Wired for destruction- Multiwire branch circuits and tandem breakers

Wenatchee Home Inspection Services

I have written about Multiwire branch circuits before and I am not a big fan of this in residential settings. Link from Mike Holt-  Understanding the Dangers of Multiwire Branch Circuits

But I wanted to use an example of one issue with these circuits that came up on a recent home inspection. This was an installation in a garage/shop.

Here we have Multiwire branch circuit; this is a circuit with two or more hot conductors sharing a common neutral (grounded conductor).

But here is the danger… in this example the multi-wire circuit is on a tandem breaker. The majority of the time, not always, these tandem breakers sit on the same leg of the electrical service. This means when you put a load on both hot conductors the load that is on the neutral is cumulative versus canceling.  This can lead to overloading the neutral and a serious potential for a fire.

In a normal two pole multi-wire branch circuit the hot conductors will be on opposing legs that are out of phase of each other and the potential load on the neutral will be the difference between the two loads.  Example- Load “A” has 7 amps and load “B” has 5 amps the load on the neutral in this scenario would be 2 amps (7-5=2), simple enough.

Now it is time to do a little math for this example in a fictional shop environment.

So let’s say you have a somewhat typical 30 gallon air compressor, for this example it has a 5 HP start load and 2 hp continuous load. 1-HP equals about 746 watts, so if we have 2 hp x 746 watts on a 120 v circuit you have a little over 12 amps of load.

Now we have a 3hp table saw the 120v amperage is probably pulling 15-18 amps or better. We will use 18amps for this equation. Add few lights, 4 100w bulbs equals a little over 3 amps.

So the math looks like this, 12 + 18 + 3= 33 amps on the neutral of this 20 amp circuit and you will start to heat the neutral wire up. Now given this maybe not for extended periods of time but you could have several things going at once and you could really cause the neutral to get overloaded for extended periods of time.  Plus it is just not right and if you happened to lose the neutral all together pop goes the weasel!

“Take calculated risks. That is quite different from being rash.”

George S. Patton

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