We're on break from a class. Paul and I are the only ones left of a bunch getting some air before going back inside.

"Extra electricity?" I reply. "What extra electricity?"

"You know, like when the light bulb doesn't need all the electricity in the circuit. Where does the rest go?"

"I'm still not getting this," I say.

"Well, if there's enough electricity for a 100W bulb, where does the extra go when you put a 40W in its place? Or why doesn't the 40W bulb burn up?"

"Oh-h-h-h. Great question! I never thought of it that way before."

Just before break I'd been teaching about loads. Electrical loads (light bulbs, motors) are consumers of electricity. A load eats up electricity and turns it into another kind of energy: light, heat, motion (as with a motor), sound or magnetism.

Earlier, I'd taught that electrical wiring is like a train. The electricity train leaves the power plant full of snapping hot electricity. When the train gets to the light bulb or motor, it "unloads" all the energy. The electricity train is then empty.

Yet the train has to return to the power plant to get more electricity energy to take to the load. The train coming from the power plant is called "hot" or "L1." The train returning to the power plant is called "neutral" or "L2." This is why there are two wires to the light bulb instead of just one.

The hot wire is the one that hurts you. The neutral wire doesn't. Don't get too confident about this, though. You don't know which is which, even though they may be labeled, unless you test with a tester. Never use your fingers or best guess.

"There isn't any extra electricity," I say in answer to Paul's question. "That's because each load 'draws' only the amount of electricity it needs. You know the term 'amp draw'? That's the amount of electricity the load needs.

"That's measured in amps. For example, the amp rating or amp draw of a common zone valve is .32A. It's printed on the end of the valve. You might think of it as a measurement of the valve's appetite. And it's a little like the amount of water that fits into a pipe. Under normal conditions, only so much flows through. How much depends on the capacity of the pipe. How much electricity depends on the size of the load."

Paul thinks for a moment. "OK. So you don't really have extra electricity. Then what's going on when someone fries a control? You know, lets the smoke out."

"That could be a couple of things," I answer. "It might be too much voltage. Just now we were talking about amps - the capacity. Voltage is the push or pressure of the electricity. If a control is made for 24V of electrical pressure and you give it 120V, it's kinda like blowing up a balloon with an air compressor for tires. It blows up, all right."

Paul smiles. "Well, what's going on when you burn a thermostat? That happens with 24V."

"That's because you 'shorted out the load.' A short means giving the electricity a shortcut so it doesn't have to go through the load. Electricity is lazy. It'll avoid the work of going through the load.

"A common way we accidentally short out the load is to let the bare wire ends of the two zone valve motor leads touch each other when the circuit is powered. Instead of going through the motor, the electricity goes from the end of one lead to the other. But it's still looking for a place to spend its energy. The thermostat anticipator will do.

"The anticipator is just a winding of tiny wire. It's a load whose purpose is to create a little bit of heat inside the thermostat. It can't handle all the electricity in the circuit if the real load doesn't take any. So if you short out the load, too much electricity goes through the anticipator and burns it. If you look at a burned thermostat, you'll see that the burn is at the anticipator."

All Good Questions

"That's yet another situation. It's an example of trying to pull too much electricity, or amperage, out of the transformer. That happens when the amp draw of the load or combined loads is more than the transformer has to offer. The image I have of that is of zone valves sucking electricity through from the transformer. It turns out that with an ordinary 40VA transformer and ordinary .32A zone valves, the transformer can handle no more than five zone valves pulling electricity at once. If there are more, and they all pull at once, the transformer secondary burns."

"What about if there's no transformer, like house wiring?"

"House wiring is line voltage, and there's no transformer. What happens when you plug in too many appliances in your kitchen or lamps in your living room? We've all seen it - the circuit breaker or fuse blows.

"A circuit breaker and a fuse have the same function. If the loads try to pull too much electricity, the circuit breaker or fuse opens, or breaks, the circuit so no more electricity can flow. Each operates as a switch. Remember, a switch is like a drawbridge. Open means that traffic or electricity can't flow.

"A fuse is a one-time switch. If too much electricity tries to go through it, the metal inside melts and creates an opening in the electrical path. It's a one-time disposable switch. Use it once and replace it.

"A circuit breaker is more like a light switch. When things are right, the switch is closed. Like when a drawbridge is closed, the traffic, or electricity can flow. When the loads try to pull too much electricity through it, though, the circuit breaker switch opens.

"No more electricity can flow unless a person goes to the circuit breaker panel and re-sets, or closes, the circuit breaker. A circuit breaker can be opened and closed many times. However, if the same one opens again and again, something's wrong with the circuit and you need an electrician to fix it."

Paul agrees. He continues, "That circuit breaker is a good idea. Why don't they put them on transformers?"

"There are transformers with circuit breakers built into them - for exactly that reason. When you make a mistake, the circuit breaker opens instead of burning the transformer. This kind of transformer costs just a little more, but the forgiveness is worth it!"

"I know it's time to go back in, but one more thing," Paul says. "Before lunch you talked about how to figure how many zone valves you can put on a transformer. How does that go again?"

"First, a transformer comes with a VA rating. 40VA is common - and it's written on the transformer. VA means volts multiplied by amps.

"When you look on a zone valve, though, it doesn't have a VA rating. It has voltage (24V) and amps. The amp rating varies from valve to valve, so you have to look. But let's say this one is .32A.

"Multiply 24V x .32A and you get 7.68VA.

"Now divide 7.68VA zone valve into 40VA transformer, and you get 5.2. Round that back to five zone valves. That's the maximum. You might want to stick to three or four just to be sure.

"But if the zone valve has an amp rating of .8A, it's a completely different story. Multiply 24V x .8A and you get 19.2VA. Divide the 40VA transformer by the19.2VA zone valve and you get only two zone valves."

"Oh, I see," Paul agrees. "Put five of those big boys on the transformer and the transformer dies."

"Right!"