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I have a small W1209 DC12V thermostat PCB that's had a small 12VDC relay attached. The print on the relay says that it can do 5A @ 240VAC and 10A at 120VAC. That makes sense, as both of those equal 1200 Watts.

However I'm wanting to wire it to a 2000 watt load at 240VAC. So have been looking at other relays I can replace the one on the PCB with.

And I'm seeing a fair number of relays which rather than having an 10A @ 240AC & 5A @ 120VAC rating they have a 10A @ 240VAC & 10A at 120VAC

So how is that possible? Is the relay that has 10A for both built for 10A @ 240, and 20A @ 120 too much for it, or is the rating fudged somehow?

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  • \$\begingroup\$ The ratings are kinda independent on each other. In the closed state it doesn't care too much about the voltage since all that matters is the amps and the resulting voltage drop and power dissipation \$\endgroup\$ – PlasmaHH Nov 5 '18 at 9:41
  • \$\begingroup\$ That's what I thought, its the Amps that's causing the build up of heat, which is why they're also rated for 10A at 30VDC. But how come one is rated for half the power at 240V when another relay is rated at the same Amps for 240? \$\endgroup\$ – Tonis F. Piip Nov 5 '18 at 9:44
  • \$\begingroup\$ The higher the voltage, the better you have arcing across the contacts. If you drive inductive loads you multiply that voltage, so the real max arc voltage is maybe reached earlier then. \$\endgroup\$ – PlasmaHH Nov 5 '18 at 9:47
  • \$\begingroup\$ If you think the current rating may be fudged, why do you chose to trust the voltage rating? \$\endgroup\$ – Dmitry Grigoryev Nov 5 '18 at 10:22
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There are a large number of design elements that affect the current and voltage ratings, technically, and then there are the approvals they choose to get. Many relays have multiple ratings, depending on the safety standard to which they are tested.

Generally "watts" is not the limiting factor, it's amperes for heating, and the voltage for contact opening. The wear on the contacts can also be limiting and that's affected by factors such as the inrush current of the load and any reactance (eg. inductance) in the load.

Generally small relays are rated for some number (100,000 is typical) life at full load, and you can derate that (eg. operate a 20A relay at 5A) to get a better lifetime.

It's not that common to find a drop-in relay that will give you substantially more life or current rating with the same load and contact metallurgy-- relays are fairly well optimized. You'll usually find the higher rated relay will draw substantially more coil current (perhaps that your existing circuit cannot safely handle) or will be substantially larger or both.

Your best bet may be to operate a larger relay or contactor with your small relay.

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  • \$\begingroup\$ Yes, the plan was to perhaps have the current relay run the coil on a larger relay, if replacement is too tedious. So is it correct that if a relay is rated for 10A at both voltages, it can do it, but if used on a higher voltage at 10A, it will degrade faster due to more arcing. So would you consider that the relays that half the rated Amps for 240 are mostly erring on the side of caution due to the increased arcing? \$\endgroup\$ – Tonis F. Piip Nov 5 '18 at 12:21

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