# How to wire a high-current relay?

Background:

I am designing a small Arduino-driven circuit to control a ~3/4 hp AC hoist motor. The hoist motor includes a starter-capacitor. The Arduino drives a transistor which in turn drives a power relay that switches the motor on and off. I am not an electrical engineer, so I'm using a Factor of Safety of 2 or higher throughout the circuit to avoid component damage/burnout.

The particular relay I had in mind is supposed to be capable of handling up to 30A sustained (more than 4 times the max-load current of my motor) at double the nominal operating voltage. My intent is to run two of them in series for safety (to handle one failed-closed relay) and for wear-leveling of the contacts.

Consulting an ampacity chart, it looks to me like I need 10AWG copper wire (about 5.3mm^2 conductor area) to handle 30A. But as you can see from the relay datasheet, the load pins are ridiculously tiny: each is roughly equivalent to 17AWG (1.6mm^2). Unfortunately Digikey does not carry a socket with solder cups or terminal lugs for this particular relay, and I'm having a very hard time soldering 10AWG wire to a 17AWG pin.

Questions:

1. How can the relay pins carry so much current with such a tiny conductor?
2. Is 10AWG really the right size of wire to carry 7A AC with an inductive load?
3. How should I go about wiring this relay to the rest of my circuit?
• This is all a matter of voltage drop. You will have the most voltage drop at the connector anyways, it doesn't matter if 10cm length of wire is 10AWG or 17AWG. Feb 25, 2018 at 20:46
• Surge current for motors can be as much as 500% for AC and relays are derated about 30% of resistive rating. Ampacity is based on heating, voltage drop affects current limit. Your relay is expect to last 1x10^3 cycles on 230Vac , possibly less with heavy hoist loads. Feb 25, 2018 at 20:48
• Spade lugs are easy to fit. Feb 25, 2018 at 20:53
• That may be rated for 30 A. (I saw the datasheet.) But I would not even consider it for more than 10 A. Best case. (Which nominally might work for 3/4 HP.) Frankly, I just think the datasheet can't be trusted since PC-mount relays like this, with pins like that, cannot handle 30 A. If you want 30 A, then what you need is called a "medium current relay" and these don't look anything like your unit. To reduce wear from arcing, consider an SSR in parallel but operated with different timing control.
– jonk
Feb 25, 2018 at 20:55
• What voltage and surge current and acceleration time? Feb 25, 2018 at 21:08

Since motors have a surge current starting up that is up to 5x the max load rated crrent, if you say the hoist motor is rated for 15A then expect up 75A starting surge which reduces while increasing in speed to match the load torque=current demand.

In this hoist motor case, I would be looking towards OMRON or PANASONIC Relays rated for 75 to 90A if you want to do more than 1k cycles, which I estimate from TE's datasheet.