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I have a few of these(http://old.ghielectronics.com/downloads/man/20084141716341001RelayX1.pdf) and I was looking at the data sheet. It states that the max inductive load is 7amp(3 on form C). Does this mean when I am using an LED light or motor that the max load is actually 7 amps instead of 3? Just want to make sure I fully understand and am safely staying within the currents the relays are rated for

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    \$\begingroup\$ Where do you 7? It's 3 for inductive load. \$\endgroup\$ – Eugene Sh. Nov 27 '17 at 19:20
  • \$\begingroup\$ Try not to rely on URLs in your post - they may disappear without warning. Instead include the manufacturer/part number so readers can look it up, and just include the URL for convenience. \$\endgroup\$ – Heath Raftery Nov 27 '17 at 19:40
  • \$\begingroup\$ Thank you @HeathRaftery that is a great suggestion and I did not think of that! \$\endgroup\$ – Sean Moriarty Nov 27 '17 at 19:55
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Look again

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The 10A 120Vac rated resistive load is best case because of the zero crossing nature that extinguishes an arc quickly so the contact temperature is reduced which degrades life span rapidly. However even short short wires have some inductance and dry contacts have very high dI/dt because disconnect time is abrupt and this results in sufficient voltage to arc across contacts but the stored energy in the wire (1/2LI^2) is very low.

Next best case is 7A 28Vdc or 240Vac R load again. This means the energy stored in the wire (L) at 28Vdc and dumped into the silver-oxide-alloy plated contacts as heat, considered comparable to 240Vac since AC voltage but can be extinguished by zero crossing currents, but still reduces the recommended limit from 10 to 7A.

The worst case is 3A high inductive load such as a motor which tends to have a surge current rating that is 8 to 10x the motor rated current on startup. This could be protected using soft-start mechanisms or PWM but then you wouldn't need relay then. With motor or solenoid relays, you can expect the lifespan to be several orders of magnitude lower than the mechanical cycle lifespan.

Check out the L/R specs on your load to be sure, also.

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  • \$\begingroup\$ Perhaps add "a LED does not have significant inductance, so the resistive ratings would apply if your load only consists of LEDs" or something to address that part of the original question. \$\endgroup\$ – Heath Raftery Nov 27 '17 at 19:46
  • \$\begingroup\$ It is always wise to derate the Max current of a relay for normal operating conditions for reasonable reliability. A 5m LEDstrip can have about 10uH but also has a much lower L/R ratio, so R load rating is a reasonable assumption. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Nov 27 '17 at 19:54
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    \$\begingroup\$ I can't thank you enough for the answer @TonyStewart.EEsince'75 I've been trying to make sense of all this and that answer makes it crystal clear. I used these relays to control LED lights that pull roughly 1.2 amps and I had some people on reddit talking about how I was going to cause a fire using those relays. I was pretty confident there was something I was missing here and that the 3 amp thing was not a normal max load. Thank you again! \$\endgroup\$ – Sean Moriarty Nov 27 '17 at 19:59
  • \$\begingroup\$ Just dont apply it to the power supply which has a large capacitive load surge and dont cycle it at some high rep rate. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Nov 27 '17 at 20:03

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