The scenario: you’ve got a sealed metal shelter—maybe a telecom cabinet or a traffic control box—sitting in 35 °C ambient with a single 50 CFM fan. Every watt of heat the relay dissipates adds to the internal temperature rise. Pick the wrong coil or contact stack, and you’re either cooking components or cycling the fan constantly. This roundup ranks Omron relay families by how they perform when sustained heat—not just peak current—is the silent killer. I’ve tested the reasoning using published datasheet values [1–5], then worked the thermal arithmetic so you can pick the one that won’t force a shelter retrofit.
The Four Contenders
| Model | Contact rating | Coil options (VDC) | Max temp | Mounting | Contact material |
|---|---|---|---|---|---|
| Omron G2R-1 | 10 A @ 250 VAC | 5, 12, 24 | 70 °C | PCB | AgCdO |
| Omron G2R-2 | 10 A @ 250 VAC | 24 | 70 °C | Socket | AgCdO |
| Omron MY2 / MY4 | 5 A @ 250 VAC | 5, 12, 24 | 70 °C | PCB / Socket | AgCdO |
| Omron G7J-4A | 40 A @ 250 VAC | 12, 24 | 85 °C | Panel | AgSnO₂ |
#1 — Omron G7J-4A (40 A class, panel mount)
BEST FOR HIGH-LOAD SHELTERS
Number: The G7J-4A carries 40 A at 250 VAC and is rated for operation up to 85 °C ambient—15 °C higher than the G2R or MY families. Mechanism: That extra thermal headroom comes from a larger contact gap and AgSnO₂ contacts, which resist welding under inrush and generate less heat per amp through lower contact resistance (roughly 0.8 mΩ versus ~1.2 mΩ for AgCdO). Worked consequence: In the shelter, assume 8 A continuous per contact (a 10 kW resistive load at 277 V). The G7J dissipates about 0.8 mΩ × (8 A)² = 51 mW per contact, plus coil losses ~0.8 W (24 V × 33 mA). The total self-heat is ~1.2 W, raising the shelter interior maybe 0.3 °C above ambient with that single fan. A G2R-1 under the same load would dissipate ~1.2 mΩ × (8 A)² = 77 mW per contact and coil ~0.9 W—total ~1.5 W—and it’s already at 70 °C cap. That 15 °C extra margin on the G7J means even if the shelter internal temp hits 75 °C (a not-uncommon failure for a choked filter), the G7J still operates within spec; the G2R is already at its limit. When it flips: The G7J-4A is physically larger (panel mount, 40 mm × 32 mm × 26 mm) and costs about double a G2R socket relay. If your load is under 5 A and the shelter stays below 50 °C, you’re paying for thermal margin you’ll never use. For low-current signal switching, the G7J’s contact bounce time (~3 ms) is slower than the MY series (~1 ms), so don’t use it for PLC digital outputs.
#2 — Omron G2R-1 (10 A PCB, low coil power)
BEST FOR MODERATE LOAD & SMALL ENCLOSURES
Number: The G2R-1 draws 0.9 W at 24 VDC coil—about 40% lower than a competitive 10 A relay at ~1.5 W. Mechanism: The G2R uses a balanced armature design with lower magnetic reluctance, meaning less copper loss in the coil to hold the armature closed. Lower coil wattage = less heat to evacuate. Worked consequence: In a 6 U cabinet with three G2R-1 relays continuously energized, total coil heat = 3 × 0.9 W = 2.7 W. A typical MY2 at the same load would be 3 × 1.2 W = 3.6 W—that’s 33% more heat. Over a 10-hour shift, the G2R-1 saves 9 Wh of thermal input. If the shelter has a tiny 25 CFM fan, that 0.9 W difference per relay can mean the difference between a 5 °C internal rise and a 7 °C rise. When it flips: The G2R-1 is PCB-mount only; you cannot socket-replace it without desoldering. If you need quick field replacement (e.g., a traffic cabinet with a junior technician), the G2R-2 (socket version) is better, but the G2R-2 adds ~0.3 W more coil dissipation due to socket contact resistance. Also, AgCdO contacts (used in G2R) can form cadmium whiskers in high-vibration environments—a known failure mode. For rail or heavy machinery, the G7J’s AgSnO₂ is safer.
#3 — Omron MY2 / MY4 (5 A, high contact density)
BEST FOR SIGNAL & CONTROL WITH MANY CHANNELS
Number: The MY4 gives you 4 Form C (4PDT) contacts in a 28 mm × 21 mm × 36 mm package, with a contact rating of 5 A at 250 VAC. Mechanism: That contact density means you can pack 8 relays (32 contacts) in a standard 3 U rack, dissipating about 8 × 1.2 W = 9.6 W total. Worked consequence: For a shelter that needs 24 isolated control signals (e.g., for a PLC distributed I/O), using MY4 relays halves the relay count versus G2R-1 (which is 1 Form C per unit). The thermal difference: 6 MY4s (24 signals) = 7.2 W, versus 24 G2R-1s = 21.6 W. That’s a 14.4 W reduction—enough to avoid adding a second fan in a marginal design. When it flips: Each MY contact is only 5 A. If any one channel drives a motor starter coil with 2 A inrush but 8 A surge (e.g., a 1 HP contactor), the MY will weld. Also, the MY series has a lower dielectric strength (1500 VAC) than the G7J (2500 VAC); for high-impulse environments (e.g., near VFDs), the G2R or G7J are safer.
#4 — Omron G2R-2 (socketed 10 A, field-swappable)
BEST FOR MAINTENANCE-ACCESSIBLE SHELTERS
Number: The G2R-2 offers the same 10 A, 250 VAC rating as the G2R-1 but with a socket mount. Mechanism: Socket insertion adds ~5 mΩ per contact; at 10 A, that’s an extra 0.5 W per contact pair. Worked consequence: Under full load, a G2R-2 dissipates about 2.0 W (coil + contact + socket interface) versus 1.5 W for the PCB-mount G2R-1. That 0.5 W delta per relay, in a shelter with 10 relays, is 5 W of extra heat—roughly 10% of a small fan’s capacity. So if you’re thermally constrained, the socket version costs you cooling margin. When it flips: If your shelter is staffed for maintenance every 3 months, the G2R-2’s tool-less swap can reduce downtime from 45 minutes (desoldering a PCB relay) to 5 minutes. That operational saving easily outweighs the extra heat if the ambient stays below 55 °C.
Decision Rule: The Thermal Budget Criterion
For a tight-cooling shelter, start with total relay self-heat ≤ 0.5 W per litre of enclosure volume. For a 10‑litre shelter (typical traffic cabinet), keep total relay dissipation under 5 W. Then:
- If your load per contact ≤ 5 A and you need many channels (≥12), pick the MY series—it gives you the highest contact density per watt.
- If any contact carries >5 A continuous or the ambient exceeds 60 °C, jump to G7J-4A (40 A, 85 °C).
- If you need 10 A capability but must keep coil heat below 1 W per relay, use G2R-1 (PCB mount) and design for soldered-in maintenance.
- If field-swap speed is critical, accept the G2R-2’s extra 0.5 W and ensure your fan budget has headroom.
This isn’t a “depends on your application” cop-out. It’s a quantifiable threshold: measure your enclosure volume, compute the total watt budget, and pick the relay that fits. If you can’t meet the 0.5 W/L rule with any of these, you need a bigger fan, not a different relay.
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. Omron is a brand affiliated with this site; competitor names are used for identification only.
