# How many loads can a single relay handle

#### Jason Andrew Langgin

Joined Feb 26, 2018
38
how many loads can a single standard automotive relay handle?

#### crutschow

Joined Mar 14, 2008
26,756
What do you consider "typical"?
This one, for example, can carry 50A on the NO contact and 20A on the NC contact.

#### crutschow

Joined Mar 14, 2008
26,756
I guess what I'm asking is on the 87 pin how many loads can you attach to it
You can add as many loads as will have a total current of no more than 40A for the NO (normally open pin) or 30A for the NC (normally closed) pin.

#### Jason Andrew Langgin

Joined Feb 26, 2018
38
You can add as many loads as will have a total current of no more than 40A for the NO (normally open pin) or 30A for the NC (normally closed) pin.
Awesome thank you that's what I figured because its really just a switch but it never hurts to ask

Another question I am going to fuse the 30 pin already , but If I was to hook this up as a trigger wire to the (reverse lights on a car) for pin 87 do I just add up all the power of the loads to determine the fuse size. I already no that's how to calc for pin (30) I just need to no for pin /87/

So basically on both ends of the relay

pin 87 hook to reverse lights = fused

And pin 30 hooked to fuse panel with (add a circuit) = fused

This will all be a total of 3 devices being powered for. Total of = 2.1Amps so would a 3 amp fuse be good for (both sides) using 16awg wire?

Or how do I calc for pin 87 being used as trigger wire?

#### K1W1

Joined Jan 13, 2018
13
Hi Jason.
A little confused with your use of the term "trigger" when referring to pins 30 and 87 on a relay.
Pins 85 and 86 would normally be referred to as the trigger pins.

Pin 30 is the power supply input, so is usually the only pin that needs fusing.
The exception would be if you want to individually fuse each device being supplied from the relay output pin 87.

An example would be if you wanted to fuse the vehicle and trailer reverse lights separately.
A 10A fuse could be fitted at the power source(battery, fuse box etc) for the wire going to pin 30.
Two wires with a 5A fuse in each would be connected to pin 87. One wire would go to the vehicle's reverse lights, and the other wire would go to the trailer plug.
A short in the trailer would only blow the 5A fuse to the trailer plug. The reverse lights on the vehicle would still operate.
The 10A fuse would protect the wire to pin 30 on the relay.

What three devices are being powered by the relay?
2.1A at 12v is only about 25Watts. Most reverse light bulbs are 21W. Are your reverse lights LED?

#### crutschow

Joined Mar 14, 2008
26,756
Typically you fuse for the wire size to prevent melting the wire or its insulation in case of a short.
Thus the smallest wire gauge on the relay will determine the fuse size if you have only one fuse on the power going to the relay (pin 30).
Here are some typical maximum fuse size recommendations versus wire gauge.
It's fine to use a smaller fuse for a smaller load current.

#### Jason Andrew Langgin

Joined Feb 26, 2018
38
Typically you fuse for the wire size to prevent melting the wire or its insulation in case of a short.
Thus the smallest wire gauge on the relay will determine the fuse size if you have only one fuse on the power going to the relay (pin 30).
Here are some typical maximum fuse size recommendations versus wire gauge.
It's fine to use a smaller fuse for a smaller load current.

View attachment 147488
Ok so no fuse on the switch side then?

#### K1W1

Joined Jan 13, 2018
13
What do you mean by "the switch side"?

Power supply to pin 30 needs to be fused.

The signal wire from the reverse light switch to pin 86 also needs to be fused. This would normally be in the vehicle's fuse box.

#### BR-549

Joined Sep 22, 2013
4,938
The trigger is being powered by the backup light power rail. He wants to fuse that trigger I believe.

It would be like fusing the coil power on a conventual relay.

#### Jason Andrew Langgin

Joined Feb 26, 2018
38
Ok so it will be 16awg wire

Pin 30 is going to be connecting to = fuse panel acc which is gong to be fused prob with 5 amp fuse I'm thinking

(Pin 86= trigger but what size fuse for this?) connecting to reverse wire

Pin 85= ground

#### crutschow

Joined Mar 14, 2008
26,756
Pin 86= trigger but what size fuse for this?) connecting to reverse wire
If you use the same wire size to the relay as the reverse wire, than the fuse to the reverse wire should suffice.

#### Jason Andrew Langgin

Joined Feb 26, 2018
38
If you use the same wire size to the relay as the reverse wire, than the fuse to the reverse wire should suffice.
Are you talking about just using the factory fuse so basically if I use same wire on relay than i don't need to worry about fusing the 86 pin cause the factory fuse will suffice? Sorry I just want to understand this correctly

#### crutschow

Joined Mar 14, 2008
26,756
Are you talking about just using the factory fuse so basically if I use same wire on relay than i don't need to worry about fusing the 86 pin cause the factory fuse will suffice? Sorry I just want to understand this correctly
Yes.
I'm assuming there is a factory fuse for the reverse wire, so if you use the same size wire to the relay coil, the factory fuse will protect the wire.

#### Jason Andrew Langgin

Joined Feb 26, 2018
38
Yes.
I'm assuming there is a factory fuse for the reverse wire, so if you use the same size wire to the relay coil, the factory fuse will protect the wire.
In the picture is maybe my dilemma
It shows that there are two exterior 40amp fuses on the reverse light wire if this is the case can I just tie another fuse say another 3 or 5 amp to the 87 pin and connect (3) different loads total=2.1amps

#### Attachments

• 365.5 KB Views: 6

#### crutschow

Joined Mar 14, 2008
26,756
Both fuses do not go to the same circuit.
You will need the vehicle schematic to determine exactly where each goes.

A 5A fuse on pin 87 should protect all the relay circuits.

#### Jason Andrew Langgin

Joined Feb 26, 2018
38
Both fuses do not go to the same circuit.
You will need the vehicle schematic to determine exactly where each goes.

A 5A fuse on pin 87 should protect all the relay circuits.
Now the only question is where do I get a schmatic at for a 2013 Dodge Charger reverse light circuit I think it should be ok a lot of people that I have seen run only a fuse on pin 30 using a fuse tap and just run all the other pins the same as usual

It's basically just running a backup camera and 4 sensors and a LCD mirror totaling = approx 2.1 amps all on 16awg wire