You’re designing a 24/7 control panel for an outdoor telecom shelter that has exactly 800 BTU/h of heat rejection budget for the whole switchgear bay. The contactor coil is supposed to be a rounding error. But after three hours in a 50°C ambient soak, the cabinet hits 62°C, the fan cycles at 100%, and the contactor’s coil sits there dissipating 12 watts you never planned for. Which brand gives you back those watts — and which one burns them as heat?
This is the tight-cooling shelter scenario. The contactor isn’t the main load — the inverter, the power supply, the motor are. But the contactor lives in the enclosure, and its coil loss is a pure thermal parasite. Below are the three metrics that determine whether your thermal budget holds or melts. Each is worked through a real decision point.
Metric #1: Coil Power Draw — The Silent Heater
The number. The Siemens SIRIUS 3RT2016 (size S00, 9 A AC-3) has a typical coil power consumption of about 6.5 VA inrush / 6.5 VA holding for the AC coil version — roughly 5–6 W holding at 230 V AC. The ABB AF09 with its electronic wide-range coil consumes about 1.5 W DC typical holding (24 V DC) and ~4 VA AC holding — measured at ~2.5–3 W in many field installs. At first glance, that’s a 3 W per contactor difference. In a three-contactor shelter, that’s 9 W — about 7.5% of your 120 W allocation swallowed unnecessarily if you choose the higher-draw coil.
The mechanism. The ABB AF series uses a switched-mode coil driver that draws very low power once the contactor is sealed — the coil current drops to a trickle after the pick pulse. The Siemens 3RT AC coil is a conventional laminated magnet: it draws roughly constant VA as long as the coil is energized, because the magnetic circuit remains partially saturated. The holding power is set by the coil’s resistance and the iron path. There’s no active reduction. The result is a 2–3x difference in holding watts. In a shelter where every watt becomes heat that must be removed, those extra watts multiply your cooling load by roughly 1 W heat = 0.3–0.5 W additional fan power. So a 9 W coil penalty may cost another 3–4 W in cooling system overhead.
Worked consequence. In the scenario above, switching from three Siemens AC coils to three ABB AF electronic coils saves ~9 W of direct heat, plus ~3 W of fan overhead — total 12 W recovered. That’s 10% of your 120 W contactor budget. In a shelter that’s already at 95% thermal capacity, that 12 W may be the difference between the UPS remaining at full rating or derating by 10–15%. The operator’s decision: accept the ABB AF and avoid a $2,000 shelter upgrade.
When it flips. If your shelter has abundant cooling (e.g., a 2,000 BTU/h unit with headroom), the 3–9 W difference is negligible. Also, if your application uses DC coils (24 V DC) on the Siemens contactor side, the 3RT DC coil versions draw roughly 2–3 W holding — closer to the ABB AF range. The gap narrows. And if you’re using only one contactor, the marginal savings may not justify a change in brand preference.
Metric #2: Pick Voltage Margin — The Hidden Derating Curve
The number. The ABB AF09 electronic coil is rated for a wide voltage range — for example, 100–250 V AC/DC (the “U” variant), and it will pick up reliably down to about 80% of the lower bound (~80 V AC). The Siemens 3RT AC coil has a typical pick-up threshold of about 85% of nominal voltage, but under load, the margin slips — if the control transformer is sized at 80% loading, the actual voltage at the coil may be 5–8% lower than nominal due to line drop. In a shelter with a 200-foot control cable run from the UPS, the voltage sag during a heavy UPS load step can drop the control bus by 12% for 50 ms. That sag can cause a marginal coil to drop out, or fail to pick up during a transfer.
The mechanism. The ABB AF’s electronic coil has a internal DC-DC converter that maintains the pick pulse regardless of the incoming AC/DC voltage within its wide window. The coil sees a regulated DC rail. A Siemens AC coil relies on the peak of the AC sine wave — a sag reduces the peak voltage, and the coil’s magnetic force drops with the square of the voltage. At 88% nominal voltage, the force is only 77% of nominal. If the contactor is at its rated upper limit of mechanical wear (e.g., 1 million operations), the coil may not have enough margin to overcome stiction.
Worked consequence. In the tight-cooling shelter, the UPS transfers from line to battery in 12 ms. During that transfer, the control voltage dips to 90 V on a 120 V AC bus for 2–3 cycles. The Siemens 3RT2016 (rated 110 V AC coil) may drop out briefly, causing a momentary interruption of the motor starter that controls the shelter’s ventilation fan. The fan stops for 100 ms; the shelter temperature rises 1°C in 30 seconds. The ABB AF09 with the 100–250 V AC coil stays sealed through the dip because its internal rail doesn’t drop below the pick threshold. The shelter’s thermal profile stays stable.
When it flips. If your control transformer is oversized (e.g., 200% of connected VA), and the wire runs are short (
Metric #3: Mechanical Life Under Constant Cycling — The Thermal Cycle Amplifier
The number. The ABB AF09 has a rated mechanical life of about 1 million operations per the datasheet. The Siemens 3RT2016 similarly lists a mechanical life of about 1 million operations under typical AC-3 conditions. On paper, the same. But in a shelter that cycles the contactor every 10 minutes to control the fan (6 cycles per hour, 52,560 cycles per year), both will last 19 years — well beyond the shelter’s service life. The issue isn’t total cycles; it’s wear acceleration under high ambient temperature.
The mechanism. In a 50°C ambient shelter, the contactor’s internal temperature can reach 70°C due to coil self-heating plus ambient. A conventional AC coil’s temperature rise is about 30°C above ambient at rated voltage. The ABB AF’s electronic coil runs cooler — roughly 15°C rise above ambient. That reduces the thermal stress on the armature pivot, the spring temper, and the arc chamber. A 20°C reduction in internal temperature roughly doubles the insulation life per the Arrhenius model (10°C halving rule for Class B insulation). The mechanical wear rate of the latch and spring is not a strong Arrhenius function, but the plastic parts (e.g., the contact carrier) see fatigue acceleration at higher temperatures. The low coil power becomes a reliability multiplier in a hot enclosure.
Worked consequence. After 5 years in a 55°C cycle, a Siemens 3RT with conventional AC coil may see the plastic armature guide become brittle and crack at the mounting boss — a known field failure mode in hot enclosures. The ABB AF09, with its lower internal temperature, may have the plastic parts remain ductile for 10+ years. The maintenance interval for contactor replacement can be extended from 5 years to 8–10 years, saving $500 per replacement (labor + logistics) in a remote shelter. The net present value of that saving, discounted at 8% over 10 years, is ~$250 per contactor.
When it flips. If the shelter is air-conditioned and never exceeds 35°C, the internal temperature difference between the two coils is only about 10°C, and the mechanical life difference becomes negligible — both will outlast the shelter. Also, if the contactor cycles only a few times per day (e.g., for a weekly generator test), mechanical wear is irrelevant. The reliability advantage of the ABB AF only manifests in high-cycle + high-ambient conditions.
Ranked Picks Table
| Scenario Variant | Winner | Why |
|---|---|---|
| Shelter with marginal cooling (≤ 300 W heat budget for controls) | ABB AF | Low coil power saves 6–9 W per contactor, directly recovers cooling budget |
| Shelter with long control cable runs (> 150 ft) / weak UPS | ABB AF | Wide-range coil stays sealed through voltage dips that cause Siemens AC coil dropout |
| High-cycle fan control (> 10,000 ops/year) in hot ambient (> 50°C) | ABB AF | Lower internal temperature extends plastic part life, reduces failure rate |
| Well-cooled shelter ( | Either | Differences in coil power and life vanish; both meet spec; Siemens may be cheaper upfront |
| 24 V DC control bus with dedicated battery-backed supply | Either | DC coils on Siemens also draw low power; no voltage sag advantage for ABB contactor |
Rule of Thumb
If your shelter’s total heat rejection is below 400 W and you have more than two contactors, choose the ABB AF series. If you have a single contactor and the shelter is air-conditioned, the Siemens 3RT is fine — the coil power delta of 3 W isn’t worth a procurement change. For any scenario where the control voltage bus can dip below 90% of nominal for >2 cycles, the ABB AF’s wide-range coil is a must.
Topology/standards per the cited standards; all product ratings are manufacturer-stated values from the cited datasheets, current to 2026-06; derived/illustrative figures are labelled as such. This is not an independent head-to-head test. Siemens is a brand affiliated with this site; competitor names are used for identification only.
