permanent live

[TwedA4]

hi guys,

would it be ok to take a permanent live feed from the big distributor block under the dash for a 300W max active sub?

be so much easier than poking a piddley wee cable through the bulk head... lol

 

[dualmono21]

which active sub is this ???
you ideally need to know what kind of current draw you will be consuming and through what size of cable it is being put down

from this you can work out if its suitable to be powered off the rear charging point

 

[TwedA4]

well using the max power at 300W and cars charching voltage (14 ish?) theres a draw of around 20 amps. there's alread a wiring kit with it and it has what looks like 2 30amp cable which is more than enough as we had the earlier model in one of our vans.

it's a blaupunkt xlf 200a.

edit: should have read all your post. there is a separate power option available which does use a cigarette lighter... lol

the unit has a 10A fuse in it too if that helps

 

[TwedA4]

one of these.

 

[SteveMcB]

You could use one of these and plug unto a spare socket, maybe towbar assembly, or piggy back off something else. Bit confused though, if it's drawing 20a why is it fitted with a 10a fuse?

 

[TwedA4]

well, thats using my maths... not sure how power is calculated for powering amps and stuff... might just be the speaker that's 300W max.

but that's all that was fitted to our last one and it was fine. until we blew the speaker. lol

edit: ok, i was doing the math wrong. it's only gonna draw 10a using RMS power which is 150.

 

[dualmono21]

its measure like this

maximum power output of the amplifier x 2 (this is because all car audio amplifiers are only generally 50% efficient )

300 x 2 = 600 600 divided by 12 is 50

so an amplifier producing 300 watt has a current draw of 50 amps
there is also an equation for cable size vs cable length

which is why i ask which unit is it ?

 

[TwedA4]

it's a blaupunkt xlf 200a.

it's a D class amp so think it's pretty efficient

 

[TwedA4]

hang on... surely if it's 50% efficient, then that's only 150W. it's not going to draw 50 amps, it'll cook its on board fuse in seconds?

 

[S4_dan]

The safest bet with any amp install is to fit direct to the battery and fuse it shortly (within 20cm) of battery terminal.
Pulling your power from the block under the dash could cause an overload on the thinner, older wires. It's not worth the risk. There are a few ways of going through the bulk head using some of the original grommets already there with wiring in them.

Dan

 

[dualmono21]

i have read the specs of the active sub

it is only 150 wrms not 300 as quoted rms is the proper way of measuring an amplifier r.m.s roots mean squared
to make 150 and maximum output the amp is only 50% efficient
so you have to put twice as much power into it to make 150wrms out of it

2 x 150 = 300 ... 300 divided by 12 is 25
so you need a cable which is capable of holding 25 amps over whatever length the cable is
but the longer the cable is the less it is capable of holding , you have to bear this in mind when installing

 

[easy going Si]

TwedA4 for a 25amp load you also have to take into account the fuse you are using if you are using glass slow blow

the equation becomes fusing factor over load which is a 1.7 multiplier for any cable I normally double the cable for the load
which gives you a 2 times safety factor i.e. 25 amp load 50amp cable

hope this helps