[sol]

In a perfect resistance-less frictionless world 1HP = 746 Watts
So at 115 Volts 1 HP = 6.5 Amps

A search through router specs shows that the manufacturers are rating their routers at 5 Amp per HP, which is pushing the honesty envelope (Yeah, they say "Peak HP"), so we'll give them a pass.

Relays are rated by amperage as well.
An "icecube" style 15 Amp 120 Volt relay is sold/rated as 1/3 HP (not 3HP but 1/3 !?)
A Printed Circuit Board relay that is rated at 10A @ 120V will fry when controlling a 2HP motor.

Whiskey Tango Foxtrot?
Obviously there are different measuring methods used by different manufacturers, anybody able to give me a clue?

Thanks!

[Greg Fill]

There is a large inrush and break current that has to be considered when using relays to control motors. Most contacts are rated with a resistive load rating and a motor/inductive load rating. The inductive load rating is typically a lot lower than the resistive rating. There are PC mountable relays that will handle your requirements. Look at the AROMAT JT1AE-TMP.

www.aromat.com/pcsd/webpages/pwr_h.htm

JT series. I have used these relays and similar Potter and Brumfield T90 for years in all our HVAC controls. You can also use a Solid state relay as well for your application.

[Al_The_Man]

I believe the amperage rating they give is resistive load, but the HP relates to inductive load rating of the contacts, inductive load much harder on contacts due to arcing. Also there is a de-rating factor if DC is used.
High DC current inductive load relays often use a 'magnetic arc blow out' method, where a permanent magnet is situated close to the contacts and effectively blow the arc out!!
Al

[HillBilly]

I have seen those magnets. Now I know why they are there. Thanks Al!

Darek

[sol]

Thank you Greg! Thank you Al!
Induction vs resistance...It comes back... through a glass darkly now.

Henrys and Farads and Lenz, Oh My!
Henrys and Farads and Lenz, Oh My! ....

[High Seas]

Thought that ditty went:
"Henrys, Farads, Lenz, and Telsa too-
What would I do without you?
Spend my daze in quiet repose
better hit the books - I suppose..."