**Transformer Taps**

There are ways to derive other voltages from a transformer by tapping into the windings at various locations. This is helpful when different voltages levels are required by different loads. Some low-voltage transformers come with taps at 24, 12 and 6 volt taps for flexibility.

Notice the connections...

Residential (single-phase) 240 volt transformer secondary windings are commonly center-tapped in order to derive 120 volts. The center-tap in this situation may be called the common, neutral, or identified conductor, and if we connect it to ground it can be called the grounded conductor. Connecting the neutral to ground diverts unwanted fault currents to the earth, and not through people like you or me.

With the normal three-wire connection to our house panel, we get the benefits of 240 volts (L1-L2) for our heavy loads (like air-conditioners, water- heaters and motors), and 120 volts (L1-N or L2-N) for our general lighting and receptacle circuits.

Balancing

It may be said that our 120 volt loads are balanced on the neutral in the above transformer. That is because the neutral only carries the difference or unbalanced current between the loads connected to the hot conductors (L1 or L2).

The above transformer secondary consists of three conductors: two hots or phase-conductors (L1 and L2) and a neutral (N). As before, the secondary has two voltage levels; 240 volts (L1-L2) and 120 volts (L1-N) or (L2-N).

Load #1 is a 1,200 watt, 120 volt load connected (L1-N). If we put an amprobe (current tester) around the neutral in this situation, we should get a reading of 10 amps (1,200w/120v).

Load #1 is a 1,200 watt, 120 volt load connected (L1-N). If we put an amprobe (current tester) around the neutral in this situation, we should get a reading of 10 amps (1,200w/120v).

Load #2 is also a 1,200 watt, 120 volt load connected (L2-N). The neutral in this situation should get a reading of 10 amps, right ! Sorry! We would get a reading of “0” on our amp-meter because the two 1,200 watt loads would be balanced. Remember ! The neutral only carries the difference or unbalanced current between the loads connected in this situation.

Now, if only one of the two 120 volt loads (#1 or #2) are turned on or running, our neutral would only carry the 10 amps of the other side. So, the maximum that our neutral would carry in this situation is 10 amps. We call this the

What about Load #3, this is a 4,800 watt, 240 volt load hooked-up (L1-L2). We don’t really need a neutral for this load because it’s a balanced load.

__“maximum unbalanced load”__and must size our neutral conductor based on this assumption.What about Load #3, this is a 4,800 watt, 240 volt load hooked-up (L1-L2). We don’t really need a neutral for this load because it’s a balanced load.

If we check the current on conductor (L1) we’ll find that it is carrying 3,600 watts. That’s one 1,200 watt, 120 volt load (#1) plus one-half of the 4,800 watt, 240 volt load (#3) for a total of 3,600 watts at 120 volts.

If we check the current on conductor (L2) we’ll find that it is also carrying 3,600 watts. That’s one 1,200 watt, 120 volt load (#2) plus one-half of the 4,800 watt, 240 volt load (#3) for a total of 3,600 watts at 120 volts.

Keep in mind that the two hot phase-conductors have to carry this load along with the two 1,200 watt, 120 volt loads. That’s a total of 7,200 watts when you add up all of the loads (1,200 + 1,200 + 4,800). If we divide the total wattage by the total voltage (7,200/240) we get a total of 30 amps. So, assuming that we have no efficiency losses we’ll require a 7.5 kVa transformer.

Remember the National Electrical Code Article 210-11(B) requires that we balance our circuits as evenly as possible when distributing our loads.