The heating industry doesn't do much with loads in series. But switches in series are the “safety committee” that makes sure the equipment fires only when conditions are safe.

We use parallel circuits for both loads and switches. Let's compare series and parallel circuits.

All of the components in a series circuit are wired together one right after the other. The circuit might consist of power supply, switch, load, switch, load, and back to power supply.

A parallel circuit could be made up of exactly the same components, but wired together differently. And the resulting performance of the circuit would be entirely different.

Loads In Parallel Vs. Loads In Series

Furthermore, it means that if there's a higher wattage load, it gets very little electricity. It would be quite dim because electricity prefers a path of less resistance - the lower wattage bulb. The lower wattage load gets almost all the electricity it wants and is bright.

Here are a couple of ways of looking at this that might help to understand it.

One is that when loads are wired in series, they are like kids in a family all sharing the same platter of food. There's only so much food, and no one gets quite enough. However, in circuits the big guy doesn't win. The little guy (lowest resistance) is closest to getting all he wants. That's because he wants less.

Another way to look at a series circuit is that the electricity has only one path. It must go through all the loads before it can be where it wants to go - back to where it came from. The electricity just gets plain tired going through all those loads, so many of the loads look kinda tired, too.

In parallel wiring, the electricity can choose among several paths. Each path has just one load. That's a lot easier for the electricity. Each load gets all the electricity it wants, so two 60W bulbs wired in parallel are fully bright.

Here's why.

Think of a parallel circuit with two loads. Imagine it as being two simple circuits that happen to share a power supply.

First wire a simple circuit of the power supply (the electricity coming from the wall outlet), a wall switch and a 60W light bulb. Finish the circuit by bringing a wire back to the power supply. This is defined as a simple circuit.

Of course the bulb is fully bright when you close the switch.

Now add a second simple circuit, using the same power supply. Just ignore the first circuit, but use its power supply to make a second circuit of power supply, switch and load. The fact that these two circuits share a power supply changes their definition from two simple circuits to one parallel circuit. Again, a parallel circuit is two (or more) simple circuits that happen to share a power supply.

With a parallel circuit, unlike a series circuit, performance doesn't depend on how many loads there are or what the relative wattages are. Each load gets all the electricity it wants because it has a direct path to the power supply, without going through any other loads.

In low-voltage (24V) control work there is a limit to how many loads you can put in the parallel circuit. That's because the power supply is a transformer, and a transformer has a limited capacity. The capacity is expressed as a VA rating.

Think of the VA rating as a contents label. If a bottle says 12 oz., you can expect to get 12 oz. out of it. If a transformer is rated at 40 VA, you can expect to get 40 VA out of it.

You can figure how many loads you can put on a 40 VA transformer just like you'd figure how many cups you can expect to fill with a 12 oz. bottle. Divide the capacity of the cup (let's say 6 oz.) into the capacity of the bottle (12 oz.). You get 2 cups.

To figure loads on a transformer you need to take one additional step. Loads are rated in A (amps) rather than in VA (volt amps). To get VA, multiply the A rating of the load (let's say the zone valve is .32A) by the voltage of the circuit (24V) and get 7.68 VA. Now, like the bottle and cup, simply divide the 7.68 VA into the 40VA of the transformer and get 5.2. That means five zone valves of .32A each could be wired to the 40VA transformer. Fewer are fine. More is asking for trouble.

House Wiring

An example is four or five bulbs above a bathroom vanity. They're in a row like holiday lights, but they aren't dependent on each other. Each light is fully bright. If one burns out, the others are not affected. The loads are in parallel with each other, sharing both a power supply and a switch.

In house wiring, electrical outlets also each have their own private access to the power supply. One way to think of an outlet is that it becomes the power supply for whatever we plug into it. Another way to think of it is that it's just a connection point and, electrically, is nothing at all. It's waiting for a load and switch - think of a table lamp - to be plugged into it.

Each ceiling light may have its own switch. Wires in the wall are going from the power source coming into the house, to a wall switch, to the ceiling light, and back to the power source. Each ceiling light and switch combination has its own private connection with the power source. Each wall outlet also has its own access to the power supply. Again the outlet is just a connection waiting for a load and switch combination to be plugged into it.

Switches In Series Vs. Switches In Parallel: When we call a circuit series or parallel, the assumption is that we're talking about loads. But series and parallel also can apply to switches. Think about this. The simple circuit portion of a parallel circuit can have more than one switch. If there are two switches, they will be wired either in series with each other, or in parallel with each other.

The fact that house wiring is loads in parallel helps to explain why electricians sometimes aren't comfortable with wiring controls. Control wiring frequently involves more than one switch controlling a load. (Recall the safety committee of switches in last month's column.) Those switches may be in series or they may be in parallel.

Switches in series is the safety committee scenario. All of the switches must be closed before the load can get power. Switches in parallel means that closing either switch allows power to flow to the load. It's possible for one circuit to have some switches in series, and some switches in parallel.

Wiring Transformers - Series, Parallel Or ...?

If you use two 40 VA transformers, how should you wire them together? The simple answer is that you don't need to. Just put three zone valves on each. That keeps the job simpler to install and troubleshoot. Plus there's the advantage that if one of the transformers fails in the future, the other three zone valves will still have power to provide some heat to the building.

If you're set on wiring the transformers together, they must be wired in parallel. That means that a terminal on one transformer must be wired to the identically labeled terminal on the other.

We've just looked into a major difference between a competent electrician and a controls technician. The electrician is thinking about parallel circuits. A controls tech has to be able to distinguish between series and parallel.

The tech knows that putting loads in series is a bad idea. He knows that switches in series means that all the switches must be closed before the circuit works. He knows that switches in parallel means that any one of the switches make the circuit come on.

And the tech knows, if the circuit doesn't work, to check out the wiring rather than blaming the controls. That's because the wiring makes all the difference.