3
DESIGN
1. Obtain the following roof information: roof edge length, roof overhang, total gutter length, total downspout
length and breaker rating.
2. Calculate the amount of cable required.
A. Multiply the roof edge length by the spacing factor found in Table 1.
B. Add the total gutter length plus the total downspout length to the result obtained in Part A to
determine the total length of cable required.
TABLE 1: REQUIRED CABLE
Roof Overhang
A
B
Spacing Factor
Heating Width Heating Height
12”
2 ft
18”
2
24”
2 ft
30”
3
36”
2 ft
42”
4
Maximum Allowable Circuit Lengths per Breaker Rating for 5W/Ft. Heating Cable
Table 3: NAMSR
Start Up @ 0°F
Start Up @ 20°F
120V
15A 20A 30A
90
ft 120 ft 175 ft 75 ft
100 ft 150 ft
Start Up @ 0°F
Start Up @ -20°F
240V
15A 20A 30A
135
ft 185 ft 275 ft 120 ft
160 ft 250 ft
4. Branch-Circuit Sizing: The ampacity of the branch circuit conductor and the rating or setting of overcurrent
devices shall not be less than 125% of the ampere load of the cable or units.
WARNING
ELECTRIC SHOCK HAZARD.
Disconnect all power before installing or servicing heating cable and accessories.
A qualified person must perform installation and service of heating cable and accessories. Heating cable must
be effectively grounded in accordance with the National Electrical Code. Failure to comply can result in personal
injury or property damage.
3. Calculate the number of circuits required. Divide the total length of cable required by the maximum heater
length allowed for the breaker rating.
TABLE 2 - ESTIMATING THE CABLE LENGTH YOU NEED
AREA
WHAT TO MEASURE
HOW TO CALCULATE
Along Roof
Overhang (A) and Length of Roof (B)
Length of roof (B) x Overhand Multiplier
Dormer
Distance around dormer
Number of dormers x Distance around dormers
Valley
Number of valleys
Number of valleys x 6 feet (1.8m)
Downspout
Number of downspouts Length of downspouts
Number of downspouts x Length of downspouts x 2
