Self-regulating or constant watt
Mission-Ready Freeze Protection.How each of the two common heat trace cables behaves on a pipe, which lines each one belongs on, and how to read the cable callout on your own drawings before a reel gets ordered.
For the person who
inherits the choice
Somewhere in your plant is a drawing, a panel schedule, or a quote that says Self-Reg 5W/ft on one line and Const-Watt 20W/ft on another. Nobody walked you through why. If that line freezes, or the cable burns out in its third season, the choice becomes yours anyway.
This guide is for reading that callout — what each cable does on a pipe, what actually decides between them, and which questions to ask before a design gets locked. You don’t have to size a circuit yourself. You have to know when the answer on the page doesn’t fit the pipe it’s on.
What you’ll be able to do
- Read a cable callout on a drawing and tell whether it suits the line it sits on.
- Ask the four questions that decide cable type — before a designer picks for you.
- Recognize the mis-specs that turn up most often in the field, including the one that burns cable out.
One cable answers the pipe.
The other doesn’t.
A self-regulating cable is two bus wires with a conductive polymer core between them. The polymer’s resistance climbs as it warms, so every inch throttles its own output against the pipe temperature right there. Cold stretch, more heat. Warm stretch — a valve in the sun, a length that runs indoors — less.
A constant wattage cable does not. Its output is fixed by construction: the same watts per foot at 30°F as at 300°F. That one difference produces every advantage and every failure mode on both sides. Fixed output holds temperatures self-regulating cable cannot reach. Fixed output also keeps pushing heat into a spot that is already hot — which is why constant wattage crossed over itself at a flange destroys itself, and self-regulating doesn’t.
One note on the number in the callout. A self-regulating cable’s 3W/ft is published at a reference pipe temperature — it is what that cable does at that condition, not everywhere on the run.
The cable on the pipe and the cable on the panel schedule are not always the same cable. Sections get replaced during an outage with whatever was on the truck, and the schedule never catches up. Read the cable’s own printing where it enters the junction box — type and output are marked on the jacket at intervals.
Four questions
settle it
Cable type is not a preference. Four facts about the line settle it, and a designer holding all four rarely has a hard decision left.
- What the pipe has to hold. Freeze protection at 40°F is a different problem from maintaining a process at 250°F.
- What the pipe sees at its worst. Steam-out, clean-in-place, an upset — exposure can sit far above the maintain temperature.
- How the area is classified. Classification and corrosive exposure narrow the list before wattage is discussed.
- How long the run is, and what the panel can start. Circuit length is limited by the cable and the breaker.
| Consideration | Self-regulating | Constant wattage | Mineral insulated |
|---|---|---|---|
| Published pipe temperature | Up to 250°F | Up to 500°F | Up to 1200°F |
| Output behavior | Varies inch by inch with pipe temperature | Fixed watts per foot everywhere | Fixed by design length and resistance |
| Overlap at valves | Permitted — it throttles where it crosses | Never — the crossed section burns out | Never |
| Outputs seen in the field | 3 / 5 / 8 W/ft | 15 / 20 W/ft | Per design |
| Draw on a mild day | Falls as the pipe warms | Unchanged | Unchanged |
| Classified areas | Approved constructions exist, per product | Approved constructions exist, per product | Metal sheath — the usual answer |
| Repair | Cut out and splice a section | Splice within a zone | Sheath is weld-repairable |
Scroll table →
Temperature figures are the ranges commonly published for each technology. Every product carries its own limits for maintain temperature, maximum exposure and area approval — the data sheet governs.
Where MI is the honest answer
MI is the third option, not a premium version of the other two. It is a metal-sheathed cable for service the polymer cables do not survive: sustained high temperature, severe classification, fire exposure. It costs more, takes trained terminations, and is right on a small number of lines at most plants — often the ones that would take the unit down. If a design puts MI on a 2-inch water line, ask why.
A design that specifies one cable type across the whole plant. Fuel gas headers, condensate returns, instrument lines and a safety shower are not one heat trace problem — they are four. One cable type across a mixed system usually means nobody ran the heat loss line by line.
Four mis-specs
we find in the field
Most bad cable choices are not exotic. These four turn up again and again on a first audit, and each was visible on paper long before it was visible on a pipe.
Constant wattage overlapped at a valve
Extra passes at a valve body are ordinary with self-regulating cable and fatal with constant wattage. The crossed section keeps pushing full output into a spot that can’t shed it, and the cable fails there — often seasons later. Valves on those circuits get designed length with spacing, or a valve jacket.
Self-regulating sized against maintain temperature only
The pipe holds 150°F all winter and the cable is fine. Then the line gets steamed out in the spring and the polymer core meets a temperature nobody wrote down. It survives the event and meggers lower every year afterward. Maximum exposure is its own line on the spec sheet — not the maintain temperature with margin added.
Wattage picked from last winter instead of from heat loss
“It froze at 5 watts, put 8 on it.” Sometimes that is the answer. More often the pipe froze because the lagging was soaked, the jacket was open, or the circuit had no power for two months. Adding watts to an insulation problem buys a bigger electric bill and the same freeze.
A run length the breaker cannot start
Self-regulating cable draws a heavy inrush cold, then settles. A circuit laid out to the last foot on paper can trip on every cold start in January — when nobody is watching the panel. Maximum circuit length per breaker size is published for every cable. It is a design limit, not a guideline.
A replacement scope written as “same as existing.” If the existing cable was wrong — wrong output, wrong temperature class, wrong for the insulation that has been on the pipe since — matching it reproduces the mistake and puts your name on it. Replacing a run is the cheapest chance you will get to correct a spec.
What your designer
needs from you
Cable selection is the output of a heat loss calculation, and that calculation is only as good as the facts it is handed. Every one of these belongs to your plant — and anyone who asks for none of them is guessing.
- Pipe size, schedule and material, with the line’s real routing — indoor, outdoor, and the run up the rack.
- Contents, and what failure looks like. Freeze protection, viscosity maintenance, and a line that must never solidify are three different specs.
- Maintain temperature, and how tightly it has to be held.
- Maximum exposure temperature — steam-out, clean-in-place, upset — and whether the cable is energized during it.
- Insulation type, thickness and jacket, as installed today, not as drawn ten years ago.
- Design ambient and wind for that location, not the regional average.
- Area classification and any corrosive exposure on that run.
- Available voltage, spare breaker capacity, and where power can land.
Those eight lines are what an engineering package is built on, and the same facts the record carries afterward: cable type, output, circuit length, breaker and setpoint, per circuit, on the panel schedule. A full package is listed deliverable by deliverable on the design services page; the three technologies sit beside their applications on the heat tracing page.
The fastest way to end a cable argument is to go read the pipe. Insulation thickness, jacket condition, what the line actually runs at, whether the circuit is even energized — a walkdown answers all of it in an afternoon and turns a specification debate into a measurement. More often than not, a disagreement about wattage is a disagreement about insulation. A circuit-by-circuit audit settles both at once.
Two references
you can print
Both are one page, built from assessment work at Northeast generating facilities. The cable on a circuit only counts once it is on the as-built record — these two say what that record has to hold. Print them, or forward them to whoever keeps the documentation.
Winterization Documentation Checklist
Records, testing, ownership — the nine boxes a regional auditor asks to see under EOP-012. The first is whether the as-built record is current: every circuit identified and located, with what it protects.
Download the checklistEOP-012 Documentation Guide
The three asks an auditor makes of a generating facility — a current as-built record, dated megger results per circuit, a named responsible party — and why a visual walk cannot produce any of them.
Download the guideAsked plainly
Can I just use self-regulating cable on everything?
Is constant wattage cable obsolete?
Which one costs less to run?
Can heat trace cable be overlapped at a valve?
The drawing says 5W/ft. Is that what the pipe gets?
How do I find out what cable is on my pipe right now?
Find out what is
actually on your pipes
A walkdown puts eyes and a meter on what is installed — cable type, condition, and whether the pipe agrees with the drawing. We don’t sell a cable brand, so nothing in the answer steers you toward a reel. If the system is in good shape, that is what you will hear.
A practical field reference drawn from heat trace assessment and installation work at Northeast industrial facilities — not a design document, and not a substitute for the cable manufacturer’s data sheet, published temperature limits, or circuit-length tables. Temperature ranges shown are the figures commonly published for each technology; the product data sheet and the area classification govern where they differ.