Current versus wire question
12-30-2005, 01:43 PM
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Current versus wire question
Hey stereo gurus and others who might know...
How much 12v current can I safely pull through 20ft of good quality, 8AWG, 735 strand wire? If I were fusing to protect the wire, what amperage fuse should I use?
I assume there is chart somewhere, but damned if I can find it
Thanks!
How much 12v current can I safely pull through 20ft of good quality, 8AWG, 735 strand wire? If I were fusing to protect the wire, what amperage fuse should I use?
I assume there is chart somewhere, but damned if I can find it
Thanks!
12-30-2005, 02:07 PM
#3
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Here's one chart
http://www.the12volt.com/info/recwirsz.asp
http://www.the12volt.com/info/recwirsz.asp
12-30-2005, 02:15 PM
#6
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Originally Posted by Corey
And that Wab Fab Wabbit beats Eric The Red by a nose hair 
I have seen that chart before, very good one.
I have seen that chart before, very good one.
He cheated, he's on Central time, that gives him a 2 hour advantage over me
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12-30-2005, 02:24 PM
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It's not an amp that I am wiring in, so that chart doesn't do much for me The "Power & Ground Cable Specs" chart is closer, but it doesn't factor in distance, so I am leary.
But thanks for the replies so far...
The "Power & Ground Cable Specs" chart is closer, but it doesn't factor in distance, so I am leary.But thanks for the replies so far...
12-30-2005, 05:03 PM
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Try this
http://www.powerstream.com/Wire_Size.htm
Conservative ratings, but definitely the best way to go.
http://www.powerstream.com/Wire_Size.htm
Conservative ratings, but definitely the best way to go.
12-30-2005, 08:22 PM
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Since I was interested in the answer myself, I looked around the internet a little bit and at first, I found the same site bulldogyota posted. They say that you shouldn't put more than 24 amps of load on an 8AWG cable to transmit power.
Personally, I thought that was a little too low, and looked up some numbers in a construction book I had. They said that you shouldn't put more than 15 amps of load on a 16/14/12 AWG cable that has a length of 25'. Nothing extreme will happen if you go a bit over that (say 20 amps) but it won't be good for whatever you are powering and it is definately a danger, not recommended.
So, my final verdict considering you wanna go that long is to get 4AWG cable and power whatever you need with that, or if you must really risk it, use 20 amps of load MAX.
May I know what you are trying to power? How much load will it use?
EDIT: I just re-read the posts from other people, who obviously trust putting more load into the wire. Just wanted to let you know, that my babbling above is from what I learned in a CONSTRUCTION book and I just tried to keep it as safe as possible. Considering construction tools are a bit more power needy (they stall, creating power problems etc.) maybe the higher amps people are telling you, are safe as well.
EDIT 2: http://en.wikipedia.org/wiki/American_wire_gauge
40a @ 60C, not considering length (20'=quite a bit tho). Also, if it ever goes through your engine comp. it might get higher than 60C, therefore less amps.
Personally, I thought that was a little too low, and looked up some numbers in a construction book I had. They said that you shouldn't put more than 15 amps of load on a 16/14/12 AWG cable that has a length of 25'. Nothing extreme will happen if you go a bit over that (say 20 amps) but it won't be good for whatever you are powering and it is definately a danger, not recommended.
So, my final verdict considering you wanna go that long is to get 4AWG cable and power whatever you need with that, or if you must really risk it, use 20 amps of load MAX.
May I know what you are trying to power? How much load will it use?
EDIT: I just re-read the posts from other people, who obviously trust putting more load into the wire. Just wanted to let you know, that my babbling above is from what I learned in a CONSTRUCTION book and I just tried to keep it as safe as possible. Considering construction tools are a bit more power needy (they stall, creating power problems etc.) maybe the higher amps people are telling you, are safe as well.
EDIT 2: http://en.wikipedia.org/wiki/American_wire_gauge
40a @ 60C, not considering length (20'=quite a bit tho). Also, if it ever goes through your engine comp. it might get higher than 60C, therefore less amps.
Last edited by marko3xl3; 12-30-2005 at 08:32 PM.
12-30-2005, 09:14 PM
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Hmmm.... lot's of very different opinions...
This a feed/charge wire to a battery isolator. As such, I have some adjustment as to how much load is seen by the wire. But, the more amps I can move, the faster the system will reset. The isolator itself is rated to 95A continuous, so I am good up to anything under that point.
I looked at the wire routing and I can get this down to 10' with a change of plans. How does that effect people's numbers?
What's interesting in the spread of numbers is that I run 6 55w rocklights, so a bit less than 30A on 10' of 12AWG wire and it doesn't even get warm...
This a feed/charge wire to a battery isolator. As such, I have some adjustment as to how much load is seen by the wire. But, the more amps I can move, the faster the system will reset. The isolator itself is rated to 95A continuous, so I am good up to anything under that point.
I looked at the wire routing and I can get this down to 10' with a change of plans. How does that effect people's numbers?
What's interesting in the spread of numbers is that I run 6 55w rocklights, so a bit less than 30A on 10' of 12AWG wire and it doesn't even get warm...
12-30-2005, 11:49 PM
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Use the chassis wiring ratings not the power transmission ratings from the website I listed. Chassis ratings are for short distances. Keep in mind that the isolation needs to cater for any heat (if you are planning high current). For 100A continious load 6 gauge will handle it fine, 8guage would be pushing it.
12-31-2005, 07:16 AM
#15
Rob,
You might trying emailing Roger Brown with your specifics...he knows Toyotas obviously and he is an electrical engineer, PE. See what he says.
Let us know what you do decide to do.
You might trying emailing Roger Brown with your specifics...he knows Toyotas obviously and he is an electrical engineer, PE. See what he says.
Let us know what you do decide to do.
12-31-2005, 07:58 AM
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Ok guys, excuse my ignorance. What distinguishes between power transmission and chassis wiring? Please, give me some examples. So far, my understanding is that with an amp, that would be power transmission.
I understand that "power" is wattage, but all circuits have "power". Am I just confusing myself and missing the obvious?
I understand that "power" is wattage, but all circuits have "power". Am I just confusing myself and missing the obvious?
Last edited by Joyota; 12-31-2005 at 08:08 AM.
01-01-2006, 02:08 PM
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Okay, let's try a slightly more esoteric question... suppose that we have two car batteries, one fully charged and one run down to almost nothing.
The grounds of the two batteries are connected together via 10 ft of 000 welding cable.
The positives of the two are then connected via the above mentioned, high quality, 8 AWG wire, 10 ft as well.
So, the batteries will equalize in voltage, but how much current will the 8 AWG wire see?
The grounds of the two batteries are connected together via 10 ft of 000 welding cable.
The positives of the two are then connected via the above mentioned, high quality, 8 AWG wire, 10 ft as well.
So, the batteries will equalize in voltage, but how much current will the 8 AWG wire see?
01-01-2006, 03:57 PM
#18
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Originally Posted by WATRD
Okay, let's try a slightly more esoteric question... suppose that we have two car batteries, one fully charged and one run down to almost nothing.
The grounds of the two batteries are connected together via 10 ft of 000 welding cable.
The positives of the two are then connected via the above mentioned, high quality, 8 AWG wire, 10 ft as well.
So, the batteries will equalize in voltage, but how much current will the 8 AWG wire see?
The grounds of the two batteries are connected together via 10 ft of 000 welding cable.
The positives of the two are then connected via the above mentioned, high quality, 8 AWG wire, 10 ft as well.
So, the batteries will equalize in voltage, but how much current will the 8 AWG wire see?
Also, the dead battery will act as an inductive load, which will create a varying current requirement as the cells charge.
01-01-2006, 07:45 PM
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Midiwall,
Your power calculations are correct, but to the wire, 20A is 20A. The voltage drop across a given length of wire for 20A of current will be the same whether its a 12VDC or a 120VDC circuit so the power loss in the wire (heat disspated) will be the same for both. The power available to the load (motor, heater, etc.) for a given current is different for different voltages as your calculations point out.
Its only when you get alternating (or at least rapidly varying) current that the "skin effect" comes into play. At DC, current flows through the entire area of the wire. As you go higher in frequency, the current flows more and more near the surface of the wire. At very high frequencies, the "wire" may as well be a thin hollow tube since very little current flows in the interior. What you end up with is a resistance that goes up with frequency for any given wire size.
Some of the variation in the wire tables is due to different safety assumptions based on use. To make a long story short, there are a lot of things to consider: type of insulation, environment the wire is in (for example, is it in free-air at low or high temps or bundled with a bunch of other heat-generating wires in a conduit), what kind of duty cycle does the load it supplies have, how long do you expect the wire to last and the effects of long-term heating, is it a flexible cord that may have strands break over time, etc. etc. That alone leads to lots of variation because of the different assumptions made.
Then you get into voltage drop and circuit function, which has its own set of considerations and influences on any current rating table. If you get too long of a run of wire in your home, 12ga wire will still be safe for a 20A circuit, but you may not end up with enough voltage for your lights or whatever to work like you expect.
As a final comment, personally, I tend to be a lot more conservative than the Crutchfield table both from a safety and a voltage drop perspective.
Your power calculations are correct, but to the wire, 20A is 20A. The voltage drop across a given length of wire for 20A of current will be the same whether its a 12VDC or a 120VDC circuit so the power loss in the wire (heat disspated) will be the same for both. The power available to the load (motor, heater, etc.) for a given current is different for different voltages as your calculations point out.
Its only when you get alternating (or at least rapidly varying) current that the "skin effect" comes into play. At DC, current flows through the entire area of the wire. As you go higher in frequency, the current flows more and more near the surface of the wire. At very high frequencies, the "wire" may as well be a thin hollow tube since very little current flows in the interior. What you end up with is a resistance that goes up with frequency for any given wire size.
Some of the variation in the wire tables is due to different safety assumptions based on use. To make a long story short, there are a lot of things to consider: type of insulation, environment the wire is in (for example, is it in free-air at low or high temps or bundled with a bunch of other heat-generating wires in a conduit), what kind of duty cycle does the load it supplies have, how long do you expect the wire to last and the effects of long-term heating, is it a flexible cord that may have strands break over time, etc. etc. That alone leads to lots of variation because of the different assumptions made.
Then you get into voltage drop and circuit function, which has its own set of considerations and influences on any current rating table. If you get too long of a run of wire in your home, 12ga wire will still be safe for a 20A circuit, but you may not end up with enough voltage for your lights or whatever to work like you expect.
As a final comment, personally, I tend to be a lot more conservative than the Crutchfield table both from a safety and a voltage drop perspective.
01-01-2006, 08:12 PM
#20
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Hmmm... I'll give you the skin effect being AC only (my bad, and message deleted) but I'm still thinking that 20A at 110v is different than 20A at 12v.
Current is the measurement of electrons past a point on the wire; and voltage is the force pushing the electrons down the wire. So given that "110" is greater than "12", then something has to be different about what an amp of current represents.
I'm digging through the 'net to get my head around this, but it's bugging me...
Current is the measurement of electrons past a point on the wire; and voltage is the force pushing the electrons down the wire. So given that "110" is greater than "12", then something has to be different about what an amp of current represents.
I'm digging through the 'net to get my head around this, but it's bugging me...
