Rule of thumb -- Volts, watts, amps

[cheech]

Hi

I am wondering if there is a rule of thumb to help estimate the number of lights/accessories I can attach to a PW ZW 275 watt transformer.  I have one that I intend to use solely to power lights and accessories on the layout.

 

There are approx 45 of these accessories, mostly PW lionel, some modern MTH, all instructiions say to power between 12-14volts and the handle on the ZW will allow me to increase/decrease power to them as needed for their individual quirks- e.g., the the Horse Corral or the Barrel Loader. All of them will not be on all of the time, but the thinking is to operate several at the same time. All will be connected by a separate 'bus', all will be on/off using the accessory on/off switch that comes with them.  NO track power to operate them.

 

I have a lot of lights around the layout. Highway lights, Flood lights, Spot lights, Building Lights, Traffic Lights...a lot of lights. Even some low voltage types that flash and operate in the 7-10 volt range. All will be on/off thru toggles. All will be connected by a separate 'bus' dedicated for lights. No track power to operate them. I expect to operate most of the lights at 10-12 volts. Most will be on at the same time.

 

Is there a rule of thumb that I can apply to go from Volts to Watts?  Specifically, how many of these lights I can have on at the same time powered by the ZW?

 

Thanks.

Ralph

[lionelsoni]

Alas, there's no such thing.  You just have to add up the loads.

For lamps, you can look up the specifications, which will probably be given for a specific voltage.  (For example, a number 53 lamp draws 120 milliamperes at 14.4 volts.)  If instead it is the power that is specified (in watts), divide by the voltage to get the current (in amperes).  Then, if you intend to run that lamp at its specified voltage, you know how much current to add to your total for the circuit.

But, if you are going to run it on a higher or lower voltage, you need to recalculate the current.  Forget about Ohm's law here:  Current and voltage are not proportional for incandescent lamps.  Instead, current varies as the .55 power of voltage.  (There's a rule of thumb for you.)  So, you can take the intended voltage, divide by the specified voltage, raise that quotient to the .55 power, and use the result to multiply the specified current to get the actual current at the intended voltage.  That's complicated; but it's how they work.

The sum of all the load currents, no matter the voltage of the four individual circuits, cannot exceed 15 amperes for the ZW, since that's where its circuit breaker trips.  As well as the current, you also need to consider the power that you're using.  Multiply the total current in each circuit by the voltage that you have it set for, to get the power.  Add the power values for all four circuits together.  That is the total power that the transformer is supplying.  For the ZW, it should not exceed about 180 watts.  (The 250 or 275 watts is the power that the transformer is consuming from the 120-volt power line, not what it can safely put out continuously.)