Consider a three-conductor cable (red, black, and white) that originates at two circuit breakers which are connected to opposite poles, and the neutral buss bar. One can connect a 120-volt load between the red and white wires, placing that load on one of the circuit breakers, and one can connect another 120-volt load to the black and white wires to place it on the other circuit breaker.
The breakers, of course, are sized appropriately to protect the wiring. But what about the current in the white wire? The white wire is the return for both loads. Won’t it be carrying more than its rated current?
The answer is that the neutral does not carry the sum of the two currents; it carries the difference. If both loads draw the same amount of current, the neutral will carry no current. The voltage on one “hot” wire is always of opposite polarity to the voltage on the other “hot” wire. Therefore, the current returning by way of the white wire for each circuit flows in the opposite direction to the current for the other circuit, so that the two currents subtract, and the white wire can never carry more current than one of the two “hot” wires. This technique is called a multiwire circuit.
Beware, however, of connecting to the wrong breaker. If the two breakers supplying the two “hot” wires are connected to the same pole, the voltage on both hot wires will be of the same polarity, and the current for each of the two circuits will return in the same direction through the white wire, and thus add together. The white wire may carry as much as twice the rating of the circuit so look for those browned white wires in those panels fella’s. This is a dangerous condition.
To check that a multiwire circuit is supplied correctly, measure the voltage between the two “hot” wires. If the two “hot” wires are connected to opposite poles, as they should be, there will be a difference of 240 volts between them. If they are incorrectly connected to the same pole, there will be zero (or nearly) volts between them.
There are some restrictions on the use of multiwire branch circuits, including these (2002 NEC 210.4):
- “All conductors must originate from the same panelboard.”
- If a multiwire circuit in a “dwelling unit” supplies “split-wired” receptacles, a means must be provided to disconnect both ungrounded conductors simultaneously. This can be done with a two-pole switch, but usually it is done by tying the handles of the two circuit breakers together.
*]The same simultaneous disconnection requirement also exists if the circuit supplies a mixture of 120v and 240v loads (whether or not in a “dwelling unit”), but the disconnection means must be the "branch circuit overcurrent
The biggest advantage of multiwire circuits is the lower voltage drop. This is effectively a 240v circuit that delivers 120v. That makes a difference on the other end of the house.
I agree Greg…but as you know I would never recommend a multiwire circuit because of the other potential issues.
I hear ya…I know some run this to an out building and try to get away with the 20A single branch circuit to an out building and forgo the grounding and additional requirements and so on…only to be surprised when I say…ahhh…not good because it is not a single branch circuit…I am sure you know what i mean…
The code would not agree with your assessment of a multiwire not being a “single branch circuit”. 225.30 " … For the purpose of this section, a multiwire branch circuit shall be considered a single circuit."
You can also feed 240v loads from a multiwire if you use a 2 pole breaker.
I agree they can be confusing for unqualified people but unqualified people should not be wiring in the first place.
Questions about 3-wire ckts:
When the difference in current between the two hot legs are “subtracting” in the neutral to carry just the unbalanced load, what is physically happening in the neutral? Is the “opposing” current “cancelling” the difference in the two currents? Is it opposing magnetic fields that get this done?
If the two hot legs were fed by single pole breakers, and one was opened, the remaining side would obviously now just revert to being a standard 2-wire 120V ckt?
On a 240V 4-wire dryer or range ckt…since they are really 120/240V ckts, are there any different considerations about the neutral in that ckt than a typical 3-wire? Aren’t those just using one of the hot legs to run 120V lights and maybe motors, so this would just be a 120V ckt tacked onto the 240V situation, so would the usual “subtracting” of unbalanced current happen on that neutral?
Isn’t the difference simply the sum of a positive number and a negative number?
Darn Saturday margaritas.
LOL… Funny Greg I do not take away from Art 225.30 the same understanding of it for the application I was talking about in my little shed example…while I agree it is considered 1 circuit under the article…I was more talking about a different thing…that refers to the (1) 20A GFCI circuit to a shed before needing to have a main disconnect at the shed and seperate grounding and so on…Think I went to DEEP here…
Multi-branch circuits as defined are (2) ungrounded conductors having a voltage between them and a grounded conductor…in my definition it is (2) circuits…sharing a Common grounded conductor…
So what i was saying…has to do with the allowance to be able to have a 20A circuit to a shed without having the need for a seperate grounding electrode due to having more than (1) circuit…not sure in you area but running a Multi-wire to a shed that only has 120V in it…to get (2) circuits would not be something our AHJ would allow.
He would demand a service panel and ground rod and other requirments…like 4 wire unless we could prove their was no potential between the shed and the house…thats actually what I was going for Greg…
In the range the degree out of phase situation is what causes the cancelation and so while their is actually a load on the neutral it is the unbalanced amount from the 120V usage to kinda simplify it.
Actually have a neat demo of that in my class except it has open wires and the students keep thinking they can touch it…WHILE its plugged in…guess they want to have a SHORT career in the electrical field…get it…short…thehehehhe
Anyway sorry I got off track on the multiwire circuit, yes it can reduce amps obviously if it was a 3 wire w/ ground multiwire curcuit…the only problem I have with it is you just never know what is going to be on the circuit and the good side of that is because of the degree’s out of phase issues on the wave form the neutral will never have BOTH hots sending back amps on it…unless of course the CURSE of the multi-wire setup is done…and someone puts BOTH hot legs on the same line…hmmmmI should stop calling them phases…phases are actually within transformers…I will stick to calling them lines…
Also because of the grounded conductors use in the old SE installations where not only could the grounded conductor have a potential unbalanced load from any 120V uses on the setup…it would also act as the Equipment Ground…so this is why now Ranges and Dryers use 4 wire setups…
EDIT- MY SPELLING SUCKS
Oh Russel…You know what I mean…thehehe…I think
Actually…man I actually did the drifting on this thread…lol…what started out about a split receptacle…lol…lead into my rant on a shed…GESSSHHH…thats a whole different situation…thehehehe
Its a weekend…and I am sitting here doing BIDS all day for work I know I dont want…(2) log homes…I dont like wiring Log Homes…
Here’s the multiwire explaination from a physics point of view.
Each hot leg of the circuit (if wired to alternate breakers on different legs as described) has a sine wave. Peak and valley. Each leg is 180 degrees out of phase with rach other. That means that when one leg is peaking the other is in the valley. The current is moving back and forth in the wire at 60 Hertz.
So, when the neutral is peaking from one leg it is also hitting the bottom of the valley on the other leg. If one leg is supplying 10 amp and the other is loaded at 5 amps, the neutral will only be ‘returning’ 5 amp because 5 amps of the 10 amp load is being cancelled out by the opposite sine wave of the 5 amp load leg.
If the two legs are the same (in phase) then the voltage is 240 and no neural is needed since each hot leg serves (alternately) as the neutral for the other.
Hope this helps;
Actually, in order to obtain 240V the phases MUST be out of phase of each other hence the reduction in any need a neutral…unless of course their is a potential 120V need on the 240V circuit.
The actual out of phase concept is the basis of for the whole idea because while one phase is at the PEAK the other is at the Valley…and if EQUAL amp draw on the phases the resulting grounded (neutral ) conductor is ZERO…which as William said…does peak and fall in relation to each sides draw on the phase…now thats getting complicated…lets leave it at that…
So basically being IN PHASE on a multiwire circuit becomes cumulitive versus negating…Wow…I know I spelled those words wrong…
Man…See…I love posts that make ya think about things… Ain’t Electricity Fun !
Yeah, Paul, sorry I messed that up. I knew but had something else on my mind. (Have a REAL bad cold and am not thinking straight).
With the 240, we have the addition of peak and valley and the phenomenon of RMS (Root Mean Square) to get the potential difference.
Thanks for the correction.
No problem my Brother…I knew what you were talking about…
Ok…for those wanting to know what RMS is here you go…lol…
**rms: **This is the root mean square value of an alternating current or voltage. It is the value generally displayed by an analog or digital electrical meter. For a sinusodial current or voltage the rms value is 0.707 times the zero to peak value of the sine wave. For a square wave current, the rms value is the same as the peak value.
Oh lets truly not discuss RMS…lol…I can hear HEADS popping now…
I inspected a house a couple of years ago where they tried to use this multi circuit technique (called “shared-neutral” down here). They made one serious error, however. They used a two-wire system (black, white and ground). They used the black on one circuit, the white on another circuit, but they used the bare ground as the neutral. I couldn’t believe what I was seeing - and the entire house was done this way.
The deal fell through when they called in an electrician who confirmed what I had said and told them that the whole house needed to be rewired.
Paul you may have a local AHJ who makes up a rule now and then but a multiwire circuit to a shed is one circuit in most of the US. No grounding electrode required. The GFCI only applies to the receptacle outlets so it can be accomplished in thew first ouitlet on the circuit once it hits the shed. That does mean you need the non-gfci burial depth.