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### Topic: 10 Amps through 3 Amp wire? (Read 3222 times)previous topic - next topic

#### jamieriddles

##### Jul 06, 2012, 06:29 am
Lets say I got a connector rated for 3 amps at 10 volts.
It can safely pass 3 x 10 = 30 watts of power correct?

In that case, can I run 10 amps at 3 volts which also equals 30 watts through it without the thing melting?

#### JimboZA

#1
##### Jul 06, 2012, 06:34 am
The rating is, as you point out, for amps, which is what does the melting afaik, not the power.

So no.
Johannesburg hams call me: ZS6JMB on Highveld rep 145.7875 (-600 & 88.5 tone)

#### jamieriddles

#2
##### Jul 06, 2012, 06:54 am
Shame.
Got the idea after seeing the ratings on a switch. Figured I could double the amps by halving the voltage:

Oh well. Thanks for the response.

#### JimboZA

#3
##### Jul 06, 2012, 06:58 am
That's just my understanding though, so maybe wait for a response for someone better qual'd at that than I
Johannesburg hams call me: ZS6JMB on Highveld rep 145.7875 (-600 & 88.5 tone)

#### Grumpy_Mike

#4
##### Jul 06, 2012, 07:04 am
You are right, don't exceed the current rating. In fact don't even go close, give it a 50% margin at least.

#### DVDdoug

#5
##### Jul 06, 2012, 07:10 am
Jimbo mentioned "melting".    A smaller diameter wire has higher resistance.    Power (heat) can be calculated as Current squared x Resistance.    So, smaller wire (higher resistance) means more power, when current is constant.

It might get a bit confusing, because higher (total) resistance means less current (when voltage is constant).     But there is usually something else in the circuit limiting current, so current is relatively constant.   And, with higher resistance the voltage "dropped" across the wire is higher.   (Power is also calculated as Voltage x Current).

Longer wire also creates more resistance.    But in that case, the additional power lost (heat generated) in the wire is spread-out along the wire so it's not a problem.     But, if the wire is too long, the voltage drop can become a problem.     Electricians use larger gage wire for higher current or for longer runs.

With a switch, there's a bit more going-on.    When the contacts are close, especially when switching off, higher voltage tends to cause an internal arc.   Thee will be more damage when the arc happens when both voltage and current are high.

#### nickgammon

#6
##### Jul 06, 2012, 09:00 am
Imagine there is a rope across a huge canyon (like in the movie Romancing the Stone). It is rated at 100 Kg. First, you might be a bit doubtful crossing it if you were exactly 100 Kg in weight. Second, 5 people can't cross it (at the same time) if they move slowly. That doesn't help.
Please post technical questions on the forum, not by personal message. Thanks!

#### Grumpy_Mike

#7
##### Jul 06, 2012, 11:58 am
Quote
It is rated at 100 Kg.

Depends what is chasing you.

#### MarkT

#8
##### Jul 06, 2012, 12:48 pm

Shame.
Got the idea after seeing the ratings on a switch. Figured I could double the amps by halving the voltage:

A switch is not a piece of wire.

Switch contacts are destroyed by arcing, the rating is to prevent that happening too quickly.  The switch _might_ be able to handle 20A while on, but switching it off at that current level will lead to arc damage.  Arc damage is strongly related to voltage and to whether AC or DC is involved, hence the separate ratings for different voltages.

[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

#### SirNickity

#9
##### Jul 06, 2012, 09:53 pm
Arcing is one problem.  Current capacity is another.  Current can be limited by the construction of the contacts.  Too thin and regardless of whether it's being switched or constant, the current flowing through the contacts can cause damage.

#### Docedison

#10
##### Jul 07, 2012, 07:16 am
Exactly, the ratings are absolute maximum of voltage and / or current not the VA rating of the switch. as to 10 A through a 3 A wire... no not really  for a usually unconsidered quality of copper, It has positive coefficient of temperature of about 3300 PPM/deg C, the real hazard being the flash point of the insulation and the rather noxious fumes given off prior to and during combustion. Given a fixed resistance of a piece of wire realize that If you kept the power the same and lowered the voltage the proportionate losses are just as large If not larger as now contact resistance becomes an issue along with connector resistance.... all the way down to the fuse... Low Voltage fuses are constructed slightly differently than their higher voltage counterparts are. So the losses at 10 A 1V / .01 ohm are 10 times as large proportionally then at 10V 1A /.01 ohm and the copper PTC makes it worse. This is the Prime reason that newer computers have Very local switchers for the 1V8 and 2V5 sources required by the processor... Too much line loss between the Main PSU and the Processor.

Doc
--> WA7EMS <--
"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

#### xvjeko

#11
##### Jul 07, 2012, 10:26 am
don't care for voltage...
just work with 0,6 of the "written" amps

#### Docedison

#12
##### Jul 07, 2012, 11:08 pm
The Short answer is NO, with caveats... a switch rated @ 0,6 (0.6?) amps could probably be used at 1.2 A IF the load was 1. Non Inductive or capacitive in nature (i.e. resistive only and non incandescent (NO Light bulbs as they are a Very low resistance until hot) and 2. it wasn't used Very frequently... Doubling the load means approximately Halving it's life time in operations before the contacts are worn enough to be an issue in the proper operation of the switch. The switch might only last for 5000 operations instead of 100,000 operations... (5%) or ~ 3 1/2 years operated twice a day. If the load is inductive capacitive or incandescent in nature or much more than twice the rated load... all predictions are impossibly difficult and you use at your own peril... Helpful?
It IS NEVER a good idea to exceed Mfr's ratings... Particularly on parts of Asian origin unless they are top quality and even then I would be concerned, in 40+ years I have had reason to use a LOT of switches and the above are my observations of my experiences.

Doc
--> WA7EMS <--
"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

#### jamieriddles

#13
##### Jul 10, 2012, 04:57 am
Great info folks.
Thanks.

#### JimboZA

#14
##### Jul 10, 2012, 06:14 am

100 Kg in weight

BZZZZZZTTT- Kilograms are mass not weight.

(But you know that, and we know that you know, and know what you mean  )
Johannesburg hams call me: ZS6JMB on Highveld rep 145.7875 (-600 & 88.5 tone)