Wire gauge help

[mike001]

I know a smart siren if using over 50 amps they say to use an 8awg cable my question is theoretically speaking if I don't have enough can I take two 12awg wires and just connect the ends at the battery and at the amp together or does the copper need to be connected along the whole length of wire?


(I know I can buy some 8awg wire I just want to know of it works)

 

[RyanZ71]

I know a smart siren if using over 50 amps they say to use an 8awg cable my question is theoretically speaking if I don't have enough can I take two 12awg wires and just connect the ends at the battery and at the amp together or does the copper need to be connected along the whole length of wire?
(I know I can buy some 8awg wire I just want to know of it works)

How long of a run will it be?

 

[mike001]

10 feet


But my question really is if you can use 2 smaller sized wires and just connect them at the ends to get the same power as a larger sized gauge wire

 

[ff.brent20]

Wouldn't suggest it, I would just run one #8 to it. Better to be safe then sorry. Just my $.02

 

[twodogs603]

What are you powering with the Smart Siren. You may not need to that large a wire if your only using a few LED's.

 

[mike001]

I'm probably not going to need it I just want to know if the concept works

 

[JohnMarcson]

It's not generally a good idea, twice the wires to trouble shoot. You are better off just getting the correct wire to begin with. It will work, it will be harder to run and harder to package and harder to troubleshoot. Gauge number equivalency can be tough too.

 

[mike001]

Ok just curious if it was an option

 

[calebsheltonmed23]

You can do anything once!

 

[tvsjr]

Yet again, a bunch of people making guesses. You know - "I don't know" is really an acceptable answer.


Actually, adding the capacity does work just fine. Keep in mind that most wire used in automotive installs is stranded... which is just a bunch of small wires. Things get interesting when you start talking about AC, high frequencies, and skin effect... but if you're dealing with DC you can negate all of that.


The capacity of a given conductor is purely a function of the characteristic resistance of the wire. If the resistance is held constant, the voltage drop will be proportional to the current flow (by Ohm's Law). Current flow through a resistor turns into heat. Thus, the current handling capacity of a wire is another way of representing the acceptable temperature rise of a conductor. Typically, acceptable temperature rise is defined as 20 degrees C over ambient. That's 68 degrees F - so a fairly substantial increase.


Now, since we know the wire is just a resistor (albeit of a very small value) and we negate skin effect, here's a pop quiz for all the electrical engineers that have posted in this thread so far. What is the characteristic resistance of two identical resistors in parallel? Hint: Req=R1R2/R1+R2. The answer: half the resistance of one resistor. So, two 50 ohm resistors in parallel give an equivalent resistance of 25 ohms.


How does this apply to wire? If the wire is identical (same size, same length, same stranding), the resistance is the same in each wire. Two wires in parallel? Half the resistance. By Ohm's Law, V=IR... so if we halve the resistance, we can double the current flow and incur the same voltage drop.


Whelen has a good cheat sheet for wire capacities:


http://www.whelen.com/install/145/14570.pdf


For your 12ga. wire, assuming the run is 20ft., the wire is capable of handling 15A. So, two identical runs in parallel, assuming proper bonding, would be capable of 30A. QED.

 

[mike001]

Thank you for that info

 

[JohnMarcson]

For your 12ga. wire, assuming the run is 20ft., the wire is capable of handling 15A. So, two identical runs in parallel, assuming proper bonding, would be capable of 30A. QED.

When I cautioned OP that the math isn't simple and that you can't just add wires together I meant the gauge numbers. With American Wire Gauge, diameters can be calculated by applying the formula D(AWG)=.005·92((36-AWG)/39) inch.... right... get right on that. The bottom line is every 6 gauge decrease doubles the wire diameter of solid wires and every 3 gauge decrease doubles the cross sectional area (measure for stranded wire).


As far as resistance.... Yes, all things equal the two smaller wires will have the same resistance as one larger given they have the same amount of conductive material. From a physics standpoint splitting the conductive material between two insulated jackets creates the same resistance as one big one. Provided you calculate correctly gauge wise and get the same amount of material, you would be ok.


In the real world multiple smaller wires might not add up to the same amount of conductive material and it may be of lower quality as their larger counterparts. You also double the failure points and can create troubleshooting problems. When someone does this it's usually because they don't have the correct wire. The smaller gauge automotive wire they do have is often of less than stellar quality.


I also would like to avoid the fun of running a group of wires if I can. If you look at an ambulance or fire truck with high amp wiring it's run with a single larger wire. It is done that way for a reason, reduction of connections and failure points. How you wire and circuit protect multiple wires becomes an issue with trouble shooting. Also, depending on how you wire it, if one wire is broken the other one may fail in a catastrophic way. From a real world stand point using multiple smaller wires in place of one bigger wire presents problems sometimes.


All things equal in a lab, you can use two smaller wires to replace one bigger one if you calculate it correctly. In the real world, avoid doing this if you can.

 

[mike001]

Gotcha will keep that in mind

 

[foxtrot5]

You can do anything once!

Quite possibly my favorite answer to any question ever!

Yet again, a bunch of people making guesses. You know - "I don't know" is really an acceptable answer.

Actually, adding the capacity does work just fine. Keep in mind that most wire used in automotive installs is stranded... which is just a bunch of small wires. Things get interesting when you start talking about AC, high frequencies, and skin effect... but if you're dealing with DC you can negate all of that.


The capacity of a given conductor is purely a function of the characteristic resistance of the wire. If the resistance is held constant, the voltage drop will be proportional to the current flow (by Ohm's Law). Current flow through a resistor turns into heat. Thus, the current handling capacity of a wire is another way of representing the acceptable temperature rise of a conductor. Typically, acceptable temperature rise is defined as 20 degrees C over ambient. That's 68 degrees F - so a fairly substantial increase.


Now, since we know the wire is just a resistor (albeit of a very small value) and we negate skin effect, here's a pop quiz for all the electrical engineers that have posted in this thread so far. What is the characteristic resistance of two identical resistors in parallel? Hint: Req=R1R2/R1+R2. The answer: half the resistance of one resistor. So, two 50 ohm resistors in parallel give an equivalent resistance of 25 ohms.


How does this apply to wire? If the wire is identical (same size, same length, same stranding), the resistance is the same in each wire. Two wires in parallel? Half the resistance. By Ohm's Law, V=IR... so if we halve the resistance, we can double the current flow and incur the same voltage drop.


Whelen has a good cheat sheet for wire capacities:


http://www.whelen.com/install/145/14570.pdf


For your 12ga. wire, assuming the run is 20ft., the wire is capable of handling 15A. So, two identical runs in parallel, assuming proper bonding, would be capable of 30A. QED.

This is a very informative answer, but as John said what works in a lab doesn't always translate well to real life.


tvsjr, based on your apperant knowledge I'm guessing you have some background in electrical engineering or something similar? If you've got some spare time I could use an hand with something that I've been bouncing around in my head for a while. Shoot me an PM if you're interested in helping.

 

[calebsheltonmed23]

Quite possibly my favorite answer to any question ever!

My agency was discussing getting a 4x4 ambulance and I asked my boss if we could go "mudding" when we weren't first up for calls. That was his response. lol. Been using it ever since.

 

[foxtrot5]

My agency was discussing getting a 4x4 ambulance and I asked my boss if we could go "mudding" when we weren't first up for calls. That was his response. lol. Been using it ever since.

Not that I have personal experience or anything, but if an ambulance were to get stuck in the mud and you try to wedge a backboard under the rear tires for traction said ambulance may or may not launch the backboard so far away that the crew may never find it. :innocent:

 

[calebsheltonmed23]

Not that I have personal experience or anything, but if an ambulance were to get stuck in the mud and you try to wedge a backboard under the rear tires for traction said ambulance may or may not launch the backboard so far away that the crew may never find it. :innocent:

Sounds like fun, in theory of course!! There was a youtube video from a local ambulance company (hospital based) that showed the truck going through quite a large puddle during a recent flood, and the camera goes to the floor and the floor is full of water. I was advised the employees were fired and the truck is totaled due to the water getting into mechanical parts. I was shown the video but don't have the link. I'm looking for it tho. lol

 

[Skip Goulet]

When I cautioned OP that the math isn't simple and that you can't just add wires together I meant the gauge numbers. With American Wire Gauge, diameters can be calculated by applying the formula D(AWG)=.005·92((36-AWG)/39) inch.... right... get right on that. The bottom line is every 6 gauge decrease doubles the wire diameter of solid wires and every 3 gauge decrease doubles the cross sectional area (measure for stranded wire).

As far as resistance.... Yes, all things equal the two smaller wires will have the same resistance as one larger given they have the same amount of conductive material. From a physics standpoint splitting the conductive material between two insulated jackets creates the same resistance as one big one. Provided you calculate correctly gauge wise and get the same amount of material, you would be ok.


In the real world multiple smaller wires might not add up to the same amount of conductive material and it may be of lower quality as their larger counterparts. You also double the failure points and can create troubleshooting problems. When someone does this it's usually because they don't have the correct wire. The smaller gauge automotive wire they do have is often of less than stellar quality.


I also would like to avoid the fun of running a group of wires if I can. If you look at an ambulance or fire truck with high amp wiring it's run with a single larger wire. It is done that way for a reason, reduction of connections and failure points. How you wire and circuit protect multiple wires becomes an issue with trouble shooting. Also, depending on how you wire it, if one wire is broken the other one may fail in a catastrophic way. From a real world stand point using multiple smaller wires in place of one bigger wire presents problems sometimes.


All things equal in a lab, you can use two smaller wires to replace one bigger one if you calculate it correctly. In the real world, avoid doing this if you can.

John: You're so right! On other threads you've seen me mention my friend's ambulance...the 1961 lwb Pontiac. Before he got through with it, it had two motor sirens (a Q and a 77GB doubletone), a Federal Director and 5 beacons: two 2-lamps, a Dietz 211-WW, and two Fireballs. One night I took the big Pontiac to the race track as my ambulance was in the shop. We had a kid hurt during the motorcycle races and started into town with him. I had the thing all lit up....there was only one switch for all those lights! First time I let loose on the Q, all the warning lights ceased to function. A minute or so later after just letting the Q roll by itself, the lights came back up, but when I rolled the Q over again, same thing happened. Fortunately my best friend who shot newsfilm had decided to ride with us to shoot some footage for a particular project. He was up front with me, so when the lights failed, he held his big sungun out the window and cleared traffic for us. The next day we all gathered at my friend's place to see where the problem was. What was wrong was that all the wires from the switch panel came together in a single junction up under the dash and were all twisted together and shoved up into a slot that was "hot" on the fuse box. Talk about a major short or fire waiting to happen. We spent the entire day totally rewiring that whole system, but we never had problems again. And that old rig was bodacious if you ran the two big sirens back to back while keeping the Director on yelp (ouch!).

 

[JohnMarcson]

And that old rig was bodacious if you ran the two big sirens back to back while keeping the Director on yelp (ouch!).

Cool setup...from back when sound mattered. Now people treat audible as a lost cause half the time. Also many times audible devices were mounted up high. Everything was in place for max sound and and max hearing damage. Now cars are so insulated, speed so high and music so loud lights rule the day. Interesting how things change.

 

[Skip Goulet]

Cool setup...from back when sound mattered. Now people treat audible as a lost cause half the time. Also many times audible devices were mounted up high. Everything was in place for max sound and and max hearing damage. Now cars are so insulated, speed so high and music so loud lights rule the day. Interesting how things change.

Oh...I agree with you completely on that one, John. The old Pontiac had the Q roof-mounted with the 77G on the rt. fender. But something that Odessa had done will quickly get your attention. They bought three of the big International rigs (Brauns, I think) several months ago. Each has two sirens: a Whelen electronic and an Eagle siren; but they also have those loud-assed train horns on top. I was coming north on Parkway Dr. in Odessa one day headed towards the nearby Albertson's. I had heard OFD tone out Squad 5 which is the far east-side Medic, but as it turned out they were out south and had to come up Parkway. Just as I started my turn into the Albertson's lot I heard this very loud horn that sounded like it was right behind me. I looked up and saw nothing, so I continued into the lot, where I parked. I walked over to the street and looked down Parkway and still saw nothing. Suddenly Squad 5 crossed University, almost a mile south and let loose on that loud train horn again! I realized that's what I had heard, and with them more than a mile away when I heard it the first time. Ouch! Much to my surpise, those little Eagle sirens are much louder than I would've expected. They're using them to clear intersections with the electronics on yelp, and NOT using the airhorn at the intersection. I could see cars not moving for the electronic siren but pulling over for the Eagle siren. Quite a surprise!