and getting something a bit higher voltage than 6 volt battery systems). And ~110 VDC too. And, from what I remember, 36 volts was common in older boats.

Another thread discussing some of this:

Why are standard battery bank voltages so low?

Anyway--What is the question about 36 volts? You can use a generic voltage drop calculator and just use 36v*3%drop=1.08 volt as your voltage drop.

-Bill

vtmaps

Re: 36 Volt Wire Chart

Joe J wrote: »

12,24,48 Easy to find........Why not 36 volt?? I'll have 16.1 amps running my panels at 36 volt. That's about 12 gauge wire a/c. Running about 25 to 30 feet max. Why is there no chart? Something against 36 volts?

Something's wrong with your numbers. In your other thread you wrote:

3 panels in series, 2 in parallel Total 6 panels: 870 watts 16.1 amps wired for 36 volts

For the purpose of calculating cable loss it is usual to use the Vmp and Imp of the combined array. 16.1 amps at 36 volts is not 870 watts, therefore your numbers are wrong. Do you know the actual Vmp and Imp of the panels?

I suspect that the 36 volts is NOT your combined Vmp. If it were, a single panel would not be able to charge a 12 volt battery. If the amps and watts are correct, then your combined Vmp is 54 volts (18 volts per panel).

These numbers (16.1 amps and 54 volts) will result in a 2.9% voltage drop and 25.2 watts lost in a 30 ft 12 AWG cable. If you go to 10 AWG cable your voltage drop will be 1.8% and your power loss will be 15.8 watts.

To answer your other question: why is there no chart for 36 volts? Same reason there is no chart for 37 volts or 38 volts. There are just too many voltages to have a chart for each. As Bill mentioned, use a voltage drop calculator.

--vtMaps

niel

Re: 36 Volt Wire Chart

what bill says is pretty much true, but i agree there is a place for 36v as many items do run at 36v. this is also a good battery voltage for 2 60 cell pvs in series if optimized with mppt, but pwm would work too with a tad of loss being it is not at the optimum 36 cell/12v. 36v is not common in controllers and most likely would be best if an mppt cc is used to maximize on the pv input voltage otherwise you would need 3 36 cell pvs in series to work it in a pwm controller. even a 72 cell pv in series with a 36 cell pv works if the currents are not too far off from one another. pvs with the 36 cell multiples are still out there, but are getting to be fewer and fewer.

the gauge of the wire you have to watch because the ac you refer to is 120v. you should determine the actual v drop % using a v drop calculator for the actual voltage encountered. as most nominal voltage values tend to be difficult to determine, i have been recommending using the total vmp as the voltage for v drop determinations. the nominal voltages are lower and would represent a higher % v drop loss. in the case of 3 12v pvs in series this is easy as it is 36v nominal.

to add to that the distance the wire is made to carry that current will also increase the v drop loss so it is imperative to use a good v drop calculator to determine the actual v drop %.

i did a quick go through based on the numbers you gave and extrapolated some other numbers to be rough and come up with a v drop % in the area of about 3.55%. now that was for a 30ft wire run and if you make it a 25ft wire run it will be at about 3%. i also used the rough vmp voltage of 55v and not the nominal 36v in the calculation which will show a loss even higher than what i came up with. some like to keep v drops less than 3% and some like it less than 2%. that 3% sounds good right? nope as you will have v drops from the cc to the battery too that add to the v drop % loss from the pvs to the cc. you want the total losses to be either the 2% or 3% and not just individual sections. the voltage from the cc to batteries is often lower with higher currents so v drop losses there can be significant and what's worse is that if the v drop is significant it can fool the cc into thinking a battery has been fully charged when in reality it isn't. for instance if you have a .2v v drop on a 12v battery set for 14.4v as the absorb voltage it will do this prematurely at 14.2v. raising the voltage by .2v to compensate does not work because as the current tapers during absorb the v drop will change and it will end in a higher than normal voltage that could hurt some batteries that are sensitive to this. this holds true unless you have a cc that does remote v sensing. the morningstar ccs are the only ones off hand that i believe have this feature, but even with that feature one should go for the lowest losses possible between the cc and batteries and is why most other cc manufacturers do not use the remote voltage sensing. do not confuse this with remote temperature sensing as this is different and recommended for any cc to have.

Joe J

Re: 36 Volt Wire Chart

vtmaps wrote: »

Something's wrong with your numbers. In your other thread you wrote:

For the purpose of calculating cable loss it is usual to use the Vmp and Imp of the combined array. 16.1 amps at 36 volts is not 870 watts, therefore your numbers are wrong. Do you know the actual Vmp and Imp of the panels?

I suspect that the 36 volts is NOT your combined Vmp. If it were, a single panel would not be able to charge a 12 volt battery. If the amps and watts are correct, then your combined Vmp is 54 volts (18 volts per panel).

Yes, you are correct. I have to watch my terminology. I was trying to say I'm wiring 3- 12 volt panels in series to produce 36 volts. Not combined Vmp. I'm also going to use #8 wire for the run.