# How can relays be rated at 10A for both 240VAC and 120VAC?

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?

• 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 Nov 5, 2018 at 9:41
• 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? Nov 5, 2018 at 9:44
• 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. Nov 5, 2018 at 9:47
• If you think the current rating may be fudged, why do you chose to trust the voltage rating? Nov 5, 2018 at 10:22