Calculating Cable Lengths

(This article is an excerpt from the 'Residential Cabling Guide' (3rd edition)).

After you have made your decisions on outlet locations and cable types, you need to determine how much cable you need for wiring the home.

There are two approaches to this problem. You can calculate the length of each cable run for each cable type and then simply sum them up. Or you can make a reasonable estimation based on the shortest and longest cable runs and the total number of outlets.

The first approach works well when you have few cable runs and it is easy to determine the length of each of them.

The second, averaging approach is used when there are many outlets and it is very time-consuming to determine the length of each of them.

Use the following estimation procedure for every cable type in your house to calculate the Total Cable Length:

1. Identify the SHORTEST cable run (A) to the outlets closest from the Distribution Device

2. Identify the LONGEST cable run (B) to the outlet farthest from the Distribution Device

3. Calculate the AVERAGE cable length (AL)

(AL) = (A + B)/2

4. Add SLACK (S) of minimum 0.9 m (36 in) at the Distribution Device plus a minimum of 0.2 mm (8 in) of excess cable at the outlet

(S) = 0.9 m (36 in) + 0.2 m (8 in) = 1.1 m (44 in)

5. Determine the Drop Length (D) at the outlets (if cable runs vertically along the studs to the outlets). For safety, assume Drop Length is equal 4.5 m (15 ft)

(D) = 4.5 m (15 ft)

6. Calculate the Total Average Cable Length (TACL)

(TACL) = AL + S + D

7. Calculate the Total Cable Length (TCL) by multiplying the Total Average Cable Length (TACL) by the number of outlets (N)

(TCL) = (TACL) * N

This article is an excerpt from the 'Residential Cabling Guide' (3rd edition).