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Can a 220V relay be used to switch 110V power? If yes, then additionally to this question, does the amperage (A) rating on the relay change when the voltage (V) is 110V instead of 220V?

To the risk of answering my own question, I think any voltage lower or equal to 220V could be switched with a 220V rated relay. Hope to be right on this, but I'm not sure about the amperage rating. My guess is that instead of 220V/5A that relay could also be suited for 110V/10A but I'm not sure.

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    \$\begingroup\$ After 40+ years in the electrical trade, I've found using a properly rated device for the application is key to a safe installation. \$\endgroup\$ – joebanana Dec 8 '17 at 4:22
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    \$\begingroup\$ Note that you also need the right coil voltage for the relay. This could be the same as the rated contact voltage, or something totally different (e.g. 24VAC or even DC). \$\endgroup\$ – Someone Somewhere Dec 8 '17 at 4:57
  • \$\begingroup\$ If you are switching mains voltage inside a machine, use listed components e.g. From Mouser or Digi-Key. If you are switching mains voltage as part of house wiring, you must use relays listed for direct use in mains wiring installation, such as RiB's, which are pretty much the same relay in a listed enclosure. \$\endgroup\$ – Harper Dec 8 '17 at 8:32
  • \$\begingroup\$ There may be a basement value on the device's specs as well, IE you need to switch a minimum of at least X volts or Y amps. Check the spec sheet. \$\endgroup\$ – Criggie Dec 8 '17 at 8:52
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    \$\begingroup\$ @joebanana: everyone will agree, but does this also mean that overdimensioning can be unsafe? \$\endgroup\$ – dlatikay Dec 8 '17 at 9:41
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The 220 volt relay could be used to switch 110 volts. However the contact current rating will not increase. Contacts rated for 5 amperes will not be safe switching more than that, certainly not 10 amperes. The contacts carry the current when closed so the voltage is not the issue but the size and material of the contacts is important. Therefore do not exceed the contact current rating no matter what the value of the voltage being switched

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  • \$\begingroup\$ Thank you for answering the question + the additional question :) \$\endgroup\$ – that-ben Dec 7 '17 at 17:17
  • \$\begingroup\$ One interesting thing I learned while at UL, there are some relays that have different current ratings at different voltages; the current + voltage were inversely related. Not very common to see this, as the current rating is essentially independent of the voltage being switched, but it did happen. \$\endgroup\$ – Jim Dec 7 '17 at 18:38
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    \$\begingroup\$ @Jim, there are two reasons for a max current rating. One is to limit ohmic heating while the contacts are closed, and the other is to limit arcing at the moment when the contacts open. The open-circuit voltage has no impact on the amount of current that the relay can conduct, but it probably does have an impact on how much current the relay can safely interrupt. \$\endgroup\$ – Solomon Slow Dec 7 '17 at 21:59
  • \$\begingroup\$ I've come across a few with a higher rating at 110V than 220V, but in a nonlinear fashion (e.g. 5A@220V, 6A@110V). They've usually had a low voltage rating as well. \$\endgroup\$ – Chris H Dec 7 '17 at 22:00
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    \$\begingroup\$ Also note that 110V AC rated does not mean 110V DC rated, and vice versa. \$\endgroup\$ – rackandboneman Dec 8 '17 at 10:44
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220 V(AC), 5 A Relay means, you can switch upto 220 V, safely across the relay and 5A is the maximum current allowed to flow through it. Current rating remains the same, even if you are switching 110 V.

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    \$\begingroup\$ Thank you for answering the question + the additional question. Since Barry had replied 8 minutes prior to you, I accepted his answer, but thank you for answering too. \$\endgroup\$ – that-ben Dec 7 '17 at 17:18
  • \$\begingroup\$ But what about using a 110V rotary switch on 220V (if the switch support much more amps than needed)? \$\endgroup\$ – JinSnow Mar 1 at 18:22
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Exceeding the voltage rating may cause the insulation to pierce while exceeding the intensity (for a long enough time) will cause the conductors to overheat. Lower voltage but higher amperage means the same power, but the dissipated power may be larger (experiment required), hence risk of overheating.

If this is a professional work, you will likely breach the code for exceeding the allowed Amp value, and you will be hold liable if something bad (like a fire) happens. If this is a lab setting, test before using, it may work.

The relay rating is for max voltage and amps, using lower voltages and amps is OK.

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  • \$\begingroup\$ Exactly, it depends whether this is permanent wiring for an elementary school, or emergency wiring to get the bilge pumps running to save the ship. \$\endgroup\$ – Harper Dec 8 '17 at 8:34
  • \$\begingroup\$ If it is about saving the ship, it will always work. My bilge pump runs at 12 Volt :) \$\endgroup\$ – user171249 Dec 8 '17 at 14:18
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Heating effect of an electrical current is i^2Rt. No mention of v. Amps are amps, regardless of what voltage is pushing them. So no, it's the same 5A rating.

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  • \$\begingroup\$ Not accurate or correct. \$\endgroup\$ – Carl Witthoft Dec 8 '17 at 16:14
  • \$\begingroup\$ @Carl Witthoft Tell us why then? \$\endgroup\$ – Laurence Payne Dec 8 '17 at 17:35
  • \$\begingroup\$ @LaurencePayne: Assumes ohmic behavior, and only heat as a problem. There are more risks with switches (arcing) that you don't mention. \$\endgroup\$ – MSalters Dec 8 '17 at 18:40
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    \$\begingroup\$ You feel arcing would be more of an issue at 100v than 220v? \$\endgroup\$ – Laurence Payne Dec 8 '17 at 19:22

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