Amperage of Relay

If I have a 240VAC 45 amp relay, and I'm running 120VAC (US power) through it, would it still have 45 amps?

The amperage is the maximum rating.
That's a lot of current, but for the same power at the lower US voltage, you'd need an even higher current rating.

Bizarre - 900+ reads in less than three minutes?

I'm running a 40 amp air conditioner, so what would be the approximate amperage I would need on a 240V relay?

I'm confused - is the unit capable of running at 240V?
You're saying 40A at 120V?

I want to run 120V 40A through a 240V relay, I'm asking how many amps I need on that 240V relay.

If the relay really is rated 240V/45A, it should be OK.*

*(But what doI know? I'm a software engineer)

I wouldn't even try to guess what the equivelent 120vac rating would be. It will not be anything like twice the amperage. So just make sure that the max voltage and max amps are within the spec and that the relay is rated for the amount of power you intend to put through it is ok for constant use. *

  • then go ask a real electrical engineer.

Through bitter experience - if you can find a way to limit the current going through the relay during switching, it'll last a lot longer.

For relay contacts, the current limit is the current limit. There is also a voltage limit, but that's independent of the current limit. So if you have a 45A, 240V relay, it can switch 45A at 240V, 120V or 12V. It can't switch more than 45A at 12V.

The current and the voltage are independent maximums.

The size of the contact, and the size of the terminals leading to it determine the current limit. Things like the spacing between contacts and the gap between the poles determine the voltage rating. Relays in this range typically don't have a power rating.

Motor loads are MUCH harder on relays than resistive loads. Limiting inrush can help a lot. Sometimes you can actually find a motor rating on a relay, and it's often much less than the equivalent power rating. So, yeah, it will work, but it may not work for all that long.

2X is often a good safety factor if you can't limit inrush.

In your case, you might monitor the AC and only turn it off when it doesn't have the compressor running. You might figure that out with a current detector, or maybe even a sound level! Most ACs don't fire up on: they come on and measure temp before they fire up the engines.