4 ms·

> I realized that these circuits coincided with breakers on the right side of the box. From this, I found out what is apparently a common pattern: we have two

> I realized that these circuits coincided with breakers on the right side of the box. From this, I found out what is apparently a common pattern: we have two 120V out-of-phase incoming lines together with a grounded neutral that is presumably the center tap on the secondary winding of the local transformer. That's exactly right... this sort of split phase approach is common, at least in the US. Larger consumers can have 240V available to them and everything else is 120V. (Larger consumers can mean things like ovens, ranges, wall AC's, normal AC's, larger shop tools... the list goes on and on.) Split-phase also has a couple interesting failure modes. A few years ago, we had a case where a 120V heater would cause fans in other parts of the house to speed up when it switched on. What was happening was that the neutral wasn't fully grounded, so when the heater switched on one phase, it pulled the neutral's voltage in a way that caused the other phase's voltage to go up. Once the electrician figured that one out, it was fixed with the quickness. (Not safe or good for electronics at all.)

colanderman 7y ago
What's doubly fun are multiwire branch circuits. Basically, instead of splitting the 240 V into 120 V at the breaker box, you take a 240 V branch circuit and split it into two somewhere down the line. This has two advantages: first, you need fewer wires going all the way to the breaker box; second, resistive loads on opposite legs of the circuit cancel each other out on the neutral, so you can (in theory) run 30 A worth of stuff off a single run of 14-gauge 4-wire Romex (= cheaper). Anyways, where the fun comes in, is if for some reason the neutral gets disconnected before the branch split. You now are effectively running the appliances on one branch in series with those on the other. Depending on the particular appliances involved, this means you could see up to 240 V on one leg of the circuit. This is, of course, very bad.
jacobush 7y ago
I lived in a place where the ceiling light would only turn on if one of the stove plates were on. If I recall correctly, the stove plate would not get hot in that setting.
organsnyder 7y ago
If you want to get a visit from the power company as urgently as possible, just call them and say the phrase "floating neutral" (of course, they'll be mad at you if it's not true). Our neighbor's (poorly-attached) service mast came down a few weeks ago, causing a short that burned out the shared wires that connect both them and us to the transformer. After the initial event (complete with loud pops, sparks, flames...), our power actually came back on; but I insisted on keeping our main breakers turned off until the power company made their repairs. Turned out to be a wise move, as we indeed had a floating neutral.

colanderman 7y ago

What's doubly fun are multiwire branch circuits. Basically, instead of splitting the 240 V into 120 V at the breaker box, you take a 240 V branch circuit and split it into two somewhere down the line. This has two advantages: first, you need fewer wires going all the way to the breaker box; second, resistive loads on opposite legs of the circuit cancel each other out on the neutral, so you can (in theory) run 30 A worth of stuff off a single run of 14-gauge 4-wire Romex (= cheaper). Anyways, where the fun comes in, is if for some reason the neutral gets disconnected before the branch split. You now are effectively running the appliances on one branch in series with those on the other. Depending on the particular appliances involved, this means you could see up to 240 V on one leg of the circuit. This is, of course, very bad.

jacobush 7y ago

I lived in a place where the ceiling light would only turn on if one of the stove plates were on. If I recall correctly, the stove plate would not get hot in that setting.

organsnyder 7y ago

If you want to get a visit from the power company as urgently as possible, just call them and say the phrase "floating neutral" (of course, they'll be mad at you if it's not true). Our neighbor's (poorly-attached) service mast came down a few weeks ago, causing a short that burned out the shared wires that connect both them and us to the transformer. After the initial event (complete with loud pops, sparks, flames...), our power actually came back on; but I insisted on keeping our main breakers turned off until the power company made their repairs. Turned out to be a wise move, as we indeed had a floating neutral.