Using a standard 120V 100A breaker on 12V?

I am also not a circuit breaker expert but do know that many have both magnetic and thermal tripping mechanisms. The magnetic mode could be interesting on dc.

e

lled

Taking a circuit breaker designed for AC and trying to use it for DC of the same voltage may result in an arcing problem. Whereas AC will extinguish itself in the zero crossing, DC will not.

"BobW"

** Yep - I have used a domestic circuit breaker with a 12 volt car battery.

No problems, even takes a dead short on the load side with ease.

...... Phil

But at TWELVE VOLTS, there will be NO arcing, dipshit!

Most switches, as well as most circuit breakers have ratings on them for both AC and DC. A switch DC rating is always lower than the AC rating. That differential as stated on most switches, will tell you the likely differential for a breaker as well. Since a circuit breaker IS a switch that self-operates if taken beyond a specific current flow..

d

y one

ntrolled

How about you listen dipshit....

If dc doesn't arc, how come points wear out? How come the old solenoid=20 washers would burn up?=20

Oh yeah it arcs...

- Tim -

I don't know what the original poster's specific application is, but... if it's for an automobile application, I'd certainly be concerned about arc issues.

A car battery has a low enough impedance, and enough stored charge, to deliver a *very* serious amount of energy in a very short time. After all, that's what they're designed to do... deliver hundreds of amperes to the starter.

I've heard quite a few stories about mechanics (professional and home-car-owner) dropping a screwdriver or wrench across the exposed terminals of a car or truck battery. It arcs, all right... it arcs seriously enough to splatter the unwary mechanic with molten metal. In some cases (according to the stories I've heard) the tool is essentially destroyed - there's little of it left.

If the OP is planning to run a high-amperage cable from a vehicle battery to a heavy load (an electric winch, or a neighborhood-annoying Overkill Boom Stereo system), and is concerned about the possibility that this cable might someday be shorted to the chassis... then I think taking the possibility of arcing into account would be a very good idea. Both the risk of arc-flash injury at the point of grounding, and the possibility that the breaker might arc-over internally and fail to interrupt the circuit promptly ought to be considered.

Best to use a breaker where you can be *sure* that the contacts open far enough to quench the worst possible arc-over in a high-amperage event. A DC-rated breaker, used within its published design limits, is probably the right thing to do.

--
Dave Platt                                    AE6EO
Friends of Jade Warrior home page:  http://www.radagast.org/jade-warrior
  I do _not_ wish to receive unsolicited commercial email, and I will
     boycott any company which has the gall to send me such ads!

e

I'm a little curious why you would want to do this. First, most residental breakers come in sizes starting at about 15 amps. For that, you could just use a standard automotive style fuse, like an ATO-15.

(ATO's for example, come in sizes up to about 50-Amps.)

I would think the bigger problem would be the connections on a typical residential breaker. Most I've seen (Square-D, Federal Pacific, Siemens, etc...) are a snap in arrangement. One side has a screw, the other snaps into the panel's busbar.

Unless you have a breaker with 2 screw (or other) terminals, I'd be more worried about ruggedness and reliability. Especially in something bouncing down the highways...

There are speciality breakers for high-current DC, mobile applications. Some manufactuers even cater to the "boom box on wheels" crowd for "cool looking" over-priced hardware. -mpm

Yes, Dave, a 12V winch is exactly the application. It's this one from Harbor Freight:

It's all hooked up and working. I used 12' of 8awg cable from the battery, through a GE THQP250 breaker.

The manual says it's got a 1.1hp motor, so with a 50A breaker, I won't be able to pull 3000lbs (746W/hp). I only need to pull about 500lbs (worst case).

Even if it burns up after the first fault, it's no big deal. The breaker was only $10. In fact, it's a dual, so if it fails I can use the other half.

Thanks for all the comments (well most of them, anyway).

Bob

--
== NOTE: I automatically delete all Google Group posts due to uncontrolled 
SPAM ==

"BobW"

** That 12 V winch motor has a big current surge ( ie full stall current) when you first apply voltage that may well trip a 50 amp domestic breaker. Just depends what amperage the magnetic device kicks in.

Under heavy loads, the current draw will exceed 50 amps - so again it will probably trip the breaker a few seconds.

A 100 amp rated breaker ( as mentioned in your first post) is just what you may end up having to use..

BTW

I hope the cable drum locks up when the motor is not energised.

...... Phil

Don't high voltage breakers have higher series resistance than low voltage ones? Something to do with the trip mechanism?

--
Return address is VALID!
Bunch-O-Stuff Forsale Here:
http://mike.liveline.de/sale.html

So far no problems with the startup surge. I will probably switch over to a (rated as) dc 100A breaker. I've found that from the right source, the Bussmann CB185-100 is about $40.

Bob

--
== NOTE: I automatically delete all Google Group posts due to uncontrolled 
SPAM ==

Mike,

The contact resistance is a concern, but I'm not sure that in a magnetic breaker there's going to be much difference between a higher voltage vs lower voltage breaker. I'm not sure about thermal breakers.

I've looked at a GE datasheet and they don't list the max dcr for their ac breakers. I can't even find a datasheet for the Bussman dc breakers.

A hundred amps is a hundred amps. The contact resistance better be effing low or that breaker is going to get extremely hot. If you lose a volt across the contacts (at 100A) then that's going to be 100W dissipated by the breaker (I confirmed this calculation with several of my HP calculators). I think the thing would melt if it were run just below its trip point for any length of time.

Bob

--
== NOTE: I automatically delete all Google Group posts due to uncontrolled 
SPAM ==

"mike"

** The current rating is the thing that matters.

A 50 amp, 120 VAC breaker has VERY low resistance.

..... Phil

A 10kV rated HV relay has near zero ohms resistance when closed as well.

It is a damned set of contacts. Nothing special. Likely Platinum points.

Breakers are available with low voltage DC ratings. These will provide a more acceptible LV sensor burden and have contacts constructed with DC arcing phenomena in mind, as far as this is practical.

Check out DC relays to get some idea of the problems involved in maintaining long-term contact integrity.

RL