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relay advice

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Forum Name: Relays
Forum Discription: Relay Diagrams, SPDT Relays, SPST Relays, DPDT Relays, Latching Relays, etc.
Printed Date: December 05, 2021 at 9:33 PM

Topic: relay advice

Posted By: coolen
Subject: relay advice
Date Posted: August 19, 2010 at 3:16 PM

I'm going to be installing an emergency warning package, that features the following:


Body Lights-12A

Siren 10A

Arrow Stick: 17A

To keep things simple, I want basically a one switch does all set up. I have a 10A switch, that along with the use of a relay will power, the lightbar, and the body lights. (22A Max)

Here's where I need some advice. The controller for the lightbar will be connected to the ign wire (additional 2A),  and the instructions reccommed connecting the siren to the ign wire as well. (additional 10A) This being the case because the siren does not have an actual off button. Will the ign wire handle the additional 12 A? If not, can i wire a relay to this particular set up?

Thanks for now.....


Posted By: oldspark
Date Posted: August 19, 2010 at 5:10 PM
Why risk it - or add more load to the Ig circuit?
(Or why even bother trying to find out?)

Use relays. One for the siren, one or more for the others.

Posted By: i am an idiot
Date Posted: August 19, 2010 at 5:18 PM
I would not try to pull 12 additional amps through any vehicles ignition circuit.  Use a relay

Posted By: coolen
Date Posted: August 19, 2010 at 7:15 PM

Thanks for the replies. I didn't think it would be a good idea to pull that much extra power through it. So, if I understand this correctly, and referencing this diagram:, I would use the ignition wire as the "remote turn on lead" (term 86), and 87 goes to and gets fused at the battery. Fuse at the battery to match the additional load..arrow stick controller and siren (15A fuse)- and term 30 can be used to power the controller and siren.

Does this sound accurate?

Thank you

Posted By: i am an idiot
Date Posted: August 19, 2010 at 8:08 PM
Accurate to me.

Posted By: coolen
Date Posted: August 20, 2010 at 4:25 PM

I was reading in the Basics section that most relays only draw very little power. Would I use a wire rated for what the relay is powering to run to the battery? Or do you just need a "signal" wire?

Thanks again

Posted By: i am an idiot
Date Posted: August 20, 2010 at 6:36 PM
The wire that goes to the battery and to the devices needs to be large enough to handle the current of all of the devices combined.

Posted By: oldspark
Date Posted: August 20, 2010 at 8:27 PM
coolen - don't confuse the "control signal" to the relay which is small - typically under 250mA for automotive relays.

That is totally separate to its "switching current".

The control/signal/coil/solenoid only need thin "signal" wire (250mA etc).

The power wire needs to be rated for the load (and fused for or under the lesser of the relay or cable rating).

A simple point of confusion, yet obvious once understood. Remember - you cannot "create" power or energy.....

Posted By: js305
Date Posted: September 05, 2010 at 4:06 PM
There is another similar relay that not many know about that will handle 70 amps. NTE (and others I'm sure) markets this under


It is a SPST with only four blades. Two are the normal quarter inch for the coil and the other two are 3/8's for the power. I have used these on most of our fire trucks for the higher amp draw devices like you describe. This is a volunteer dept and everyone wants everything to turn off and on with the ignition. I use one for the radios, one for the lights, and one for the siren. All of them only draw maybe an extra amp through the ignition. I have never had one fail in 26 years on this department.

I usually run an 8 gauge wire and carefully solder it directly to the 3/8 inch blade and add a little heatshrink. It's pure copper and solders easily. There are sockets available but I have seen some metal fatigue and resistance build up with these forty and fifty amp devices, especially old lightbars with a half dozen rotators.

Joe in Texas

Posted By: howie ll
Date Posted: September 05, 2010 at 5:43 PM
JS you're quite correct, I've seen these on UK Fords as well 80amp rating with the same terminal sizes.

Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

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