Can I use an extension cord to connect between my truck connector and my trailer cable connector ? If yes do I have to worry about a voltage drop if the distance is too long. I park my trailer in a trailer barn that I built several years ago, and this year I added next to it an attached truck port where I now park my truck. Each time we want to work in the trailer or have heat/lights on in it, I have to back the truck out and back it up to the trailer to plug it in. I am thinking that if I had an extension cord, I could leave the truck parked in the truck port and then just using it , plug into the trailers cord. I have space to run under the side of the two buildings, but haven't measured the distance I would need for the extension. If I can do this; what is the maximum length I can use for the extension cord, (naturally I will use the shortest that I can)Jack L

Don't understand why you need to have your truck plugged into the trailer to have trailer lights or heat???Where's the trailers battery? Can't you just plug in the trialer to shore power?

Your trailer 12VDC Parasitic Drains will run your truck start battery down just as easy as it will run down your trailer battery. Not a good idea to me.I would install 120VAC to your "trailer barn" and plug the trailer shore power cable into an installed RV 30AMP pedestal type receptacle or at least into the 120VAC Receptacle using a RV30A-15A long 'dogbone' type adapter (WALMART). This will run your trailer on-board converter/charger unit to keep your trailer batteries charged up. Rule number one in my book is never mess with the truck 12VDC start battery system as the truck is the only way you can get back home with...Roy Ken

The truck engine is idling during the time you need to be in the trailer, and the trailer has no battery but does have a working converter I assume. Also no portable generator handy.Need for heat and doing some work in the trailer suggests this could be for a longer time than just a few minutes. So another way is to attach an inverter to the truck's battery and plug the trailer's shore power cord via as long a 120v extension cord as you need into the inverter. The truck engine still has to be idling. The hood needs to be up with the inverter hanging out on the fender to prevent the inverter overheating from engine heat and shutting down.If the power requirement is not so large for 120 or 12 it needs the truck's power, then one of those power box combo inverter/battery things would let you plug the shore power cable into that, then recharge the power box in the stick house.

:HPlugging in truck/trailer umbilical cord only provides power for trailer exterior lights (turn/stop/running)---brakes----and a 'charge line' for trailer battery.Charge line is really only helping to maintain trailer battery voltage IF truck is running.Trailer battery is providing the DC for interior lights and furnace to run.Then just hook up a portable battery charger via extension cords OR remove battery and take it to a portable batter charger if unable to do so in trailer

Hopefully someone can answer the simple questions I asked and not try to suggest any thing else.With my truck plugged into the trailer, everything in the trailer will run perfectly and my truck battery won't run down as long as I have the truck runningJack l

Electrical truck to trailer connection question

22 Replies

allen8106
Explorer
Nov 25, 2014
Jack,

While definitely no expert in my humble opinion as long as you have the 7 pin connectors you should just be able to what you are thinking of. The #4 pin is for 12 volt charging. As long as you have a wire hooked to both #4 pins you can plug the connectors in as you suggest. My only recommendation would be to up size the wire by two wire sizes.
NanciL
Explorer II
Nov 24, 2014
Lantley wrote:
Now that you have built the barn follow through and install electricity in the barn. 20 amp circuit would do 30/50 amp would be better.
Leave battery in RV and keep trailer plugged in 24/7.
Installing electric would be the professional thing to do and eliminate monkeying around with connecting RV to truck etc.
With electric installed you could go in and out of the trailer as you please without having to do anything.

That sure would be nice and I would do it in a heart beat, but there is no power any where near it.
It would solve all my problems and I wouldn't have people second guessing me too !

Jack L
jrnymn7
Explorer
Nov 24, 2014
http://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=2.061&voltage=12&phase=dc&noofconductor=1&distance=60&distanceunit=feet&eres=8&x=42&y=5
YamaDooed
Explorer
Nov 24, 2014
If your just hooking up a just a two wire charge cable.Go buy a 30' 12g extension cord cut the ends off and wire to your male and female seven pin.
2112
Explorer II
Nov 24, 2014
To answer your question, yes you can do what you are asking.
You can purchase the two mating connectors, the proper length of cable and fabricate your umbilical. What is the largest gauge wire the connectors will accept? My guess is you may be able to squeeze an 8 AWG in it but I do not know.
Doing in this way you will have the voltage drop across the wire from the battery, relay, wire to the connector, connector itself, the wire you added and another connector.

I have a question. Do you own a set of jumper cables? Too short?
Determine how short and go to the auto part store and get a second set at least that length. Find two short pieces of 1/2" or so copper pipe. connect one set to the TT battery terminals, you don't need the battery. Connect the other set directly to the truck battery. Connect the two jumper cable ends together by clamping them to the copper pipes.

This will provide the least voltage drop and cost you less with less work.
Just my humble suggestion.
Lantley
Nomad
Nov 24, 2014
Now that you have built the barn follow through and install electricity in the barn. 20 amp circuit would do 30/50 amp would be better.
Leave battery in RV and keep trailer plugged in 24/7.
Installing electric would be the professional thing to do and eliminate monkeying around with connecting RV to truck etc.
With electric installed you could go in and out of the trailer as you please without having to do anything.
ol_Bombero-JC
Explorer
Nov 24, 2014
Guy Roan wrote:
I can't help the OP but I don't understand why people feel they have to chime in when they evidently don't know the answer.
He/she asked two simple questions and the first four replies didn't give him the answers he was looking for but felt they had to chime in.

Guy

Now - both you and I have also chimed in....without the answer!..:S

~
CA_Traveler
Explorer III
Nov 23, 2014
The trucks alternator and charging voltage will be mostly determined by the trucks battery. If it drops to say 13.6V then there is less voltage difference to charge the trailer battery. The low voltage involved means that you need bigger wire.

Sure you can calculate the voltage drop of the wire but there are other factors including the batteries and truck wiring. Can you connect directly form the truck battery to the trailer battery? ie The shortest possible distance.

I would beg, borrow or steal (not really) a cord or other wiring and try it. Even if the trailer charging is limited you might get enough amps to allow usage of the trailer while maintaining the battery.
Guy_Roan
Explorer
Nov 23, 2014
I can't help the OP but I don't understand why people feel they have to chime in when they evidently don't know the answer.
He/she asked two simple questions and the first four replies didn't give him the answers he was looking for but felt they had to chime in.

Guy
NanciL
Explorer II
Nov 23, 2014
CA Traveler wrote:
NanciL wrote:
Can I use an extension cord to connect between my truck connector and my trailer cable connector ?
Yes
If yes do I have to worry about a voltage drop if the distance is too long. Yes Use a 10ga cord, 25' should be long enough but measure first. There will be some additional voltage drop.

Even if you don't want to hear this: I would never work inside an enclosed area with a engine running due to carbon monoxide among other things. If power is not available my choice would be a portable 120V gen outside.

Many thanks CA Travler

I appreciate your answers to my questions.

I agree with you 100 percent on not working in an enclosure with the truck running, but in this case the trailer is in one building and the truck is in the next one, and there is wide open spaces on both of them. Visualize two car ports side by side

Jack L