Determining Heat Cable Needs
I remember a discusion of heating cables a while back.
Here is a short artical that I found on the subject,
that may be of some use.
Gordon Tanner
Maple Valley
Determining Heat Cable Needs
Larry A. Sagers
USU Regional Horticulturist
Electric heat cables are a convenient and dependable
way to heat hotbeds. Cables
are covered with either lead or plastic to prevent
electrical shorts. Roof heating
cables and heat tapes are not suitable for direct
burial.
Cables vary in length and wattage rating. Some are
rated at 6.7 watts per foot while
others are rated at 3.5 watts per linear foot. Cable
is installed to produce a certain
number of watts per square foot of bed area. Plan on
allowing 12-15 watts per
square foot.
Lay the cable on loose, level soil or sand. Be careful
not to damage the covering
of the cable. Avoid kinks that could damage or break
the cable, and don't cross
cable over itself. Never cut
the cable to shorten its length.
The spacing between the loops of cable determines the
number of watts supplied
per square foot of bed. Use the following formula to
determine the spacing
between loops:
Spacing (inches) = (15 x watts per foot of cable)
÷ wattage required per sq. ft.
of bed
To determine the watts per foot of cable, divide the
total number of watts for the
cable by its length in feet.
Example: Spacing for a 400-watt cable 60 feet long to
produce 15 watts per square
foot of bed area would be:
Spacing = (15 x [400 ÷ 60]) ÷ 10
spacing = (15 x 6.7) ÷ 10
spacing = 10 inches between loops
The factor used here is 15, spacing for 12 watts per
square foot of bed area would be 8 inches between
loops.
If the heating cable is too long, use spacing slightly
closer than calculated, but
always distribute the cable uniformly throughout the
bed. Irregular spacing makes
temperature control difficult. Generally, a 60-foot
cable should heat about 36
square feet of bed area.
After the cable is positioned, cover with sand or fine
soil. Over this, place
half-inch-mesh hardware cloth. The hardware cloth
protects the cable from
damage when soil above it is being worked for
planting. Over this, add the layer
of soil for planting or another layer of sand if all
planting is to be done in beds.
Thermostats. Use a thermostat to maintain uniform
temperatures. Some cables
have built-in thermostats that keep the soil
temperature close to 74 degrees F. For
larger beds, a separate, adjustable thermostat is
best.
In general use, thermostats have an operating range
between 30 and 120 degrees F.
Bury the remote temperature bulb beneath an inch of
soil midway between two
loops at the center of the bed. Don't allow the bulb
to contact the heat cable.
© Geoffrey Sagers / Software Ink.
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