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Voltage Drop/Wire Sizing
Voltage Drop is the decrease in the voltage along your 12V electrical wire.  The longer the cable, the larger the drop.

 All lamps in your lighting plan require 11-12V to glow correctly.  The factors that determine acceptable lamp performance (minimum voltage drop) are:  
  • Shortest possible cable lengths
  • Transformer regulation
  • Cable (or wire) gauge
  • Lamp load
What happens if I do not account for Voltage Drop?

 Your lamps will decrease in brightness the further they are from the transformer.
How do I minimize Voltage Drop and maximize Light Output?
  1. 1.Keep wire lengths as short as possible
  2. 2.Select the correct wire gauge (the smaller the wire, the greater the voltage drop)
How do I calculate Voltage Drop for each fixture?

 You must know these three values and plug the numbers into this basic formula:

Voltage Drop = A x B
C
  1. A.= Length of the wire
  2. B.= Total # of watts for all lamps on that wire run
  3. C.= Cable constant (determined by wire size)

If the voltage drop for any fixture is greater than 2V,
you must change the layout, or the cable(s)
or the transformer so that that fixture is not dimmed.
WIRE SIZE
 CABLE CONSTANT
#10
 11920
#12
 7500
#14
 3500
#16
 2200
#18
 1380

How do I keep wire lengths really short?

 Use these design guidelines to minimize voltage drop.
  • Avoid a single long cable with all your fixtures attached to it! (“Standard Run”)
  • Create several runs going in 2 or more directions, attached to the same transformer terminals. (“Multiple Runs”)
  • Use heavier gauge cable for the primary cable attached to the transformer. Add lower gauged cable off the end of the heavier cable forming a “T”. (“T-Run”)
  • Use a single cable that begins and ends at the transformer terminal to reduce voltage drop and produce more uniform light output. Polarity must be matched. (“Loop Run”)
  • Center the transformer in the middle of the run.
  • Substitute slightly lower voltage lamps to reduce overall wattage
  • Use multiple 12V transformers with separate runs but all fed from a single 120V source.
  • Add small transformers to the end of the run.

How do I select the right wire gauge?

 To prevent voltage drop, you must select the right cable for a 12V system.  Distance of the run and wattage load on the run will determine the cable gauge/size. (NOTE: Total Cable Length refers to each single run from a transformer. Multiple runs are commonly used from a single transformer.)
A quick guide is:
  • Don’t use 16-gauge cable for more than 150 watts or 100 feet
  • Don’t use 14-gauge cable for more than 200 watts or 150 feet
  • Don’t use 12-gauge cable for more than 250 watts or 200 feet
50% is the minimum light output for the last fixture on any run. If you know the value of light output at the end of your wire length, the % Light Output Chart below identifies the required wire size, length of straight run and wattage load.

Low Voltage Cable
Selection Guide
Total cable length (ft) Max Watts on ONE Cable Run
#16
wire		 #14
wire		 #12
wire		 #10
wire
300 NA NA 0-60 0-90
200 NA 0-60 90 130
100 75 120 175 275
80 95 140 250* 350
50 150 215* 250* 360*
40 155* 215* 250* 360*
30 155* 215* 250* 360*
20 155* 215* 250* 360*
10 155* 215* 250* 360*
*maximum wire capacity
% LIGHT OUTPUT

Cable Size		 Distance
from
Transf
(ft)		 Load (Watts)
50
 100 150 200 250 300
12
 10 99 98 97 95 94 89
20
 98 95 89 83 80 79
30
 97 89 82 82 72 70
40
 95 83 79 71 63 56
50
 90 80 73 63 54 52
80
 83 71 56 45 35 26
100
 80 63 52 35 25 20
200
 63 35 20 10 7 5
300
 52 20 8 5 NA NA
10
 10 99 99 98 97 96 92
20
 99 97 92 91 88 84
30
 98 92 89 84 81 80
40
 98 91 84 80 78 73
50
 97 88 81 78 72 65
80
 90 80 73 63 55 51
100
 88 78 65 55 45 40
200
 79 55 40 36 18 11
300
 65 40 21 11 8 6
voltage drop chart for #8-2 wire
watts
40
 80 120 160 200 240 280 300 340 384
amps
3.3
 6.7 10 13.3 16.7 20 23.3 25 28.3 32
distance
25
0.1
 0.2 0.3 0.4 0.5 0.6 0.7 0.8 The NEC (National Electrical Code) limits all secondary wire runs to a maximum of 20 amps
50
0.2
 0.4 0.6 0.9 1.1 1.3 1.5 1.6
75
0.3
 0.6 1 1.3 1.6 1.9 2.2 2.4
100
0.4
 0.9 1.3 1.7 2.1 2.6 3 3.2
125
0.5
 1.1 1.6 2.1 2.7 3.2 3.7 4
150
0.6
 1.3 1.9 2.6 3.2 3.8 4.5 4.8
175
0.7
 1.5 2.2 3 3.7 4.5 5.2 5.6
200
0.8
 1.7 2.6 3.4 4.3 5.1 6 6.4
225
0.9
 1.9 2.9 3.8 4.8 5.8 6.7 7.2    
250
1
 2.1 3.2 4.3 5.3 6.4 7.5 8    
275
1.1
 2.3 3.5 4.7 5.9 7 8.2 8.8    
300
1.2
 2.6 3.8 5.1 6.4 7.7 9 9.6    
325
1.3
 2.8 4.2 5.5 6.9 8.3 9.7      
350
1.4
 3 4.5 6 7.5 9        
375
1.5
 3.2 4.8 6.4 8 9.6        
400
1.6
 3.4 5.1 6.8 8.5 10.2        
425
1.7
 3.6 5.4 7.3 9.1          
450
1.8
 3.8 5.8 7.7 9.6          
475
1.9
 4.1 6.1 8.1 10.1          
500
2
 4.3 6.4 8.5            
525
2.1
 4.5 6.7 9            
550
2.2
 4.7 7 9.4            
575
2.3
 4.9 7.4 9.8            
600
2.5
 5.1 7.7 10.2            
625
2.6
 5.3 8              
650
2.7
 5.5 8.3              
675
2.8
 5.8 8.6              
700
2.9
 6 9              
725
3
 6.2 9.3              
750
3.2
 6.4 9.6              
775
3.3
 6.6 9.9              
800
3.4
 6.8 10.2              
825
3.5
 7                 
850
3.6
 7.3                
875
3.7
 7.5                
900
3.8
 7.7                
925
3.9
 7.9                
950
4.1
 8.1                
975
4.2
 8.3                
1000
4.3
 8.5                


voltage drop chart for #10-2 wire
watts
18
 35 70 105 140 175 210 245 280 288
amps
1.5
 2.9 5.8 8.8 11.7 14.6 17.5 20.4 23.3 24
distance
25
0.1
0.2 0.3 0.5 0.6 0.8 0.9 1.1 1.3 1.3
50
0.2
 0.3 0.6 0.9 1.3 1.6 1.9 2.2 2.5 2.6
75
0.2
 0.5 0.9 1.4 1.9 2.4 2.8 3.3 3.8 3.9
100
0.3
 0.6 1.3 1.9 2.5 3.2 3.8 4.4 5 5.2
125
0.4
 0.8 1.6 2.4 3.2 3.9 4.7 5.5 6.3 6.5
150
0.5
 0.9 1.9 2.8 3.8 4.7 5.7 6.6 7.6 7.8
175
0.6
 1.1 2.2 3.3 4.4 5.5 6.6 7.7 8.8 9.1
200
0.6
 1.3 2.5 3.8 5 6.3 7.6 8.8 10.1 10.4
225
0.7
 1.4 2.8 4.3 5.7 7.1 8.5 9.9    
250
0.8
 1.6 3.2 4.7 6.3 7.9 9.5      
275
0.9
 1.7 3.5 5.2 6.9 8.7 10.4      
300
1
 1.9 3.8 5.7 7.6 9.5        
325
1.1
 2 4.1 6.1 8.2 10.2        
350
1.1
 2.2 4.4 6.6 8.8          
375
1.2
 2.4 4.7 7.1 9.5          
400
1.3
 2.5 5 7.6 10.1          
425
1.4
 2.7 5.4 8            
450
1.5
 2.8 5.7 8.5            
475
1.5
 3 6 9            
500
1.6
 3.2 6.3 9.5            
525
1.7
 3.3 6.6 9.9            
550
1.8
 3.5 6.9 10.4            
575
1.9
 3.6 7.2              
600
1.9
 3.8 7.6              
625
2
 3.9 7.9              
650
2.1
 4.1 8.2              
675
2.2
 4.3 8.5              
700
2.3
 4.4 8.8              
725
2.3
 4.6 9.1              
750
2.4
 4.7 9.5              
775
2.5
 4.9 9.8              
800
2.6
 5 10.1              
825
2.7
 5.2 10.4              
850
2.8
 5.4                
875
2.8
 5.5                
900
2.9
 5.7                
925
3
 5.8                
950
3.1
 6                
975
3.2
 6.1                
1000
3.2
 6.3                

voltage drop chart for #12-2 wire
watts
20
 40 60 80 100 120 140 160 180 192
amps
1.7
 3.3 5 6.7 8.3 10 11.7 13.3 15 16
distance
25
0.1
 0.3 0.4 0.5 0.7 0.8 1 1.1 1.2 1.3
50
0.3
 0.5 0.8 1.1 1.4 1.6 1.9 2.2 2.4 2.6
75
0.4
 0.8 1.2 1.6 2 2.4 2.8 3.2 3.7 3.9
100
0.5
 1.1 1.6 2.2 2.7 3.2 3.8 4.3 4.9 5.2
125
0.7
 1.4 2 2.7 3.4 4.1 4.7 5.4 6.1 6.5
150
0.8
 1.6 2.4 3.2 4.1 4.9 5.7 6.5 7.3 7.8
175
1
 1.9 2.8 3.8 4.7 5.7 6.6 7.6 8.5 9.1
200
1.1
 2.2 3.2 4.3 5.4 6.5 7.6 8.6 9.7  
225
1.2
 2.4 3.7 4.9 6.1 7.3 8.5 9.7    
250
1.4
 2.7 4.1 5.4 6.8 8.1 9.5      
275
1.5
 3 4.5 5.9 7.4 8.9 10.4      
300
1.6
 3.2 4.9 6.5 8.1          
325
1.8
 3.5 5.3 7 8.8          
350
1.9
 3.8 5.7 7.6 9.5          
375
2
 4.1 6.1 8.1 10.1          
400
2.2
 4.3 6.5 8.6            
425
2.3
 4.6 6.9 9.2            
450
2.4
 4.9 7.3 9.7            
475
2.6
 5.1 7.7 10.3            
500
2.7
 5.4 8.1 10.8            
525
2.8
 5.7 8.5              
550
3
 5.9 8.9              
575
3.1
 6.2 9.3              
600
3.2
 6.5 9.7              
625
3.4
 6.8 10.1              
650
3.5
 7                
675
3.7
 7.3                
700
3.8
 7.6                
725
3.9
 7.8                
750
4.1
 8.1                
775
4.2
 8.4                
800
4.3
 8.6                
825
4.5
 8.9                
850
4.6
 9.2                
875
4.7
 9.5                
900
4.9
 9.7                
925
5
 10                
950
5.1
 10.3                
975
5.3
                  
1000
5.4
                  

How do I select the right transformer?

 Low voltage lighting systems and fixtures use a transformer to step the 120VAC line voltage down to 12VAC.  To simplify installation, many transformers have multiple taps on the low-voltage side of the unit that provide 12V, 13V, and 14V output (up to 22V).  Most transformers include built in programmable on/off timers. Others may include a photo sensor that activates the lights from dusk to dawn.
 To determine the proper size transformer:
  1. 1. Add up the wattages of all the fixtures to be connected to that transformer ONLY
  2. 2. Divide the total by .8
  3. 3. Round up to the nearest available transformer size
    Ex:    Twelve fixtures with 18 watt lamps.
    12 x 18 = 218. 
    216 / .8 = 270. Closest transformer is 300 watts.


How do I select the right tap on the transformer?

 If the total wattage of all fixtures on the line = 100 watts AND the total distance of cable line is 200 feet AND you are using 12 gauge wire, THEN use a 17V tap to eliminate voltage drop. How did we get that?  See the charts below for more information.

transformer tap chart for #8-2 wire
watts
40
 80 120 160 200 240 280 300 340 384
amps
 3.3 6.7 10 13.3 16.7 20 23.3 25 28.3 32
distance
25
12
12 12 12 13 13 13 13 The NEC (National Electrical Code) limits all secondary wire runs to a maximum of 20 amps
50
12
 12 13 13 13 13 13 14
75
12
 13 13 13 14 14 14 14
100
12
 13 13 14 14 15 15 15
125
13
 13 14 14 15 15 16 16
150
13
 13 14 15 15 16 16 17
175
13
 13 14 15 16 16 17 18
200
13
 14 15 15 16 17 18 18
225
13
 14 15 16 17 18 19 19    
250
13
 14 15 16 17 18 19 20    
275
13
 14 16 17 18 19 20 21    
300
13
 15 16 17 18 20 21 22    
325
13
 15 16 18 19 20 22      
350
13
 15 16 18 19 21        
375
14
 15 17 18 20 22        
400
14
 15 17 19 21 22        
425
14
 16 17 19 21          
450
14
 16 18 20 22          
475
14
 16 18 20 22          
500
14
 16 18 21            
525
14
 16 19 21            
550
14
 17 19 21            
575
14
 17 19 22            
600
15
 17 20 22            
625
15
 17 20              
650
15
 18 20              
675
15
 18 21              
700
15
 18 21              
725
15
 18 21              
750
15
 18 22              
775
15
 19 22              
800
15
 19 22              
825
16
 19                 
850
16
 19                
875
16
 19                
900
16
 20                
925
16
 20                
950
16
 20                
975
16
 20                
1000
16
 21                

transformer tap chart for #10-2 wire
watts
18
 35 70 105 140 175 210 245 280 288
amps
1.5
 2.9 5.8 8.8 11.7 14.6 17.5 20.4 23.3 24
distance
25
12
12 12 13 13 13 13 13 13 13
50
12
 12 13 13 13 14 14 14 15 15
75
12
 13 13 13 14 14 15 15 16 16
100
12
 13 13 14 15 15 16 16 17 17
125
12
 13 14 14 15 16 17 18 18 19
150
13
 13 14 15 16 17 18 19 20 21
175
13
 13 14 15 16 18 19 20 21 21
200
13
 13 15 16 17 18 20 21 22 22
225
13
 13 15 16 18 19 21 22    
250
13
 14 15 17 18 20 22      
275
13
 14 16 17 19 21 22      
300
13
 14 16 18 20 22        
325
13
 14 16 18 20 22        
350
13
 14 16 19 21          
375
13
 14 17 19 22          
400
13
 15 17 20 22          
425
13
 15 17 20            
450
14
 15 18 21            
475
14
 15 18 21            
500
14
 15 18 22            
525
14
 15 19 22            
550
14
 16 19 22            
575
14
 16 19              
600
14
 16 20              
625
14
 16 20              
650
14
 16 20              
675
14
 16 21              
700
14
 16 21              
725
14
 17 21              
750
14
 17 22              
775
15
 17 22              
800
15
 17 22              
825
15
 17 22              
850
15
 17                
875
15
 18                
900
15
 18                
925
15
 18                
950
15
 18                
975
15
 18                
1000
15
 18                

transformer tap chart for #12-2 wire
watts
20
 40 60 80 100 120 140 160 180 192
amps
1.7
 3.3 5 6.7 8.3 10 11.7 13.3 15 16
distance
25
12
12 12 13 13 13 13 13 13 13
50
12
 13 13 13 13 14 14 14 14 15
75
12
 13 13 14 14 14 15 15 16 16
100
13
 13 14 14 15 15 16 16 17 17
125
13
 13 14 15 15 16 17 17 18 18
150
13
 14 14 15 16 17 18 18 19 20
175
13
 14 15 16 17 18 19 20 21 21
200
13
 14 15 16 17 18 20 21 22  
225
13
 14 16 17 18 19 21 22    
250
13
 15 16 17 19 20 21      
275
13
 15 16 18 19 21 22      
300
14
 15 17 18 20          
325
14
 16 17 19 21          
350
14
 16 18 20 21          
375
14
 16 18 20 22          
400
14
 16 18 21            
425
14
 17 19 21            
450
14
 17 19 22            
475
15
 17 20 22            
500
15
 17 20 23            
525
15
 18 21              
550
15
 18 21              
575
15
 18 21              
600
15
 18 22              
625
15
 19 22              
650
16
 19                
675
16
 19                
700
16
 20                
725
16
 20                
750
16
 20                
775
16
 20                
800
16
 21                
825
16
 21                
850
17
 21                
875
17
 21                
900
17
 22                
925
17
 22                
950
17
 22                
975
17
                  
1000
17
                  

Real Life Voltage Drop Example

 You want to install 10w lights about 10 feet apart down a 250' sidewalk leading to a driveway. The initial plan calls for the transformer to be installed at one end of the run.

 What size transformer do you need?
 With a cable run of 250 feet and fixtures spaced 10 feet apart you will need 25 fixtures. If each lamp is 10w the total load will 250w. At minimum you will need a 2 X 300w (600w total) transformer with 12, 13 and 14V taps.

What gauge wire do you need?
 Move the transformer to the mid-point of the run so that the cable runs would be only 125' each. Each 125' of 10/2 gauge low voltage cable would be fed from one of the two 300w outputs and should be center-fed i.e. like a "T" with the one arm folded; this reduces the overall length of each leg (spliced at the center point) to 62 1/2'. This results in a 120 to 130w load on each 300w output, which should eliminate any problems with voltage drop (using the 13V or 14V tap should give you about 10 - 11.5V for each fixture.

What are the voltage drop considerations?
 If the transformer stays at the end of the 250’ run, use the ABC voltage drop formula to estimate how many volts will be lost at the last fixture. In this case, multiply the Cable Constant x 2 for long runs.

Voltage
Drop =		 A x B
C x 2
  1. A.= Length of the wire
  2. B.= Total # of watts for all lamps on that wire run
  3. C.= Cable constant (determined by wire size)

      WIRE SIZE
      		 CABLE CONSTANT
      #10
      		 11920
      #12
      		 7500
      #14
      		 3500
      #16
      		 2200
      #18
      		 1380

      So running 250 feet of #10 cable with 250 watts would result in an unacceptable 5-10v drop at the end of the run.

       What are possible solutions?
       Be creative and break up the run! Remember to always use a voltmeter or multimeter to measure actual voltage drop and plug into the necessary tap on the transformer to provide 10.5-12 volts to all fixtures.
      • Run 1:  Run #10 low voltage cable for the first 10 fixtures and plug it into the 12v tap on the transformer. Using the ABC formula, you would get a .83 volt drop [(100 x 100)/11960}.
      • Run 2 Option 1:  Add the remaining 15 fixtures at a 250' a serial run.  Using the ABC formula, you would get an unacceptable voltage drop of 3-6V.
      • Run 2 Option 2:  Loop the wire at the end of the run back to the transformer to ease the voltage drop. Total cable would be 600’ #10.
      • Run 2 Option 3:  Use the "T" run for these 15 fixtures with low voltage cable by running #8 wire 175' to the center and then branch off 75' each way with #10 cable. Total cable would be 250' of #10 cable and 175' of #8 cable.
       
       
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