Can 2 relays be used in parallel to use for double the rated load current?

Question

I am trying to use 2 X 20Amp load current rated relays in parallel (both COMMOM tied together, both NO tied together, both NC tied together, both coils tied together) to run 40A load current. From electrical/theoretical standpoint it should work but I just have never seen any such implementation. My limitation is size as I can find a dual 20A relay in a much smaller height than a single 40A relay.

Answer 1

Solid state relays are better than electro-mechanical releys for the application you are trying to do. Some solid state relays will handle 45 Amps.


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Answer 2

In theory, no problem. In practice it won't work for long.

You have a few issues. First of all they won't close or open at exactly the same time, so 40A will be going through 1 of them for a short time when it is turned on and off. 1 20A relay can almost certaintly handle 40A for a fraction of a second - the problem is the contacts. The voltage induced across the contacts will be L*(di/dt) - so it will be double what it should. This will cause a bigger arc across the contacts than they are designed for and they will wear out very quickly.

One thing you can try doing is to split the load. For example, if you have 4 amplifiers that require 10A each, put 2 of them on 1 relay and 2 on the other relay. You can also try looking into power FETs - they are usually much more space efficient than relays and will be more reliable anyway.

Answer 3

Relays In Parallel

Answer 4

The risk is that both relays do not actuate exactly simultaneously. From there it depends on the load. If the load draws a huge inrush current, two relays might not work out. Probably you will be fine though.

These relays are separately fused right? If not, just put a 20A fuse in series with each. Then, if one relay fires and not the other it won't start a fire. The worse that can happen is that you blow fuses and then you will have to find a 40 relay.

Answer 5

Answer is "NO", for several reasons:
The 2 relay coils in parallel may not close at the identical moment. The mechanical operation of these 2 relays are different, especially should these relays be from a different manufacturing lot. The contact resistance of each relay probably is not equal. RECOMMEND you obtain a 40 ampere electronic relay to fit your confined space.
DROCK 1.

Answer 6

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Relays with current ratings above 10 amps or so are generally called contactors. Contactors are often available with one or more auxiliary contacts that have a lower current rating than the main contacts. A very common contactor configuration includes 2 or 3 normally open main contacts and 1 N.O. auxiliary contact. The auxiliary contact is usually used as a "seal in" contact in a 3-wire, momentary-contact push-button control circuit. Edit 1 I believe that the notation "C/O" means "changeover" or "double throw." That configuration is not usually available in contactors. A contactor with 1 N.O. and 1 N.C. contact is also likely to be difficult to find.

Answer 7

No.

The relays will never simultaneoulsy close, thus one of them will always take a 'spike' of full load current (twice the rated value). Those contacts will burn out rather quickly, and then the other relay will fail even quicker.

Even if you have a double-pole relay you shouldn't current-share.

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Answer 8

i am no expert on this situation, but i do have minimal experience in the electrical field, and moderate experience in automotive, but my thought that is it may work, but if one of the relays fails for some reason, would it not cause double the load on the other and then have risk of fire???

Answer 9

The answer is NO. One contact will always make and/or break before the other one will. If you are using 40 amps through 20 amp contacts in parallel... one will eventually burn over time. Then the other one will burn also!
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