transformer wiring question kw

[Anonymous]

I have a kw on my layout, running 2 separate loops. I have a separate #12 bus wire for each post. If I add another transformer for another loop and accessories, should I pull another common or 'u' wire or can I share what is there?

[willpick]

It depends on how far the new loop will be from the transformer-- if it's close, then you could run ok. personally, I'd run the extra buss just to be safe, as you may be drawing a bunch of amps, and while that #12 is adequate now, it will probably be at the limit if you don't add the extra buss-- just my opinion[:)].

[lionelsoni]

I agree that you should add more wire, but not that it depends on the length of the wire. The ampacity of 12 AWG is 20 amperes, so 14 AWG would have been big enough (for safety) since the ZW circuit breaker trips at 15 amperes. Two ZWs could supply 30 amperes together, however, which calls for 30 AWG for the common, or separate common runs for the two loops.

One trick that you could use is to run the two transformers out of phase with each other, in which case the return currents would cancel rather than add. This is the same trick used in wiring houses with 120 and 240 volt service. If you should do this, don't connect the track of the loops together becahse, even if you don't follow the common but bad practice of running between separately powered blocks, sooner or later you will do it by accident.

Another thing to avoid is using two smaller wires in parallel instead of a larger one. Two 14 AWG wires can carry the same 30 amperes as one 10 AWG, but it is difficult to connect them symmetrically so that they share the current equally. And, if one comes loose, you will not notice any problem until the overload occurs and it is too late.

[phillyreading]

I wouldn't go higher than a #10 AWG, the ratings for the wire is usually based on voltages at 120 volts or higher & most transformers don't go over 27 volts. You are dealing with low voltage & usually low wattage so even a #12 AWG should do unless you are talking about running the wire for over 300 feet or have two ZW's in parallel and most layouts are not that big. With transformers an important thing to remember is that you can not get more output than you have input, if a transformer draws 500 watts from the outlet you may have up to 485 usable watts to play with, the transformer windings always take up some power. In my opinion using a higher gauge wire is not always needed, get the proper circuit breaker rated for 30 volts AC or less, don't use house circuit breakers as the voltage rating is too high and won't protect anything.
Lee Fritz

[lionelsoni]

The voltage has nothing to do with the size of the wire nor with whether a circuit breaker will open at its rated current.

Wire should be insulated for the voltage it is used at; but the size needed for the conductor depends only on the current that you put through it. Number 10 AWG is good for 30 amperes, whether at 16 volts, 120 volts, or 7200 volts. It will get just as hot in each case if you put more than that current through it.

A circuit breaker has no idea what the voltage is on the wires it is protecting as long as it is closed. Both wires are connected together; so they are at the same voltage. Only when it opens can there be any effect from the voltage, which then is the difference between the voltage on the two wires. The only difference between a high-voltage circuit breaker and a low-voltage one is that the high-voltage one can interrupt the higher voltage difference without arcing or destroying itself. They will both try to open at the same current. You can perfectly well use a high-voltage breaker in a low-voltage application, but not the other way around.

[phillyreading]

To Lionelsoni,
I disagree with you on the wire issue as I was at one time trying to become a journeyman electrician, low voltage don't need as large a wire as long as the watts are low. Also it is most important to have the proper rated circuit breaker both voltage & amps or you will fry what you want to protect!! The inside metal element is designed to trip at a certain heat range and will not do so if you use higher voltage rated breakers so needless to say that 20 volts will not have enough power to trip a 1000 volt circuit breaker.
Have you studied Residential & Commercial Electrical construction?
Have you looked at the cord going into the ZW transformer? I don't think that it can handle even 10 Amps.
Lee F.

[fwright]

Lee

I think you are confusing transformers and wiring. A step-down transformer takes high voltage, low current and changes it to low voltage, high current. The 10 amps is on the 20 volt side of the toy train transformer. Translated to the input side, the 10 amps becomes 1.7 amps (plus 20-30% for internal losses) in the power cord. We use higher voltage to transmit electricity to reduce the resistance losses in the wire. Resistance losses are related to the current in the wire, not the voltage (loss = I^2 times wire resistance). As Bob states accurately, the only impact the voltage has on a wire is the insulation required to protect the wire.

We use 12-24 volt systems (with relatively high currents) in our toy trains, boats, autos because those voltages are easier to achieve with batteries and because the internal resistance of the human body is high enough that not enough current will pass to cause damage at those voltages. About the only thing in a house that draws 10 amps or more are appliances and hair dryers. A 60 watt light bulb draws half an amp, but you would need 5 amps for the same light output at 12 volts. In household work, if the current draw is going to be much more than 10 amps, we generally raise the working voltage of the device to 240 volts to keep the current within our normal household wiring limits.

The same with circuit breakers or fuses. As long as the voltage rating is not exceeded, a 10 amp fuse will blow at 10 amps regardless of whether it has 120 volts or 12 volts passing through it. The voltage rating determines at what point the blown fuse might arc over and conduct anyway (similar to the dreaded practice of using pennies to replace blown fuses when houses had fuses instead of circuit breakers). Typically, the in-line fuses used in 12 volt wiring are rated at 125 volts.

The all-too-common practice of downsizing wiring because of lower voltage is a performance-robbing practice (due to voltage drops) and carried to extremes, a fire hazard. Good practice would actually call for increased wire sizes in 12 volt circuits because a 1 volt drop due to wire resistance is so much more significant to the end device.

As with household wiring, circuit breaker ratings should never be higher than the current rating of the wiring. Otherwise, you can get the case where the resistance of the wire regulates the current at an unacceptably high level for the wire size (can be fire-starting heat) but never trips the breaker.

And yes, I have read and used electrical codes, as well as the recommended practices for marine wiring. I have wired more than one building electrical panel.

yours in wiring
Fred W

[lionelsoni]

Thank you, Fred.

Lee, I have wired houses; and I was doing some recreational reading in the 2005 NEC Handbook just the other day. I have never tried to become a journeyman electrician.

Bob Nelson, BSEE, MSEE

[johnandjulie13]

Hello Bob:

You were "recreationally" reading the NEC handbook? The National Electrical Code handbook? Wow. You must have some reading list. [;)]

Just kidding. Seriously, thanks to you and Fred for the valuable information. It is amazing how much I am learning just by lurking around these forums. I continue to "reacreationally" read "Teach yourself electricity and electronics." I am currently reading about AC and impedance and admittance. But I still have a looooong way to go.

Thanks again,

John

[ADCX Rob]

QUOTE: Originally posted by fwright

...Good practice would actually call for increased wire sizes in 12 volt circuits because a 1 volt drop due to wire resistance is so much more significant to the end device...

Fred W

Our cars, starters & alternators included, would be wired w/ zip cord & bell wire if they were based on 240 volt systems... remember the size of the battery cables on 6 volt cars & tractors? Light aircraft are 24 volts for a reason too...

Rob

[lionelsoni]

Rob, you reminded me of the interesting behavior of the generator light on my 6-volt VW beetle long ago. It would glow dimly when the headlights were on, not because the regulator contacts that it was wired across were open (they weren't) but because the lamp was connected to the 6-volt line that fed the headlights instead of having its own wire to the regulator back in the engine compartment; and the voltage drop between the battery and the front of the car was enough to light it up a little.

[phillyreading]

I will go along on the wire sizes but don't use higher voltage circuit breakers or fuses, or like I mentioned before you may fry something before you know it. Don't be cheap with the current protection device, get the proper rating. ONE very important thing NEVER use a DC breaker on AC because AC has more kick for the same volts, don't use automotive switches either because they are rated in DC. Just for your info I hold an NIASE certification in automotive electrical and have been to three years of Vo. Tech school for residential & commercial electrical.
Lee Fritz

[ben10ben]

I certainly don't have the qualifications of most of the other people on this thread, but to me, anything rated for use with DC should be safe for use with AC. I do know that DC has more of a tendency to arc because it is a constant flow of current, while AC has no current flow for an instant every 1/120 of a second(or the inverse of twice the frequency, if you're not using 60 hz).

[lionelsoni]

You're right, Ben. But I think that this is hopeless.

[dwiemer]

Bob and Fred,
Thanks for your valuable additions to this forum, it is how some of us less learned ones can become educated. Bob, like your idea of light reading [(-D] Personally, I like the Feynman Lectures on Physics. I am sure I will be asking you more than a few questions in the very near future as I get my plans together for my layout.
Dennis

[phillyreading]

Ben,
To put it plainly AC has more kick to it because it alternates 60 times a second, DC is usually battery voltage and is constant, the real differance with AC voltage: for 120 volts AC at 60 hertz the momentary voltage reading on an oscilliscope will be near 178 volts
and for DC at 120 volts will be only 120 volts. Hope this can clear up why you should not use DC rated switches on AC, just to restate it plainly AC will burn out DC switches rated at the same voltage. Lee

[lionelsoni]

170 volts.

An example of switch specifications:
NKK series M miniature toggle switches
6A @ 125VAC & 3A @ 250VAC
4A @ 30VDC for On-None-On and On-None-Off; 3A @ 30VDC for all other circuits

http://www.alliedelec.com/Images/Products/Datasheets/BM/NKK_SWITCHES_OF_AMERICA/NKK-Switches-Of-America_Industrial-Control_8708613.pdf

[ben10ben]

phillyreading,
I know that the peak voltage is actually very high, but it happens for such a brief instant that it's not really all that significant. That's why we talk about the RMS(root mean square) AC voltage. 1 RMS volt of AC will have the exact same effect on a circuit in terms of current(or "kick" as you call it) as one volt of DC. This is why most people are concerned with the RMS voltage and the peak voltage is almost never mentioned.

Voltage and current are not the same thing. If one RMS volt cause the same amount of current to flow through a circuit as one volt of AC, then, for our purposes, it is the same thing. Arcing is the enemy of switches, and what causes them to fail. The amount of arcing is directly dependent on current, so, if the current is the same for a given RMS AC voltage and DC voltage, the amount of arcing will be the same, and everything works just as it was intended.

As I mentioned earlier, and as our resident electrical engineer confirmed, DC actually has more of a tendency to arc than AC, so it can destroy switch contacts faster.