Why do 40 amp relays have 12awg wire?!

OutdoorTxs

OutdoorTxs

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Im looking to add a 40amp relay to a 40amp circuit under the hood for so me lights, and every relay harness I find has pigtails with 12g wire. Shouldn't 40 amps require 8g wire?
I've searched high and low and cannot find a way to attach 8g wire to a 40amp relay, short of soldering it straight to the relay.
Even 80 amp relays have 12g wire on the harness's I've found. This is bugging me....lol
 
The relay is only a switch. The 12awg wires only activate the switching mechanism in the relay. The 40 amp circuit is switched on by the lower amperage circuit and allows for the connection via the 8awg (or applicable wiring) connections. Hopefully that makes sense.
 
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Doesn't the relay go in-between the battery and the load? If the load is 40amps, shouldnt the relay have 8g wire to the battery, and to the load? Then I can use smaller wire to run into the truck for the actual switch
 
OutdoorTxs said:
Doesn't the relay go in-between the battery and the load? If the load is 40amps, shouldnt the relay have 8g wire to the battery, and to the load? Then I can use smaller wire to run into the truck for the actual switch
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OutdoorTxs said:
Doesn't the relay go in-between the battery and the load? If the load is 40amps, shouldnt the relay have 8g wire to the battery, and to the load? Then I can use smaller wire to run into the truck for the actual switch
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Yes, the switch is connected via the 12awg wires and the battery and load gets connected to the relay with the appropriate wires to meet the amperage of the load. I.e- 8awg for 40 amps. Temp and distance can affect resistance but in most vehicle runs it is not a concern.
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Ok, so that was my question. The pig tails, off the harness for a 40amp and even 80amp relay, are 12g wire. Even the leads going to the battery, and the load are 12g wire. I dont understand why the leads to battery and load, on a 40+ amp relay, are not 8g wire or bigger. This has me really confused lol
 
OutdoorTxs said:
Im looking to add a 40amp relay to a 40amp circuit under the hood for so me lights, and every relay harness I find has pigtails with 12g wire. Shouldn't 40 amps require 8g wire?
I've searched high and low and cannot find a way to attach 8g wire to a 40amp relay, short of soldering it straight to the relay.
Even 80 amp relays have 12g wire on the harness's I've found. This is bugging me....lol
Click to expand...
I have had the same issue when looking for large relays. By the book, a full 40 amp load would require min. 6 gauge copper wire, and that would be good for a total length of 20 feet to maintain a voltage drop of less than 3% (.36 Volts) Much of the accessory industry greatly undersizes wire size and switches etc. Often they are also using CCA (copper clad aluminum) wire which has a lower rating than full copper. the heat generated by overloading the wire causes failure. Keeping wiring as short as possible and making good connections is critical. First thing is to know exactly what amperage your load will be drawing. Lighting, especially LED draws much less than it used to. Check specifications for each item to be connected, you can also connect an amp meter in line and test the lamp if you do not have specs. 40 amps seems high for lighting. a 40 amp load will also make a good hit on your alternator when totaled in along with all other vehicle loads, so make sure the total load of the vehicle and added accessories is within capacity of your alternator. If you have more than one light that you want to control from one switch and the amperage of each lamp is high, consider adding a relay for each light, that will bring down the load on the relay and associated wiring to within specs of a smaller rely and its socket wiring. the relay triggers draw a tiny amount and can be gathered to one standard switch for control.

For plug in relays and connectors, the largest one I could find was this one from Amazon The three large wires (input, normal open and normal closed) are not listed by size, but the ones I received measure out averaging 0.10 inches in diameter for the conductor. This equates to 10 gauge and looks to be all copper, one step up from most others I have found. (the wire would be rated at 20 amps) The relay is stamped at 80 amps (greatly overstated in my opinion). I have been using these to control the power supply to all my two way radios so I can shut them down with one switch. As the highest load passing through when transmitting is a bit less than 30 amps and is it is a non continuous load I have not seen any issues of warm wire or anything.
 
KonzaLander said:
You can find unpinned relay sockets and build your own harness with the proper size wire.

Here is an example. Basic 4 pin relay that is being marketed for HID lighting wiring.
View attachment 248884
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I like the modular design and O-ring seals. Specs say the relay and its coils are rated to 40 amps, however the terminals look light, and are only rated for 14-16 gauge wire which would not be capable of 40 amps. I am sure the quality is better than the Amazon one overall, but surprised at the minimal size of the terminals for a 40 amp load.
 
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OTH Overland said:
I like the modular design and O-ring seals. Specs say the relay and its coils are rated to 40 amps, however the terminals look light, and are only rated for 14-16 gauge wire which would not be capable of 40 amps. I am sure the quality is better than the Amazon one overall, but surprised at the minimal size of the terminals for a 40 amp load.
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Good catch! I am not sure how I missed that detail.
 
KonzaLander said:
Good catch! I am not sure how I missed that detail.
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I like to run my electrical devices and wiring at 80% of continuous load to prevent issues, relays seem to last much longer when they run at 50-60% capacity, thinking it results in less pitting of the contacts when they make or break contact. I have a background in commercial/industrial electrical design and there is no way you can get away with what the automotive industry does regarding sizing of components...lol
 
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I suppose one could purchase an 80 amp relay that has the oversized blades for the load portion similar to the one from Amazon, and completely eliminate the plug in base. Solder some short 6 gauge pigtails to the blades and connect into your wiring system with an anderson plug for the large gauge wires and a duetch plug for the 16 or 14 gauge control wires. would give you solid connections all rated for the 40 amps, a heavy duty relay and you could build a spare to swap in on the trail if needed.
 
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I've never been happy with the supplied pigtails.
As @OTH Overland mentioned, its not so much the relay ratings as it is arcing (pitting) on the contacts. The issue I run into isn't that the relay cant flow but the arc on the contacts. On my higher draw items I now go with larger relay sizes. My compressor is the best example because of its cycling. My light relays have way more life. The compressor relay tends to have about a one year life span using a standard 40 amp "Bosch" design. Tear down shows the contacts wore due to arcing. I now run a 100 amp contactor on the compressor. Its just a relay on steroids.
As someone else mentioned, Its also a good idea to run the skirted waterproof relays.
Almost all of my automotive electrical supplies come from Del City or Waytek. I rarely buy these things on Amazon unless I know who made it.
 
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Some good info. Thank you guys. I guess going with over sized relay, and just soldering connections is the best option.
 
OutdoorTxs said:
Im looking to add a 40amp relay to a 40amp circuit under the hood for so me lights, and every relay harness I find has pigtails with 12g wire. Shouldn't 40 amps require 8g wire?
I've searched high and low and cannot find a way to attach 8g wire to a 40amp relay, short of soldering it straight to the relay.
Even 80 amp relays have 12g wire on the harness's I've found. This is bugging me....lol
Click to expand...
Relays don't have wires, only contacts. Could you provide a picture of what you have?
Wire gauges depends on length and current and duration so it's not that easy.
 
MazeVX said:
OutdoorTxs said:
Im looking to add a 40amp relay to a 40amp circuit under the hood for so me lights, and every relay harness I find has pigtails with 12g wire. Shouldn't 40 amps require 8g wire?
I've searched high and low and cannot find a way to attach 8g wire to a 40amp relay, short of soldering it straight to the relay.
Even 80 amp relays have 12g wire on the harness's I've found. This is bugging me....lol
Click to expand...
Relays don't have wires, only contacts. Could you provide a picture of what you have?
Wire gauges depends on length and current and duration so it's not that easy.
Click to expand...

I know relays have contacts, I'm referring to the harnesses.

This is an example of 40amp relay with included harness using undersized wire. https://amzn.to/3ZYIakd
 
Okay, so that is not easy to explain but technically it's not undersized.
It's not suited to permanently work with the maximum current of the relay, which sounds similar but it's not the same...
If you really want to draw 40A, which would be quite a bit of power, I recommend getting a stronger relay anyway and buy it from a quality source with a quality harness if you need that. You can't trust cheap relays!
With that setup you are good for 15-20A temporary load.
 
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