P500 When do we need a relay?

[Deleted member 3748]

Is there a rule of thumb to determine how many watts an accessory can draw before we should use a relay?

 

[Admin]

 

[ohanacreek]

What's the switch rated for?
What's the wire you want to fed the switch rated for?
What are you trying to power and for how long?

 

[Deleted member 3748]

What's the switch rated for?
What's the wire you want to fed the switch rated for?
What are you trying to power and for how long?

20 amp switch, 12 ga wire, 120 watt lights. These components check out. Do your add a relay if you think you're going to burn up your switch or wire?

 

[Montecresto]

20 amp switch, 12 ga wire, 120 watt lights. These components check out. Do your add a relay if you think you're going to burn up your switch or wire?

My light bar came equipped with a relay in the harness.

 

[393stroker]

You add a relay if your load exceeds the ampacity of your switch. You size you wire according the ampacity of the device example (lights).

 

[393stroker]

The way you find your true amps is watts divided by volts. Example 120w divided by 12v = 10a.

 

[swsebek]

For what you have a fuse would be fine

 

[ohanacreek]

There are a number of reasons to use a relay, if you exceed the capacity of any part of the circuit you need one.

Best example is a winch contactor, it is a HEAVY duty relay that through its design reverses the polarity to reverse direction of feed, its a 200+amp circuit controlled by a circuit that is probably around a 3-5amp fuse.

You should use them for light bars over 180 watts, or anything over @20amp draw, if its close or right at it always better to err on the side of caution then burn a 10-20k SXS to the ground because you over estimated the circuit capacity.

(I know how to calculate amp draw but I had a 180watt bar draw 22amps, they aren't always efficient/accurate when you get the Amazon specials)

 

[Montecresto]

For what you have a fuse would be fine

Apparently Aux-Beam doesn't agree as they include a relay in their wiring harness.

 

[Deleted member 3748]

Apparently Aux-Beam doesn't agree as they include a relay in their wiring harness.

Thanks everyone. The 180 watt rule of thumb is what I was wondering about.

 

[Montecresto]

Thanks everyone. The 180 watt rule of thumb is what I was wondering about.

Just so you'll know, my 12" auxbeam is a 72W and it came with a built in relay in the harness with an included 30AMP inline fuse.

 

[ohanacreek]

Thanks everyone. The 180 watt rule of thumb is what I was wondering about.

Not a rule of thumb, I just wouldn't personally risk a $10-20k machine over a $5 relay and $5 worth of wire.

A true 180w draw at 12v (not 13.8 which is what most systems actually are close to when the alternator is charging) is 15amps.

You don't pass the 20amp mark till 240w, BUT like I said I have had a 180w bar draw 22amps. (It kept blowing a 20amp fuse and I was using 10ga wire and a 20amp rated switch so I threw a meter on it)

Better safe than sorry.

A lot of these kits use a cheap $2 switch, a lot of small gauge wire and a cheap relay because it's more profit to them instead of including a 20amp switch and 12ga wire or 10ga wire. They only include a few feet of the heavy gauge wire for the bar or light kit.

It's also easier to get through small gaps in dashes.

 

[Deleted member 3748]

Not a rule of thumb, I just wouldn't personally risk a $10-20k machine over a $5 relay and $5 worth of wire.

A true 180w draw at 12v (not 13.8 which is what most systems actually are close to when the alternator is charging) is 15amps.

You don't pass the 20amp mark till 240w, BUT like I said I have had a 180w bar draw 22amps. (It kept blowing a 20amp fuse and I was using 10ga wire and a 20amp rated switch so I threw a meter on it)

Better safe than sorry.

A lot of these kits use a cheap $2 switch, a lot of small gauge wire and a cheap relay because it's more profit to them instead of including a 20amp switch and 12ga wire or 10ga wire. They only include a few feet of the heavy gauge wire for the bar or light kit.

It's also easier to get through small gaps in dashes.

I haven't hinted any way about how I'm planning on wiring these lights. I was just wondering if anyone had some ideas to go by... like "anything over 50W and I use or relay" or "anything under 150W blah blah blah" I'm not worried about burning up my SxS... I'm worried about melting my Squatch Lights switch! I ordered a relay with my light bar so I have one anyway. All of the info you guys gave is appreciated and helpful. I see that I just need to follow the normal rules for electrical design and I should be good. Thanks to everyone for the help.

 

[Deleted member 3748]

The way you find your true amps is watts divided by volts. Example 120w divided by 12v = 10a.

This is one of the few things I learned in school that is use ALL the time.

 

[swsebek]

Apparently Aux-Beam doesn't agree as they include a relay in their wiring harness.

If i sold light bars I would add a relay to protect myself as acompany.But I know alot of people who never use them on there light bars.And they do fine.

 

[Montecresto]

If i sold light bars I would add a relay to protect myself as acompany.But I know alot of people who never use them on there light bars.And they do fine.

And, what's wrong with protection?? That's what relays, fuses and circuit breakers are all about.

 

[swsebek]

Is it a need?no,20 amp fse would work.

 

[Montecresto]

Is it a need?no,20 amp fse would work.

My Auxbeam came with a 30 amp in line fuse on 10 gauge wire from a grounded relay. I'm installing it like they sent it with the redundancy of protection. As another poster has already pointed out, an ounce of prevention is worth a pound of cure.

 

[swsebek]

You are absolutly correct 100 percent......But the thread is wether he needs a relay....He stated 12 gauge wire,120 watt light with rocker with fuse beside positive on battery...For that install the relay is not needed....and will work fine.....yes a relay is fine too I agree it will give piece of mind to those who want it.....

 

[Montecresto]

You are absolutly correct 100 percent......But the thread is wether he needs a relay....He stated 12 gauge wire,120 watt light with rocker with fuse beside positive on battery...For that install the relay is not needed....and will work fine.....yes a relay is fine too I agree it will give piece of mind to those who want it.....

I think ohanacreek said it well in post 12, words of wisdom.