relays for subs?

Ok, in short, I want to wire relays into my sub output lines to switch between two subs. Can I just use regular automotive relays, or is the amperage too high for normal relays? I believe I have an 80 amp fuse from my battery to my amplifier, but didn't know if that would change or not after the amplifier.

The relay contacts must exceed the sub current (AC rating).

The sub current will be the square-root of P/R where P is the output power (RMS Watts) and R is the sub impedance.

EG - 400W into 1R (R=Ohm), i = root(400/1) = root or 400 = 20, hence 20A, hence use (preferably) a 30A or greater AC-rated relay.


Single pole can be used is the sub is common-grounded; otherwise dual pole for bridged etc.


FYI - the amps fuses have nothing to do with output current EXCEPT that they may determine max output power.
EG, 80A means 1152W @ 14.4V; 1280W @ 16V etc (eg 14.4V x 80A = 1152W RMS).

Great, thank you very much. I'm running an Alpine MRP-M500 using two subs, at 2 ohms, so if I did my math right (15.something amp load), I shouldn't have any problem using just any relay form O'Reilly's, as most are 30 or 40 amp, correct?

From what I see, the Alpine MRP-M500 can supply 500W RMS into 2 Ohms (14.4V supply) at under 1% THD.
That's rt(500/2) = 15A. (Cool.)

But it also says Total Max power = 1100W. But that is NOT under "Output Power". But is 1100W RMS? (Yes - I think so because Alpine are unlikely to use any other measure in that case. And if "Music Power", it's 550W RMS which is too low for 500W output - and output can presumably be much higher but exceed 1% THD).
But I'd assume that max power is input which assuming 90% efficiency means 990W => Rt(990/2) = 22A.


Yeah - you are right.... A "normal" car relay should handle it where normal means the typical 30A contacts (not 15A).   

Mind you, that assumes the contacts are rated for AC current, but that is usually ok. (Even 12V relays tend to plate their contacts as if switching higher voltages (>20V??) and I think that helps the AC rating...???)


Good luck!

Hmm. Well I guess everything looks like it should hypothetically work, so I guess I'll be picking up a couple of relays soon, and giving it a try..