# Wire size



## Joker1 (Jul 5, 2015)

What gage wire do you use to connect the high voltage side of the contactor to DC to DC converter?


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## major (Apr 4, 2008)

Joker1 said:


> What gage wire do you use to connect the high voltage side of the contactor to DC to DC converter?


It depends on the current your particular converter will draw and the branch circuit fuse you use. You can search for _ampacity_ to find tables for wire size based on the type of insulation it uses. Often the manual which comes with the device will detail the installation.


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## EVisa (Jun 30, 2014)

I've used 6mm2 cable. The ticker the better, but I couldn't find any terminals that would fit on the dc-dc and accept heavier cable. The cabling doesn't get hot at all. My dc-dc converter is a Meanwell 350W (29.2 A).


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## dougingraham (Jul 26, 2011)

Joker1 said:


> What gage wire do you use to connect the high voltage side of the contactor to DC to DC converter?


It depends on several factors. If your input voltage is higher then lighter gauge wire could be used.

If we plan for peak output of the DC-DC being 80 amps at 12 volts and the DC-DC is 90% efficient that means an input wattage of 1067 ((80*12)/.9). If your high voltage battery is 400 volts then the input current will peak at 2.67 amps. If your high voltage battery is 144 volts then the input current will peak at 7.41 amps. 12 gauge wire will be more than adequate in this case. Going to 10 gauge would give you a safety factor of *2. The typical DC-DC converter is just an AC input switching supply and those are usually specced for a minimum voltage of 90 so if your traction pack sags below 90 volts you could have issues.

A typical conversion would be fine using 12 gauge on the input side of the DC-DC converter but going to 10 gauge will handle just about any situation.


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## Joker1 (Jul 5, 2015)

Thanks I will go out and buy 10 gauge wire to hook-up the contactor to the DC to DC converter. Again thank you for taking time to answering my question.

Joker1


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## dougingraham (Jul 26, 2011)

You are welcome.

Don't forget to fuse the circuit to the DC-DC. What size you ask? Probably 10 or 15 amp. But you need a high voltage fuse. A 12 volt automotive fuse would not be suitable.


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## Joker1 (Jul 5, 2015)

Thanks for the heads up about the fuse, but I was wondering why do you have to fuse it at all?

Joker1


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## dougingraham (Jul 26, 2011)

Joker1 said:


> Thanks for the heads up about the fuse, but I was wondering why do you have to fuse it at all?
> 
> Joker1


You place a fuse as close to the power source as possible. In our case this is the battery. The purpose is to prevent a fire in the case of a wiring short. You want to size the fuse to be perhaps 25 to 50 % greater than the load but smaller than the ampacity of the wire you are protecting. In an EV where you have to run a positive and negative wire from the battery it would be best practice to fuse both wires. So far I have done this only on the wires to the charger but I plan to do this to all the devices attached to the battery. And strictly speaking the best practice would be to have fuses at the charger side of the wires as well for when it is the power source while charging the battery. ( have a circuit breaker on one of my chargers but not on both legs.)

If you have fuses in the battery leads but the short is in the 10 gauge wire to your DC to DC converter the 10 gauge wire is going to look like the filament of a light bulb and will be the fuse but possibly not before starting something on fire. 10 gauge wire has a fusing current of 330 amps at around 10 seconds.


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## major (Apr 4, 2008)

Joker never has said what the converter rating is or what the system voltage is, but #10 wire sounds like overkill to me. It will just be extra cost and difficulty on the install. Did anybody look for a manual for the device? What size fuse or breaker is recommended for it? Using an oversize fuse can result in a hazard should the converter be overloaded or malfunction.


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## arklan (Dec 10, 2012)

my car is under a ton with no power steering or ac so this may not apply to everyone
but the high voltage wires from my dc converter are about the same as speaker wire or normal dash wire
the 12v side is of course better than this


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## Joker1 (Jul 5, 2015)

I want to get this right, when you say battery do you mean the 12 Volt battery?


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## major (Apr 4, 2008)

Joker1 said:


> I want to get this right, when you say battery do you mean the 12 Volt battery?


I guess it depends on who's saying what. Do you know how to use the quote function?


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## Joker1 (Jul 5, 2015)

NO I don't


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## major (Apr 4, 2008)

Joker1 said:


> NO I don't


Click on the "quote" icon in the lower right corner of the original post. Then a screen will appear with the quote. Type in your reply below the quote and hit send. It will appear like this one and people will understand to whom you are replying.

edit: Use the quote icon with Quote spelled out, not the symbol "


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## Joker1 (Jul 5, 2015)

major said:


> Click on the "quote" icon in the lower right corner of the original post. Then a screen will appear with the quote. Type in your reply below the quote and hit send. It will appear like this one and people will understand to whom you are replying.
> 
> edit: Use the quote icon with Quote spelled out, not the symbol "


Thanks for telling how to use the quote function.


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## Joker1 (Jul 5, 2015)

dougingraham said:


> You place a fuse as close to the power source as possible. In our case this is the battery. The purpose is to prevent a fire in the case of a wiring short. You want to size the fuse to be perhaps 25 to 50 % greater than the load but smaller than the ampacity of the wire you are protecting. In an EV where you have to run a positive and negative wire from the battery it would be best practice to fuse both wires. So far I have done this only on the wires to the charger but I plan to do this to all the devices attached to the battery. And strictly speaking the best practice would be to have fuses at the charger side of the wires as well for when it is the power source while charging the battery. ( have a circuit breaker on one of my chargers but not on both legs.)
> 
> If you have fuses in the battery leads but the short is in the 10 gauge wire to your DC to DC converter the 10 gauge wire is going to look like the filament of a light bulb and will be the fuse but possibly not before starting something on fire. 10 gauge wire has a fusing current of 330 amps at around 10 seconds.


I want to get this right, you do mean the 12 Vote battery?


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## dougingraham (Jul 26, 2011)

Joker1 said:


> I want to get this right, you do mean the 12 Vote battery?


Mostly I am talking about the traction (high voltage) battery. But all circuits connected to batteries need to be fused. Batteries cannot be turned off.

Major was correct when questioning my 12 gauge suggestion with 10 gauge as a safety factor. I gave the equations I used to estimate the loads and am assuming you would plug in your actual numbers and then look at an ampacity chart to find your wire size. For almost everyone 12 gauge is more than heavy enough. If you are running a 96 volt system and have a large load on the DC-DC you might need 10 gauge. Say you are running the rear defogger and have the fans turn on high and all the lights on with the wipers running with the seat heaters going and you push in the cig lighter and roll the windows up to stall all at the same time. And then the vacuum pump kicks on. You could see 100 amp surge. If you have a 1000 watt DC-DC it will run up to around 80 amps on the 12 volt side and the rest will have to come out of your 12 volt battery. If your traction pack is 96 volts it will see a load of about 11 amps. If your traction pack is 200 volts the load would be a little over 5 amps. If your traction pack is 300 volts the load would be around 3.5 amps. And you could easily use 16 gauge as it is rated for 3.7 amps when used to transmit power but 22 amps for chassis wiring.


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## Joker1 (Jul 5, 2015)

My conversion is 144 volt Ford Ranger pickup if that helps with any advise I ask for in the future. I already used 10 gage wire from the contactor to the DC/DC converter, but I put the fuse on the 12 volt side not the high voltage side so I will have to get another block and fuse and place it in-between the contactor and the DC/DC converter right? 

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dougingraham said:


> Mostly I am talking about the traction (high voltage) battery. But all circuits connected to batteries need to be fused. Batteries cannot be turned off.
> 
> Major was correct when questioning my 12 gauge suggestion with 10 gauge as a safety factor. I gave the equations I used to estimate the loads and am assuming you would plug in your actual numbers and then look at an ampacity chart to find your wire size. For almost everyone 12 gauge is more than heavy enough. If you are running a 96 volt system and have a large load on the DC-DC you might need 10 gauge. Say you are running the rear defogger and have the fans turn on high and all the lights on with the wipers running with the seat heaters going and you push in the cig lighter and roll the windows up to stall all at the same time. And then the vacuum pump kicks on. You could see 100 amp surge. If you have a 1000 watt DC-DC it will run up to around 80 amps on the 12 volt side and the rest will have to come out of your 12 volt battery. If your traction pack is 96 volts it will see a load of about 11 amps. If your traction pack is 200 volts the load would be a little over 5 amps. If your traction pack is 300 volts the load would be around 3.5 amps. And you could easily use 16 gauge as it is rated for 3.7 amps when used to transmit power but 22 amps for chassis wiring.


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## dougingraham (Jul 26, 2011)

Joker1 said:


> My conversion is 144 volt Ford Ranger pickup if that helps with any advise I ask for in the future. I already used 10 gage wire from the contactor to the DC/DC converter, but I put the fuse on the 12 volt side not the high voltage side so I will have to get another block and fuse and place it in-between the contactor and the DC/DC converter right?


Yes. Note that you need high voltage fuses. Automotive 12 volt fuses fail rather spectacularly when you try to use them at high voltages.


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## Joker1 (Jul 5, 2015)

How Big of fuse should I use? 



dougingraham said:


> Yes. Note that you need high voltage fuses. Automotive 12 volt fuses fail rather spectacularly when you try to use them at high voltages.


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## dougingraham (Jul 26, 2011)

Joker1 said:


> How Big of fuse should I use?


This will depend on things I don't know the answer to. It should selected so that it does not fail due to the inrush current most of these devices have when you initially apply power. You probably want a slow blow type rated for 25 to 50 percent more than your peak load condition. But it needs to be small enough to open if your wires get crossed.

Fuse suggestion

Probably the 10 or 20 amp.


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