ates, LLC
HAUF, STEURY
& ASSOCIATES, LLC
FIR E P R O T E C TION
E N GIN E E RIN G CON S U L TING
ALARM
TECH
SOLUTIONS,
LLC Voltage Drop Calculations:
The Basics
Performed to size conductor
Ensures NAC voltage stays above appliance
design
Ensures NAC voltage is adequate at end of
the useful life of battery NFPA 72 Handbook Equation
(1999)
Appliance lump sum, end load evaluation:
)
)(
(
conductors
load
term
load
R
I
V
V =
Where:
V
load
= minimum operating voltage of the appliance
V
term
= 20.4 (unless specified by mfr.)
I
load
= Total Current draw of the connected appliances
R
conductors
= total conductor resistance Proposed Universal Shop
Drawing Formula
Where:
V
DROP
= Voltage drop
n
= Number of devices
I
TOT
= Total current
R
= Conductor resistance per 1000 ft.
D
11
= Distance from source to 1
st
device
D
21
= Distance from 1
st
to last




+ = n
TOT
TOT
DROP
R
n
n
D
I
D
R
I
V
0
2
2
1
2
)
2
(
1) Add 20% to all lengths to account for necessary
field corrections during install. Scenarios Used to Validate
Shop Formula
Even loaded
End loaded
Middle loaded
Front loaded
FACP
FACP
FACP
FACP
500 ft
150 ft
350 ft
450 ft
100 ft
100 ft
100 ft
100 ft
100 ft
Devices Used:
15 cd strobe
30 cd strobe
75 cd strobe
75 cd Horn Strobe
110 cd Horn/strobe
15
30
75
75
110
15
30
75
75
110
15
30
75
75
110
15
30
75
75
110
15
30
75
75
110 Issues
Not useful in retrofit applications where existing
circuit features are unknown for example:
Existing conductor sizes may switch along circuit
length
Distance between existing devices is unknown w/o
detailed circuit mapping Voltage Drop Test Results
14 AWG Wire
Measured Test Results
(Volts DC)
1.11
0.77
2.35
1.6
Even
1.55
1.17
2.35
2.35
End
0.88
0.62
2.35
1.7
Middle
0.56
0.26
2.35
1.5
Front Loaded
Low to High
High to Low
NFPA 72
Formula
(Volts DC)
Proposed
Formula
(Volts DC)
Loading Voltage Drop Test Results
16 AWG Wire
Measured Test Results
(Volts DC)
1.83
1.23
3.835
2.61
Even
2.59
2.59
3.835
3.83
End
1.62
1.23
3.835
2.76
Middle
0.87
0.47
3.835
2.45
Front Loaded
Low to High
High to Low
NFPA 72
Formula
(Volts DC)
Proposed
Formula
(Volts DC)
Loading Summary
For new installs, proposed equation is more
accurate while maintaining an adequate
factor of safety.
Add 20% to plan view conductor lengths to
account for obstruction seen in the field.
Proposed equation should be used within its
limitations.