How many amps?

Submitted: Thursday, Jul 02, 2009 at 14:36
ThreadID: 70327 Views:5105 Replies:10 FollowUps:10
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Hey electrical gurus, how many amps does a 12V 20W lamp draw? What's the formula for working this out? I'm going to build an overhead console for my 4runner and am trying to work out what type of lights to put in.

Louie
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Reply By: Member - Matt (Perth-WA) - Thursday, Jul 02, 2009 at 14:40

Thursday, Jul 02, 2009 at 14:40
watts divided by volts 20/12 is 1.666666
AnswerID: 372766

Reply By: Member - Troll 81 (QLD) - Thursday, Jul 02, 2009 at 14:41

Thursday, Jul 02, 2009 at 14:41
Hey

Go into the members/file download section. There is a excel file in there somewhere that you type in the watts and it tells you the amps....very handy
AnswerID: 372767

Reply By: oldtrack123 - Thursday, Jul 02, 2009 at 14:49

Thursday, Jul 02, 2009 at 14:49
HI Louie

Easy. divide watts by volts gives amps

P=EI
I=P div byE [I=P/E
P=IxI x R
OHMS law I=E/R. Interpose as required
AnswerID: 372769

Follow Up By: tim_c - Thursday, Jul 02, 2009 at 15:08

Thursday, Jul 02, 2009 at 15:08
Not trying to sound rude, but that doesn't seem particularly helpful...

I'd suggest that anyone who knows what each of those letters represent would already know the formulae:

P: Potential difference (measured in Volts)
E: Energy (measured in Watts)
I: Current (measured in Amperes/Amps)
R: Resistance (measured in Ohms)
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FollowupID: 640023

Follow Up By: oldtrack123 - Thursday, Jul 02, 2009 at 15:26

Thursday, Jul 02, 2009 at 15:26
Hi
Ok I,l accept that I should have posted details.
I will give the standard CORRECT ones now.
Yours are not correct.

E=applied voltage in VOLTS
I= current in AMPS
R= resistance in OHMS
P= power in WATTS
ALL fractions to be shown as decimal points
DO not use milli,mega,kilo,etc,always convert to basic units.
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FollowupID: 640026

Follow Up By: tim_c - Thursday, Jul 02, 2009 at 16:15

Thursday, Jul 02, 2009 at 16:15
You're right, it doesn't work putting in the info I suggested... I was just putting in the definitions of the letters as I'd been taught without actually checking against the formulae.

Isn't there a saying about incorrect information being even less helpful than a vague answer?!
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FollowupID: 640035

Follow Up By: oldtrack123 - Thursday, Jul 02, 2009 at 16:34

Thursday, Jul 02, 2009 at 16:34
tim_c posted:
Isn't there a saying about incorrect information being even less helpful than a vague answer?!
Hi Tim
I'll pay that one,WE see many of that,
Now I wonder if that's why some run into trouble with the power formula.
It does seem to be a recurring problem.
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FollowupID: 640036

Follow Up By: olcoolone - Thursday, Jul 02, 2009 at 17:10

Thursday, Jul 02, 2009 at 17:10
A simple question and it gets so technical...why?

Too much information is dangerous to people who don't understand it.
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FollowupID: 640039

Follow Up By: oldtrack123 - Thursday, Jul 02, 2009 at 20:42

Thursday, Jul 02, 2009 at 20:42
Hi
I think the question was formula for finding amps from watts.
THE FORMULAE GIVEN ALLOWED FOR CALCULATIONS FROM VARIOUS KNOWNS.

Surely not too much info
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FollowupID: 640068

Reply By: Louie the fly (SA) - Thursday, Jul 02, 2009 at 15:06

Thursday, Jul 02, 2009 at 15:06
Thanks.

Louie
AnswerID: 372772

Follow Up By: Member No 1- Friday, Jul 03, 2009 at 09:51

Friday, Jul 03, 2009 at 09:51
you any the wiser now?...hahaha...so how big does the wiring need to be
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FollowupID: 640138

Reply By: Member -Dodger - Thursday, Jul 02, 2009 at 17:03

Thursday, Jul 02, 2009 at 17:03
Loui
have a look at inserting LED lights. Maybe your solution.
I used to have a handle on life, but it broke.

Cheers Dodg.

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AnswerID: 372779

Reply By: Best Off Road - Thursday, Jul 02, 2009 at 21:05

Thursday, Jul 02, 2009 at 21:05
Louie Old Son,

I;m not sure this has been answered in a way to help you work things out in the future.

I'll have a crack.

Watts = Volts x Amps
Amps = Watts divided by Volts.

That is to say a Kettle that draws 2400 watts at home will use 10 amps (because it is on a 240 volts source and 2400 watts divided by 240 volts = 10 amps).

However should you use the same kettle (assuming you had an inverter big enough) in the bush it would draw 200 amps, 2400 divided by 12 = 200.

When I say amps, I should say amps per hour. So the kettle that draws 200 amps, if only run for 3 minutes would take 10 amps out of your battery. Gas would be better LOL.

Jim.

AnswerID: 372823

Follow Up By: oldtrack123 - Thursday, Jul 02, 2009 at 22:11

Thursday, Jul 02, 2009 at 22:11
Hi Jim
As you say A 2400watt kettle on 12v would draw @200amps plus some extra for inverter efficiency loss.
THE formula for "amphrs" is ;- Load amps X time in hrs
200 x3divided by 60= 10 amphrs

Also this is incorrect""When I say amps, I should say amps per hour" you should say amps!!

Also this "if only run for 3 minutes would take 10 amps out of your battery."
It will take10 amphrs from the battery.
Correct terminology is ,I believe, is important in tecniical threads .
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FollowupID: 640090

Follow Up By: Welldone WA - Friday, Jul 03, 2009 at 01:03

Friday, Jul 03, 2009 at 01:03
A wise old electrician once told me there are only 2 fundamental rules/laws of electricity:

1/ "Electricity is contained in smoke'' . If you have an electrical device that suddenly emits smoke , this is a sign that the electricity is escaping! If enough smoke escapes than the said item will cease to function , 'cause all the electricity has gone!

2/When wiring,it is important to remember: "Red to Black = Blow to Bits!" Failure to follow this rule can lead back to rule 1/

Cheers ;))

Welldone
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FollowupID: 640123

Reply By: Member - Tony S (WA) - Friday, Jul 03, 2009 at 00:43

Friday, Jul 03, 2009 at 00:43
as the old woman of Prague said - "that is That is"
The rig

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AnswerID: 372863

Reply By: Member - Mike DID - Friday, Jul 03, 2009 at 10:12

Friday, Jul 03, 2009 at 10:12
If it's an incandescent bulb, keep in mind that when it's first plugged in, it will draw up TEN times its running current for a fraction of a second i.e. a 20 watt bulb will draw 16 amps at startup.

Handy to know if you're wondering why it's blowing 2 amp fuses occasionally.
AnswerID: 372893

Follow Up By: oldtrack123 - Friday, Jul 03, 2009 at 10:35

Friday, Jul 03, 2009 at 10:35
Hi Mike

Now that's something that very few, including many who should, know.
It is the very reason so many incandescant bulbs blow @ the instant of being switch on.
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FollowupID: 640143

Reply By: Louie the fly (SA) - Friday, Jul 03, 2009 at 17:36

Friday, Jul 03, 2009 at 17:36
Thanks everyone for all your advice. And yes, it was ALL helpful. I just have one more question, can I run electricity and water in the same pipe? Just kidding, I know I can.

Think I'll fit either LED's or 20W halogen downlight lamps into the console as they seem to suit my needs.

Louie
AnswerID: 372946

Reply By: Member - Roger B (VIC) - Friday, Jul 03, 2009 at 23:45

Friday, Jul 03, 2009 at 23:45
The Military used to teach it this way: Virgins Are Rare.

Voltage = Amps x Resistance.

and Women Are Virgins.

Watts = Amps x Volts.

Nice and simple....
AnswerID: 373006

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