cut off switch question...

[Dale M.]

As for "normal" light switches.... Most DO NOT have a DC "rating" on them only a AC rating, that has to tell you something.....

Dale

They do not have a dc rating because they are made for 115v a/c.... I use them in my tears with no problems.

The recommendation is that you cut the watts in half when using for 12v. That leaves you with 72 amps with 12v... since I use a 30 amp fuse, I am not concerned.

Think you need to do more research or get facts straight...... You state 72 AMPS....... NO "general application" wall switch is rated at that value at 115V AC or 12V DC .... Most are only rated at 15 AMP or 7.5 AMP DC (by your statement) ..... Even if you divide by 2 (115 AC watts) and then divide by 12 volts (DC voltage) to get 72 amps its not true....

I would also like to see documentation that the DC rating on a switch is 1/2 of AC rating...

Dale

[madjack]

.................................................................................I would also like to see documentation that the DC rating on a switch is 1/2 of AC rating...

Dale

...can you say..."old electrician tale" ...let me second Dale on amp ratings on common wall switches...15-20A @ 120vac is most common with some rated for 30A @ 220vac...almost anything over that is a special build using some sort of hi-amp contactor system...I am not an electrician or electrical engineer, just what I have noted during my construction career and in doing HVAC work............
madjack

[Larwyn]

After a bit of research I am reminded that the true issue here is the air gap between the contacts of the switch. Switches intended strictly for AC operation can have a very small gap between the contacts. Whether AC or DC there is arcing when the switch is opened, but by definition AC goes to zero 120 times a second (in the US),somewhere between full voltage and this zero crossing ionization of the air between the contacts cannot be maintained so the arc is easily extinguished. While a DC switch depends entirely on the dielectric strength of the atmosphere between the contacts to cause the arc to extinguish as there is no zero crossing.

If the switch is truly dependent solely on the dielectric properties of the air gap in the switch then it should follow that the voltage level is the main factor in determining whether or not the switch will work properly. The dielectric requirements for the insulation on a conductor is determined by the voltage at which the cable is to be utilized and not the current or the wattage. Just as the clearance (air gap) of high voltage buss bars are determined by the voltage. Since the switch is actually seeing the peak voltage of the AC sine wave (approximately 169 volts) then operating the switch at only 12 volts DC leads me to believe that the switch could easily handle whatever current the switch contacts were designed to carry, unless that switch were very carefully designed to utilize the characteristics of the AC sine wave.

On the deep cycle battery in a TD, I see little reason for a more elaborate switching method than simply turning a wingnut a few turns counterclockwise and removing the cable. It costs nothing, makes it easy to provide plenty of air gap and if there is not too much light, you get to see the nice little spark which has caused all this talk.

[Miriam C.]

AC switches are not designed for DC. The DC contact is heavier because it is constant current. You should never use a switch for AC on DC unless it is DC rated. Truthfully you can use them but the contacts will wear out faster. Will it cause a fire------dunno.

Disconnect switches are cheap unless you are doing marine grade or industrial grade. I think I got mine at Harbor Freight for $3 bucks ish.

EDIT= What Larwyn said! had to answer the phone before finishing. The wing nut might work for a lot of us but some folks hide the battery............ Or like me, make the hidie hole too short making it a pain to retrieve.........

[danlott]

I've just got to ask (because I didn't add a master switch and now I feel insecure )

I guess I thought you put a master switch in to prevent drawdown during long term storage...and if that were the case...am I goofing up? I put in a three stage charger with the intent to just leave it plugged in and fully charged during non-use. Is there a reason you want to disconnect the battery for towing?

I'm so confused

I see 2 reasons to have a kill switch. The first would be unwanted drawdown of the battery, either while storing your trailer or while on a camping trip. Some 12V fixtures will still draw power even if not turned on, a radio is a good example. If you get in the habit of shutting off the kill switch while you are camping your battery could possibly have more power for a longer time. Of course you would have to turn the switch on when you needed to use power.

The second reason would be for battery protection. It depends where the switch is placed in your system. I plan on installing one between the charger/converter and the battery. I also plan on charging with the tow vehicle. While towing I will have the kill switch turned off. I do not think that I would ever run the risk of having 120 AC hooked up and also have the tow vehicle hooked up at the same time, but if I did a kill switch would help possibly prevent power going to the battery from 2 different sources.

Dan

[madjack]

I used a double pole double throw switch to provide AC power to the charger and DC power from the charger directly to battery...I used the Master Switch to connect between the battery and fuse block, this allowed me to power the charger to battery circuit with out providing power to the rest of the system..............
madjack

[Arne]

I stand by my statement. A normal house switch can be used on a 1,500 watt heater. That is a 13 amp draw. If you cut that in half, with 12v, you can draw 62 amps.... My fuse in only 30 amps on 12v.... but, if you don't want to use it, don't...

quote: Best Answer - Chosen by Voters
It makes diddly squat difference. Maybe if you're a perfectionist or something like that, but in reality so long as the device behind the 15 amp switch draws less than 15 amps, it's no a problem. Even if the circuit was 1000 amps, you could still use a 15 amp switch (if you drew less than 15 amp through it.)

[Dale M.]

I stand by my statement. A normal house switch can be used on a 1,500 watt heater. That is a 13 amp draw. If you cut that in half, with 12v, you can draw 62 amps.... My fuse in only 30 amps on 12v.... but, if you don't want to use it, don't...

quote: Best Answer - Chosen by Voters
It makes diddly squat difference. Maybe if you're a perfectionist or something like that, but in reality so long as the device behind the 15 amp switch draws less than 15 amps, it's no a problem. Even if the circuit was 1000 amps, you could still use a 15 amp switch (if you drew less than 15 amp through it.)

You really do not have any concept of electrical formulas or physical design of electrical circuits. Your comments are dangerous....

I personally would NOT let you design or build any electrical circuits for me. Or would I accept anything built by you ...

This is not a personal attack, this is a warning to other to beware of comments and suggestions that you make about electrical circuits...

Dale

[Larwyn]

Just as for informational purposes; when I was working with an outfit that built and tested electrical substations I wired a regular old everyday single pole AC rated light switch into DC lighting circuits ranging from 48 Volts DC to 130 Volts DC in the states of Texas, New Mexico, Arizona, California, Arkansas, Pensylvania, and New York. In every case the switch was specified and drawn on drawings approved for construction by a profesional electrical engineer for the state in which the work was done. The only time I ever saw such a switch fail was the one time in Mesa Arizona where a "Quiet Switch" was supplied by the customer. We first advised against it's use and then set quietly back and watced it smoke and glow the first time it was turned off. It was then replaced with a "normal house switch" which more than likely is still in service today.

The job I retired from involved the maintenance of electrical substations. We had around 21 sustations and every one of them had DC emergency lights controlled by a standard single pole light switch just like the ones used in houses on AC lighting circuits. All these DC lighting circuits were connected to the 130 Volt station battery. I never had to replace one of those switches and most of them had been in service long before I even went to work for that company.

As far as I am concerned, based on extensive experience with using these switches at a much higher voltage level than that used in a TD, anybody who says they will not work is most likely somewhat short of being correct. Of course , what do I know? I may have been extrememly lucky to have experienced 27 years of exposure to this "ticking time bomb" with only one incident, that one involving the wrong type of the wrong type of switch.

[Arne]

I guess with have both been lucky, then...... :}

[Dale M.]

Just as for informational purposes; when I was working with an outfit that built and tested electrical substations I wired a regular old everyday single pole AC rated light switch into DC lighting circuits ranging from 48 Volts DC to 130 Volts DC in the states of Texas, New Mexico, Arizona, California, Arkansas, Pensylvania, and New York. In every case the switch was specified and drawn on drawings approved for construction by a profesional electrical engineer for the state in which the work was done. The only time I ever saw such a switch fail was the one time in Mesa Arizona where a "Quiet Switch" was supplied by the customer. We first advised against it's use and then set quietly back and watced it smoke and glow the first time it was turned off. It was then replaced with a "normal house switch" which more than likely is still in service today.

The job I retired from involved the maintenance of electrical substations. We had around 21 sustations and every one of them had DC emergency lights controlled by a standard single pole light switch just like the ones used in houses on AC lighting circuits. All these DC lighting circuits were connected to the 130 Volt station battery. I never had to replace one of those switches and most of them had been in service long before I even went to work for that company.

As far as I am concerned, based on extensive experience with using these switches at a much higher voltage level than that used in a TD, anybody who says they will not work is most likely somewhat short of being correct. Of course , what do I know? I may have been extrememly lucky to have experienced 27 years of exposure to this "ticking time bomb" with only one incident, that one involving the wrong type of the wrong type of switch.

I to have some experiences with DC power systems, 38 years to be exact.... I worked in telecommunications all that time and have logged may hours (more than I care to remember) wiring 52 volt DC and 130 volt DC powers system. These are the systems that powered almost everything in a telephone offices.... And even 500 volt DC power systems (back up power for satellite earth stations). Some of the battery back up system were capable of putting out 10,000 AMPS or more on a surge.... Its enough to literally vaporize a wrench if you make a mistake on the UNFUSED side.....

We also had 52 volt DC emergency lighting, and we uses a normal wall style light switch, BUT it ALWAYS controlled a "contactor" (relay to the uninformed) so it was never loaded more them its design/engineered specifications....

Also...

Check "DC rule of thumb"....

http://www.carlingtech.com/products/swi ... amp-rating

Dale

[madjack]

...well, one more electrical thread heading south inna handbasket...since Doug got his question answered in the first couple of posts, all the rest has become ??????? as a result I am going to lock this one...by the way...amp capacity is amp capacity...regardlesss of wattage or voltage...say goodnite Gracy...........
madjack