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Engineering Data
Voltage, Phase and Temperature Controls
The heater is half the installation. What switches it, what measures it, and what stops it are the other half, and they are where most tank heating projects come apart.
Start from what you have at the tank
Available voltage, phase and spare breaker capacity are constraints, not preferences. A 40 kW heater on a 480 V three phase service draws about 48 A per leg; the same 40 kW on 240 V single phase draws around 167 A, which is a much larger breaker, much larger wire and possibly a service upgrade. Check the panel before deciding the kilowatts.
Elements are built for a specific voltage and the relationship is squared, not linear. Power scales with the square of applied voltage, so a 480 V element run on 440 V produces roughly 16 percent less heat, and a 240 V element energized at 480 V dissipates four times its rating and fails immediately.
Choosing how to switch it
Contactor, on and off. Cheapest and simplest. A mechanical contactor closes and opens the full load. Temperature swings around the setpoint by several degrees and every cycle is a thermal shock to the element. Fine for tanks with large thermal mass and loose tolerance.
SCR power controller, proportional. Solid state, switches rapidly to deliver a proportion of full power. Tight temperature control, no mechanical wear, dramatically less thermal cycling of the element. More expensive, and it needs to be sized and cooled properly. The right answer when tolerance is tight, when the element is expensive, or when the heater cycles frequently.
Staged or stepped. Splitting a large load into stages and bringing them on progressively. Reduces inrush, gives coarse proportional behavior with contactors, and lets part of the system keep running if one stage fails.
Sensor placement decides control quality
A perfect controller reading the wrong spot controls the wrong thing. Put the process sensor in well-mixed liquid, away from the heater itself and away from the tank wall. A sensor too near the element reads element heat and shuts off early, leaving the bulk of the tank cold. A sensor in a dead corner lags badly and the tank overshoots.
A thermowell lets the sensor be replaced without draining the tank. On any tank that is a nuisance to drain, specify one.
Full load current by rating and voltage
| Rating | 120 V 1φ (A) | 240 V 1φ (A) | 240 V 3φ (A) | 480 V 3φ (A) |
|---|---|---|---|---|
| 1 kW | 8.3 | 4.2 | 2.4 | 1.2 |
| 3 kW | 25.0 | 12.5 | 7.2 | 3.6 |
| 6 kW | 50.0 | 25.0 | 14.4 | 7.2 |
| 9 kW | 75.0 | 37.5 | 21.7 | 10.8 |
| 12 kW | 100.0 | 50.0 | 28.9 | 14.4 |
| 18 kW | 150.0 | 75.0 | 43.3 | 21.7 |
| 24 kW | 200.0 | 100.0 | 57.7 | 28.9 |
| 36 kW | — | 150.0 | 86.6 | 43.3 |
| 48 kW | — | 200.0 | 115.5 | 57.7 |
| 75 kW | — | — | 180.4 | 90.2 |
| 100 kW | — | — | 240.6 | 120.3 |
| 150 kW | — | — | 360.8 | 180.4 |
Nominal values for balanced resistive loads at rated voltage. Conductor sizing, overcurrent protection and continuous-load derating must follow the NEC and local code. Dashes indicate combinations that are impractical rather than impossible.
Control components
| Component | What it does | Practical note |
|---|---|---|
| Contactor panel | Switches the full heater load on and off | Simple, low cost, high thermal cycling of the element |
| SCR power controller | Proportional power output, solid state | Tight control, no mechanical wear, needs correct sizing and cooling |
| PID temperature controller | Reads the sensor and drives the contactor or SCR | Autotuning, alarm outputs, ramp and soak on better units |
| Capillary bulb thermostat | Self-contained mechanical temperature control | No panel needed, wide deadband, limited accuracy |
| High-limit over-temperature cutout | Independent backup that breaks power on overtemperature | Must be separate from the control sensor to be worth anything |
| Low liquid level cutoff | Breaks power when liquid falls below the element | The single most valuable protection on an open tank |
| Flow switch | Breaks power when circulation stops | Mandatory on circulation heaters |
| Type J or K thermocouple | Wide range temperature sensing | Inexpensive, robust, less accurate than an RTD |
| RTD, 100 ohm platinum | Precise temperature sensing | Better accuracy and stability, preferred for tight control |
| Thermowell | Lets a sensor be replaced without draining the tank | Adds a small measurement lag; worth it on most tanks |
| Ground fault protection | Detects insulation breakdown to ground | Especially valuable in wet and corrosive environments |
Electrical and control questions
What voltage should I order the heater for?
Whatever is actually available at the tank, measured rather than assumed. Element power varies with the square of applied voltage, so a mismatch is not a small error. Higher voltage and three phase reduce current for the same kilowatts, which means smaller wire and a smaller breaker — if 480 V three phase is available, it is almost always the better choice for larger heaters.
Can I run a 240 V heater on 480 V?
No. Power scales with voltage squared, so 480 V on a 240 V element dissipates four times its rating and destroys it within seconds. The reverse, a 480 V element on 240 V, is not dangerous but produces only a quarter of rated output.
Do I need an SCR controller or is a contactor enough?
A contactor is adequate when the tank has large thermal mass, the temperature tolerance is a few degrees, and cycling is infrequent. Choose an SCR when the tolerance is tight, when the heater cycles often, when the elements are expensive or hard to replace, or when the process is sensitive to temperature swing. SCRs also dramatically reduce thermal fatigue on the element, which often pays for itself in heater life.
Where should the temperature sensor go?
In well-mixed liquid, away from the heater and away from the tank wall, at a depth representative of the bulk. Too close to the element and the controller reads element heat and cuts off with the tank still cold. In a dead corner and the control lags and overshoots. Use a thermowell so the sensor can be replaced without draining.
Is a high-limit cutout the same as the thermostat?
No, and the difference matters. The thermostat is the control device; the high-limit is an independent backup with its own sensor, wired to break the contactor coil directly. If they share a sensor, a single sensor failure removes both control and protection at once. On any tank where overheating causes damage or hazard, the high-limit must be independent.
Can you supply the heater and the control panel together?
Yes, and it is the better outcome. Specified together, the watt density, sensor placement, contactor or SCR rating and safety interlocks all line up. Bought separately from different vendors, those three routinely do not match, and nobody owns the gap.
Send the nameplate on your panel
Available voltage, phase, and spare breaker capacity. We will size the heater to fit the service you have rather than the one you would need.