I'm trying to select a relay for a IoT project that will control a water boiler. I took a look at the HF32F-G relay, but some of the specifications in the datasheet weren't making any sense to me.

Specifically, the given contact rating for a resistive load is 10A@250VAC, which naturally translates into the specified max switching apparent power of 2500VA.

However, the specified figure for max switching real power is merely 300W, which doesn't seem to agree with the given contact rating for a resistive load of 10A@250VAC.

Can anyone shed some light on this apparent discrepancy?


2 Answers 2


The difference is AC vs DC.

For DC, the rated voltage is only 30V, which at 10A comes to a 300W max load.

For AC, the rated voltage is 250V, which at 10A comes to a 2500VA max load. Note that this is Volt-Amps which for a pure resistive load is equivalent to Watts.

The datasheet is not saying that for AC, you can have 2500VA active power, but only 300W resistive power. Rather it is giving two specifications, one for AC and one for DC.

I've tried to highlight how these specifications link together in the annotated diagram below.

Contact Ratings

  • \$\begingroup\$ Side question, do you know why the DC voltage rating is so low then? \$\endgroup\$
    – Linkyyy
    Sep 11, 2018 at 20:30
  • \$\begingroup\$ You're probably right... I can't believe I overlooked this :) \$\endgroup\$
    – Sagie
    Sep 11, 2018 at 20:44
  • 7
    \$\begingroup\$ DC ratings on switching contacts are generally MUCH lower then AC ratings because with AC the current drops to zero twice each cycle so the arc tends to self extinguish, with heavy DC you have to take special measures. \$\endgroup\$
    – Dan Mills
    Sep 11, 2018 at 20:45

Depending Vdc Idc or AC ratings of output/input , DC relays being more efficient can have current gains of upto 3000 or so and high power gains by trading off qualty or DC input voltage or some other parameter.

This example has a maximum DC power switching 300W/450mW ~ 600.

DC contact ratings must be reduced for inductive loads due to arc load which is why V* I is not equal to W. Ac has a zero crossing to quench current arcs.

  • \$\begingroup\$ another zero feedback troll -1 \$\endgroup\$ Sep 11, 2018 at 21:51
  • 1
    \$\begingroup\$ Not sure what that is about. Have a +1 to make up for it :) \$\endgroup\$ Sep 11, 2018 at 22:32
  • \$\begingroup\$ It's extremely terse and starts talking about "gain" which won't help OP. Only the last sentence is helpful, but does not add anything to Tom's answer. \$\endgroup\$
    – pipe
    Sep 12, 2018 at 7:05

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