Relay switching power rating - Page 1
Forum section: electronics / projects

author: Red Squirrel
author_rank: Super Contributor
timestamp: May 12, 2023, 09:33:20 am
content: I was looking at relays for a potential project involving switching up to full main loads (120v 15a 1800w) and noticed that relays have a max switch power. So even though the max current may be 20a and max voltage 300v for example, it does not mean you can switch that amperage at that voltage. I found however that these ratings are rather low. for example check out this one:https://www.citrelay.com/Catalog%20Pages/RelayCatalog/J151.pdfMax Switching Power 5540VA, 560WMax Switching Voltage 300VACMax Switching Current 20AThe large difference between the VA and W rating in first line is what I'm mostly confused about. Normally VA is fairly close to watts so I would expect the rated wattage to be somewhere along 4,000w at least.Am I missing something or interpreting this wrong? I know DC and AC can also have different ratings as DC is harder to stop an arc but for this purpose I'm only focusing on AC. Is the VA rating for AC and the watt rating for DC? So as long as I'm switching AC I'm ok to go by the VA number?

author: DBecker
author_rank: Frequent Contributor
timestamp: May 12, 2023, 01:52:21 pm
content: There are many designs where normal operation is switching the relay at near zero power, yet they need to handle continuous high current. For these applications relay contacts are optimized for low resistance rather than erosion resistance.A good example is an EV "charger" (EVSE). Line power isn't applied until low voltage signalling is established, and the release latch switch shuts off the charger in the vehicle before the supply side relay opens.

author: sparkydog
author_rank: Regular Contributor
timestamp: May 12, 2023, 05:07:16 pm
content: Quote from: Red Squirrel on May 12, 2023, 09:33:20 amI have avery similarproject, for which I ended up selectingElesta SIP 512 relays, which are rated for 250VAC / 16A or 4,000VA on the power contacts. (These are force-guided relays that also have a pair of signal contacts that are lower-rated. For my application, I'm running 12V on those to verify operation. SIP 6 comes in 512, which has one each NO+NC signal contacts, and 422 which has two NC signal contacts.) They're not cheap (or small), but "relays" rated for high current (especially force-guided) can run into three-figure territory.If you don't need the monitoring capability provided by a FGR, perhaps theOmron G4A-1A-Ewould be suitable? It's reasonably inexpensive for what it's doing (~$10) and offers 20A switching capacity. (Oddly, the data sheet doesn't give a rating in VA or W, butdoesstate that "this Relay is suitable for power load switching of air-conditioning compressors and power supplies", so it's probably okay for your purposesas long asyour load is at least 100 mA.Otherwise, maybe you should look at "contactors" rather than "relays". Functionally, they're essentially identical, except "relays" are expected to switch signals or modest levels of power, while "contactors" are meant to switch large amounts of power (e.g. condensers, warehouse lighting, industrial loads). They're also notcheapby the standards of $1 relays, but basic models (2-pole 40A) can be found for ~$20.As DBecker noted, the reason or lowswitchingratings compared tocarryingratings is arcing and contact wear. On that note,PLEASEuse a TVS (or an RC snubber) across the relay coil.DO NOTuse a flyback diode. If you can use "nothing", even better, but you'll need an isolated supply that can handle the back-EMF.Of course, I am assuming there's a reason you've ruled out SSRs or TRIACs...

author: bdunham7
author_rank: Super Contributor
timestamp: May 12, 2023, 05:42:54 pm
content: Quote from: Red Squirrel on May 12, 2023, 09:33:20 am1 Pole : 20A @ 277VAC & 28VDC, General Purpose28VDC x 20A = 560W (DC)277VAC x 20A = 5540VA (AC)

author: Red Squirrel
author_rank: Super Contributor
timestamp: May 13, 2023, 01:06:22 am
content: So as long as I am running AC I can go by the VA rating then? That's what I was starting to think, but it was not super clear.The relays that have a signal contact sound interesting too, as being able to confirm they actuated would be a plus. May look into those.