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-   -   Use a shared nuetral or not? That is the question. (http://www.diychatroom.com/f18/use-shared-nuetral-not-question-21346/)

 Cyndi 05-22-2008 03:59 PM

Use a shared nuetral or not? That is the question.

I am pulling out #12 wire, two circuits with a shared neutral and replacing with #10 wire. What are the benefits of keeping a shared neutral verses running each circuit with its own neutral?

It seems to me that when you run a shared neutral, it limits you to the usage because you have to make sure the loads are always balanced on the two circuits, you have to make sure that the two circuits are on opposite legs of the panel and use a common trip.

Would it be less complicated to run each circuit with its own neutral so that you don't have to be concerned about the loads??

What is the main reason that two circuits are run with a shared neutral..is it to save on wire costs or utility costs??

Thanks. Cyndi

 Termite 05-22-2008 04:21 PM

What you're talking about is called a multiwire branch circuit, and what you've said is dead-on. Having the hot wires on opposite phases is critical. Balancing loads isn't critical because a pair of properly phased multiwire branch circuits act as two separate circuits, although they share the neutral...As long as the loads are within the limits of the circuit I think you're find.

Electricians run multiwire branch circuits to save on wire (for 3 wire you're only paying for 1 extra wire over 2 wire) costs and time it takes to pull two runs instead of one. It simplifies things in a lot of cases...Some people make it complicated though, and for them two separate 2 wire circuits are often best! It will not affect utility costs in any way.

 Cyndi 05-22-2008 04:55 PM

The following info was really hard for me to get. Since I have two circuits one of which is 170 feet and the other is 390 feet does the following apply where I need to use a shared neutral or would it be better to run a separate neutral for each circuit in the same conduit. By reading this reply to an earlier post...it looks like there are other benefits to running a shared neutral.

Ok Cyndi

Let me see if I can explain this easily. A shared neutral multiwire is just like a 240 volt circuit with no neutral when considering voltage drop....if for example you have a load on each hot wire that is the same in amps and both loads are at the exact same distance from the panel. This is the ideal situation as only the one way distance is calculated in the voltage drop because there is no return current on the neutral or at least very little.
So lets say I had two of your 1200 watt 120 volt heaters at 300 feet in the same water trough. Each heater plugged into gfci receptacles served by the hots wires of the multiwire. If I use a voltage drop calculator it assumes the one way distance of 300 feet to be one half the distance round trip for 120 volts So it calculates the distance to be 600 feet for 120 volts.

The multiwire is serving two 10 amp balanced loads at 300 feet not 600 because current is not returning to the panel over the neutral. So when I enter this into a calculator using 120 volts 10 amps I set the distance to 150 feet so that it will calculate 300 feet not 600. This gives me a drop of 6 volts or 114 volts at each heater if we have 120 volts at the panel. This is a 5% voltage drop. You don't want much more than that on resistive heater loads.

Now in contrast lets just run one heater a 1200 watts (10 amps) on 120 volts 300 feet. When I calculate this I will get a result from a round trip of 600 feet because all the current is traveling on the neutral back to the panel. That will be 12 volts of drop or 108 volts at the heater 10% voltage drop. Notice this is double the drop of a multiwire branch circuit with balanced loads at the same distance.

Now it gets a little trickier when we vary the loads and the loads aren't at the same distances but the voltage drop advantages are still there balanced loads or not.

If however no load is operating on one leg..... for instance lets say you unplug one of those heaters on the multiwire example I gave.. you now have lost all the advantages of the multiwire because you now just have one 120 volt circuit operating and all its current is returning to the panel and source so your at 600 feet of voltage drop instead of 300. The advantage in voltage drop comes when cancellation of the neutral current is occurring when both legs of the multiwire are operating loads.

So if you have multiwires to those heaters in the pasture be sure to have both legs operating something like the heaters and the thermostats set the same. Balance the loads between legs best you can.

 Speedy Petey 05-22-2008 06:20 PM

Use the MWBC. There is not that much to think about and they can and do lessen voltage drop.
The ONLY thing to remember is to keep the breakers on different legs. If you use a 2-pole breaker (now required if you are under the 2008 NEC) this is a non-issue.

Balancing the loads on the circuits is also a total non-issue.

We use MWBC's for more than just cost savings. There are other more important reasons as well.

 Cyndi 05-22-2008 06:39 PM

Thank you for the info Speedy Petey! Also, do both circuits need to be #10 wire or can I run #12 for the 170 foot run and #10 for the 370 foot run with the shared neutral??? The only reason I ask is because I have plenty of #12 wire and need to buy all of the #10 wire as needed. I just need to know how much to put on the shopping list !!!! In other words...can the #10 share the same neutral with the #12 wire or should I run all #10 for these two long runs??

Cyndi

 Speedy Petey 05-22-2008 06:53 PM

I would do #10 all the way. 170' is far enough to justify it.

 Cyndi 05-22-2008 06:59 PM

Thanks again!!! Cyndi

 Stubbie 05-23-2008 01:59 AM

1 Attachment(s)
Hello Cyndi

I see my earlier reply was rather confusing. So let me say that the multiwire in #10 for your distances is much better than two circuits and separate neutrals for voltage drop. In my example earlier with both loads balanced and set at the same distance from the panel you will have exactly 50% less voltage drop than if you ran two circuits with two neutrals to those same loads. Lets say the distance one way is 200 feet and the two 120 volt loads are 10 amps. And the voltage drop is 5% considering separate circuits utilizing two neutrals. If I serve those two loads on a multiwire with shared neutral my voltage drop will be 2.5% a 50% reduction because of balanced loads on both legs (at the same distance) and total cancellation of return current for the entire distance back to the source on the shared neutral. This screnario allows for calculating the voltage drop only one way and not round trip as you would with two circuits and individual neutrals.

Look below at this graphic from Mike Holt........ Multiwires can reduce circuit voltage drop by as much as 50%. Look at 210.4, 300.13(B), and 408.41 for other requirements when installing multiwire branch circuits.

In the graphic notice you have four duplex receptacles two of them are on one leg (red) and the other two are on the opposite leg (black). Notice also that the distances are not the same... the duplexes on the red leg are closer to the panel than those on the black leg. Now lets plug an 8 amp 120 volt heater into the farthest duplex on the black leg and also an 8 amp heater into the farthest receptacle on the red leg. Lets also say the distance to the receptacle on the black leg is 200 feet and the receptacle on the red leg is 100 feet from the panel. The voltage drop on the conductor to the heater and on the neutral on the black leg will be calculated at 300 feet... distance to the receptacle and then back on the neutral to the receptacle on the red leg where the return current from both loads becomes common to the shared neutral and since both loads are balanced no return current returns to the source on the shared neutral after that point. So the last 100 feet is omitted from the calculation due to cancellation of all neutral current at the 300 foot point on the black leg. The voltage drop to the receptacle on the red leg is calculated at 100 feet one way not 200 feet round trip. This is due to total cancellation of neutral current returning to the source at that point in the multiwire branch circuit.

Separate circuits ran to these loads not sharing a neutral between opposite legs would be calculated at 400 feet for the black leg and 200 feet for the red leg.....to the load and all the way back to the source.

Multiwires cancel return current and if perfectly balanced all return current will cancel at some point in the branch circuit. When loads are not balanced the calculation becomes rather complicated.

But as Speedy said don't concern yourself with balanced load....just know that multiwires will improve voltage drop regardless. It is just that when properly designed those advantages can be fully utilized.

 jrclen 05-23-2008 09:46 AM

I was wondering how this project was coming. Hello all. :thumbsup:

 Cyndi 05-23-2008 11:18 AM

Good Morning Stubbie!!!

I get it now. Thanks a million!!! The project is going slower than I had hoped but since the inspector was out, I have been pretty sick and am still recovering. It's all good though. :)

I have also been delaying somewhat until you are not so busy because I am sure that I will need your input!! Hope that's o.k.

I will have some help pulling the #2 wire this weekend and then the rest will fall into place....I hope!!!

I am so glad to see you post again. I miss you when you are not there!!!!!

Cyndi

 Cyndi 05-23-2008 11:32 AM

I read somewhere in this forum that some GFCI's don't work on MWBC's. I had planned on putting GFCI receptacles at each location on the MWBC's that run out to all of the pastures.

Would it be better to install GFCI breakers in the panel for these circuits or should I proceed as planned??

 Stubbie 05-23-2008 11:34 AM

I am incredibly busy at the present time but I check in almost every day. Seems the forum runs without missing a beat whether I'm here or not....:)

I'm also glad someone misses me.........:thumbsup:

 jbfan 05-23-2008 11:39 AM

Quote:
 Originally Posted by Cyndi (Post 125317) I read somewhere in this forum that some GFCI's don't work on MWBC's. I had planned on putting GFCI receptacles at each location on the MWBC's that run out to all of the pastures. Would it be better to install GFCI breakers in the panel for these circuits or should I proceed as planned??
No. Use the receptacles. When you wire the gfcis to the line side, then continue to the next gfci, you no longer have a mwbc.
This is done all the time.

You would really hate being 370 feet from the panel and have to walk back to reset the breaker.

 Cyndi 05-23-2008 11:49 AM

jbfan-

Just for clarification.....are you saying that when I install GFCI's at each location on MWBC's I will no longer have MWBC's or if I put GFCI breakers for the MWBC's I will no longer have MWBC's?

Also, like I said.......someone on this forum stated that Some GFCI's don't work on MWBC's. Is there a certain type that I should get in exchange for the standard ones that I already purchased??

 Stubbie 05-23-2008 11:57 AM

1 Attachment(s)
Hi Cyndi

Here are your options I really have no preference. I would gfci with a double pole breaker. Only disadvantage is if it nuisance trips it is a long way back to the panel to reset it. I usually wire in an indicator light somewhere so you can monitor the condition of the multiwire circuit at the house or barn. If the light is on gfci is not tripped.... if off gfci is tripped.

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