Wire gauge calculation

[JRRNeiklot]

I'm trying to decide on wire sizing.  Am I correct that in calculating wire run from panels to controller, I use the panel voltage?  Most online calculators only have an option for 12 or 24 volt.  I'm going to have 60 volts at the panels.

Thanks.

 

[DLTooley]

The higher the voltage the less wire you need, that’s one of the big advantages of higher voltage systems.

Total amps is also a limitation.

I would recommend finding a general source that specifically answers your question as well as all the other aspects of your system.

I do not know of such a source, someone else may.

 

[desert_sailing]

Here is a nifty chart you may find helpful.'

http://assets.bluesea.com/files/resources/newsletter/images/DC_wire_selection_chartlg.jpg

https://www.bluesea.com/resources/1437

the length for the chart is based on a "round trip" full circuit run.

 

[Weight]

Most panels used on vehicles have 10 awg. That has low voltage drop at the amperes from most panels used on vehicles.

 

[MrAlvinDude]

The basis for all of the voltage drop calculations is Ohm's Law.

Volt = Amp x Ohm.

Some Wire charts will show how many ohm can be expected by each gauge of wire.
This chat does show Ohm values: https://www.powerstream.com/Wire_Size.htm


For instance 10 Awg has 0.9989 ohm per 1000 feet (or 3.276392 ohm per 1000 meters).

So one foot is: 0.9989 ohm / 1000 = 0.0009989 ohm
Also know as 0.9989  milli ohm (mΩ)

If you have a 10 awg wire of 6 feet, the ohm value for that wire is  6 x  0.9989 mΩ  = 5.9934 mΩ  (milli ohm)
5.9934 mΩ = 0.0059934 ohm


Next you need to know the expected amps, or the max amps, in order to determine the max voltage drop.

At 10 amp on that 10 awg wire (6 feet long) the voltage drop will be:  10A x  0.0059934 ohm = 0.059934 Volt 
0.059934 Volt  = 59 milli Volt (mV)  an easier/shorter number is 0.06 Volt

So a 6 foot, 10 awg wire, at 10 amp load, would hardly have any voltage drop, as 0.06V is not much.


At 50 amp on that 10 awg wire  the voltage drop will be:  50A x  0.0059934 ohm = 0.2967 Volt 
0.2967 Volt  = 296 milli Volt (mV) 
an easier/shorter number is 0.3 Volt
So a 6 foot, 10 awg wire, at 50 amp load, will see a 0.3 voltage drop.

and this voltage drop is the same regardless if origin is 12V or 60V


It is not recommended to do more than about 55 amp on a 10 Awg wire.
If one would however to do a 70 amp load on a 10awg wire, the numbers would be:

At 70 amp on that 10 awg wire (of 6 feet) the voltage drop will be:  70A x  0.0059934 ohm = 0.419538 Volt 
0.419538 Volt  = 419 milli Volt (mV) 
an easier/shorter number is 0.5 or 0.4 Volt
So a 6 foot, 10 awg wire, at 70 amp load, will see just above 0.4 voltage drop. But the wire might get very hot, because 55A is the recommende toplimit for a 10 awg wire. 

and this voltage drop is the same regardless if origin is 12V or 60V




hmmm.... perhaps it would be easy to put these numbers into as spread sheet.


Do you have any specific numbers that you need to look at?

 

[MrAlvinDude]

I have made a google spreadsheet where it is easy to enter the numbers, and get a calculation of voltage drop for any given length of wire.

I don't have much experience with google documents or google spreadsheets, so please let me know if you experience any trouble using this sheet. Ok?

https://docs.google.com/spreadsheets/d/1KncneOE-_E4YgIXjwT7FOVpx-MGnUbVUFzZjNwXiZmk/edit?usp=sharing


One issue that I do expect is, that only one person might be able to  enter numbers at any time. As in everyone will see the same numbers, that anyone enters.
So please copy the sheet to you own Google drive (I have currently no clue how that is done),
or wait for calculations, until you see no change in numbers. Ok?


This is basically the content of the spreadsheet:


Calculate expected voltage drop in a wire

[font=Tahoma, Verdana, Arial, sans-serif]Chart for Ohm per 1000 feet: https://www.powerstream.com/Wire_Size.htm[/font][/SIZE]
Ohm per 1000 feet [font=Tahoma, Verdana, Arial, sans-serif]0.9989 [/font]Ohm 
Ohm per foot 0.0009989 Ohm = 0.9989 milli ohm (mΩ) per foot

Max amp load 10 Ampere
Voltage drop per foot 0.009989 Volt =  9.989 milli Volt (mV) per foot

Wire length in feet 6 feet 
Voltage drop in wire 0.059934 Volt = 59.934 milli Volt (mV) for the wire

Note only: this wire is 10 Awg

 

[JRRNeiklot]

Do you have any specific numbers that you need to look at?

I have 750 watt panels, the wire run is about 75 feet one way.  The panels are 30 volt, 8.53 amp, wired in series, so 90 volts, 8.53 amps.

Thanks.

 

[MrAlvinDude]

For a 10 awg wire, with plus wire 75 feet, minus wire 75 feet, total 150 feet, and max of 8.53 amps, I get the following result:

Calculate expected voltage drop in a wire 

Ohm per 1000 feet 0.9989000 Ohm 
Ohm per foot 0.0009989 Ohm 0.9989 milli ohm (mΩ) per foot 

Max amp load 8.53 Ampere 
Voltage drop per foot 0.008520617 Volt = 9 milli Volt (mV) per foot 
                             0.01 Volt per foot 

Wire length in feet 150 feet --- If you are using both plus and minus wire, then remember to double the length 
Voltage drop in wire 1.27809255 Volt 1= 278 milli Volt (mV) for the wire 
                           1.28 Volt for the wire 

Note only: this wire is 10 Awg 

Maybe interesting info: 
Energy loss per foot 0.07268086301 Watt = 72.68 milli Watt (mW) per foot 
                               0.1 Watt per foot 

Energy loss in the wire 10.90212945 Watt = 10,902.13 milli Watt (mW) in the wire 
                               10.9 Watt in the wire

Or in other words, the expected total voltage drop is 1.28 Volt , for the complete 150 feet of wire (75 feet plus + 75 feet minus) on a 10 awg wire, at a maximum of 8.53 amp


Just for my own curiosity, I also calculated how many watts is lost in the wire,
with 1.28V and 8.53 amp = 10.9 watt, lost in the 150 feet of 10 awg wire.

 

[MrAlvinDude]

If you were wanting to save a bit on the cost of wire, say using 16 awg wire in sted, then I get the following numbers:

Calculate expected voltage drop in a wire					
					
Ohm per 1000 feet 4.0160000 Ohm	
Ohm per foot 0.004016 Ohm = 4.016 milli ohm (mΩ) per foot	
					
Max amp load 8.53 Ampere			
Voltage drop per foot 0.03425648 Volt = 34 milli Volt (mV) per foot	
                            0.03 Volt per foot	
					
Wire length in feet 150 feet --- If you are using both plus and minus wire, then remember to double the length		
Voltage drop in wire 5.138472 Volt = 5138 milli Volt (mV) for the wire	
                         5.14 Volt for the wire	
					
Note only: this wire is 16 Awg 			
					
Maybe interesting  info: 					
Energy loss per foot 0.2922077744 Watt = 292.21	milli Watt (mW) per foot	
                              0.3 Watt per foot	
					
Energy loss in the wire 43.83116616 Watt = 43,831.17 milli Watt (mW) in the wire	
                               43.8 Watt in the wire

Or in other words, a complete voltage drop of 5.14 Volt for the length of 150 feet, of 16 awg wire, at max 8.53 amp. 

And a loss of 43.8 watt of energy in the 150 feet, of 16 awg wire, at max 8.53 amp


A 16 awg wire has a recommended a max rate of 22 amp. 
But because of the length of wire, then the total voltage (and watt) loss in the wire, maybe getting to a point ,where a heavier wire might very well be worth the extra cost. 

 

A 12 awg wire gets these numbers: 

Calculate expected voltage drop in a wire					
					
Ohm per 1000 feet	1.5880000	Ohm	
Ohm per foot	0.001588	Ohm	1.588	milli ohm (mΩ) per foot	
					
Max amp load	8.53	Ampere			
Voltage drop per foot 	0.01354564	Volt	14	milli Volt (mV) per foot	
			0.01	Volt per foot	
					
Wire length in feet	150	feet	If you are using both plus and minus wire, then remember to double the length		
Voltage drop in wire	2.031846	Volt 	2032	milli Volt (mV) for the wire	
			2.03	Volt for the wire	
					
Note only: this wire is	12	Awg 			
					
Maybe interesting  info: 					
Energy loss per foot	0.1155443092	Watt	115.54	milli Watt (mW) per foot	
			0.1	Watt per foot	
					
Energy loss in the wire	17.33164638	Watt	17,331.65	milli Watt (mW) in the wire	
			17.3	Watt in the wire

[font=Tahoma, Verdana, Arial, sans-serif]Or in other words, a complete voltage drop of 2.0[/font][font=Tahoma, Verdana, Arial, sans-serif] Volt for the length of 1[/font][font=Tahoma, Verdana, Arial, sans-serif]50 feet,[/font][font=Tahoma, Verdana, Arial, sans-serif] of 12 awg wire, at max 8.53 amp. [/font]

[font=Tahoma, Verdana, Arial, sans-serif]And a loss of 17.3[/font][font=Tahoma, Verdana, Arial, sans-serif] watt of energy in the 150 feet,[/font][font=Tahoma, Verdana, Arial, sans-serif] of 12 awg [/font][font=Tahoma, Verdana, Arial, sans-serif]wire, at max 8.53 a[/font][font=Tahoma, Verdana, Arial, sans-serif]mp


Are these calculations of expected voltage and watt loss of use to you? [/font]

 

[JRRNeiklot]

Thank you very much! I'm using 8 awg wire, so I should be good to go.

 

[MrAlvinDude]

Yes you are.
8 awg is very good in this particular context.