CH4
Member
I get the concept of a single hot/neutral wire pair. What I do not understand is two hots and single neutral.
Seems like the neutral would heat up .. why does it not?
Seems like the neutral would heat up .. why does it not?
Why can two hots feed a single neutral?
- Thread starter CH4
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billconner
Well-known member
AC. The 2 hots are 180 degrees out of phase - or should be.
pattenp
Well-known member
The neutral only carries the amp difference between the two opposite legs. Example, If one leg is carrying 10A and the other 15A, the shared neutral is seeing 5A. Look up multiwire branch circuit.
SlappyWhite
Well-known member
To work it out mathematically you can draw up the circuit and use Ohm's law... Not really worth the effort IMO as it comes down to the neutral carrying the difference (current) not the combination.
***
To also blow your mind, so to speak
. The entire service to the house is done this way, two hots and a single neutral. Not sure for NEC but here reduced size neutrals for the service is a thing. As an example a 100 amp service could have two #3 copper hot wires and a smaller gauge for the neutral.And even more so as @Bert_ noted, three phase....
alfredeneuman
Well-known member
The entire transmission system from the power plant on down is done this way.
3 or 2 hots sharing the same neutral. When 2 hots only are used the transformer manfactures the neutral.
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SlappyWhite
Well-known member
Actually three hots (three phase) and no neutral...
alfredeneuman
Well-known member
In a delta system.
Not in a wye system.
wyliesdiesels
Well-known member
Depends on the type of system.
My PoCo has both primaries- 3-wire Delta and 4-wire Wye
TRWham
Well-known member
Transmission and distribution are 2 different things. The transmission system has no neutral- just the three hots, and neutrals may or may not be used in distribution. It doesn't make sense for the PoCo to string 4 wires everywhere when a neutral can be created at a PoCo or customer owned transformer if it is needed.
Very common in older residential areas for a 4160/2400V distribution system to exist.
rlitman
Well-known member
Sort of. 3-phase adds some complexity here. Harmonic currents are a reason why a 3-phase Y neutral may be spec'd to 150% of the circuit ampacity even though technically 100% would seem to be sufficient.
Depends on the load. I have heard some discussion weather it's as much an issue as we think sometimes.
Bad Habit
Well-known member
In the old days computer power supplies were horrible about this and caused a lot of problems in the early data centers and their power distribution systems. In many case the buildings these were put into actually had undersized neutrals which greatly exasperated the problems (i.e. fire)
exranger06
Well-known member
It becomes easier to understand once you realize that the hot legs don't only supply power, they also serve as a return path. Likewise, a neutral isn't just a return path, it also supplies power. In AC, the current is constantly going back and forth: one moment the hot leg is the "supply" and the neutral is the "return," and the next moment the neutral is the "supply" and the hot leg is the "return." And they switch back and forth like that 60x per second.
Neutrals are only used on 120V circuits. A 240V circuit doesn't use any neutral at all. A 240V circuit just uses the two hot legs: One hot leg supplies current, and the other hot leg is the return path. And they also switch back and forth 60x per second.
"But wait, don't some 240V circuits have a neutral too, like my electric stove and clothes dryer?"
A stove or dryer circuit is not just a 240V circuit. It's a 240/120V circuit. There are some components that run on 120V. The light bulb inside the oven, for example, runs on 120V, while the heating elements run on 240V. On a dryer, the motor that spins the drum runs on 120V, while the heating elements run on 240V. So, again, neutrals are ONLY for 120V, NOT 240V.
Even though a multiwire branch circuit supplies 120V, it works just like a 240/120V circuit. Again using an electric stove as an example:
Let’s say you turn the oven on, and 20A starts flowing through the heating elements. Since the heating elements are 240V, you have 20A flowing up one hot leg, and 20A flowing down the other hot leg. There is NO current flowing through the neutral; like I said, we don’t even need a neutral for 240V.
So we have:
Hot leg: 20 A
Other hot leg: 20 A
Neutral: 0 A
Then you open the oven door and the light inside turns on. Let’s say the light uses 1 A of current. So that 1A flows up one of the hot legs (doesn’t matter which one) and flows back down through the neutral. So now we have:
Hot leg: 21 A
Other hot leg: 20 A
Neutral: 1 A
The neutral is only there to carry the DIFFERENCE in current between the 2 hot legs. In the first scenario, we had no difference in current, therefore the neutral current was zero. In the 2nd scenario, we have a difference of 1A between the hot legs, so the neutral carries 1A.
Now, getting back to MWBCs: A MWBC has 2 hot legs and one neutral (just like the oven circuit). If both legs of the MWBC are carrying the exact same amount of current, there will be no current on the neutral. If you have 13A of current on one hot leg, and 7A of current on the other leg, the neutral will have 6A of current (13A – 7A = 6A: neutral carries the difference). If you shut off the 7A on the 2nd hot leg, now the neutral current jumps up to 13A (13A – 0A = 13A).
So, there is no risk of overloading the neutral. Even if you were close to maxing out the circuit: For example, say you have a 15A MWBC, and you have a lot of stuff plugged into it and both legs are using 14A each, the neutral will have little to no current on it. If one leg of the circuit is almost maxed out (say, ONE leg has 14A) and the other has no current on it, then the neutral will also be almost maxed out (the neutral will have 14A on it). But there's never a time where the neutral has MORE current than either of the hot legs.
Now, all of this works as long as you use either a double-pole circuit breaker, or two single-pole breakers that are handle-tied together. This is required by code, and here's why: This ensures that the two hot legs are actually on the two hot legs of the service (and they're 180 degrees out of phase). If both legs came from the same hot leg in the panel (and were in phase), THEN you could definitely overload the neutral. In that case, if you had 13A on one leg, and 7A on the other, you'd have 20A on the neutral. Instead of the currents on the hot legs cancelling each other out and reducing the current on the neutral, they'd just add MORE current on the neutral. That's why it's against code to use two single-pole breakers and just stick them in whichever spaces on the panel; they could both end up on the same hot leg.
engineer2
Well-known member
You may find older homes (even into the 90's) that had shared neutral circuits on separate breakers that were next to each other. It would be wise to update the breakers or buy some handle-ties.
ddawg16
Well-known member
I personally don't like MWB ckts. If the neutral comes loose, bad things can happen.
Pretty common to find them not at all next to each other, for that matter. Prior to the handle tie requirement (in 2008 NEC), there was no reason they needed to be next to each other, other than sanity.