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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