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Old 11-16-2008, 12:05 AM   #46
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Conduit Fill and Derating


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Originally Posted by jamiedolan View Post
So does this in effect mean that 2 -20A circuits only really have the capicity for 20A total and not 20A each for a total of 40A? I am confused about how the double pole breaker ensures that the neutral is not overloaded.

I must be missing something.

Jamie

This is what the transformer looks like as a diagram

The red and black connecting to the secendary are your two hots

They share a wire (neutral) going back to the center of the tranformer.

When these two potentials are balanced on the neutral, it is not overloaded.

The neutrals purpose is to balance out the circuit. With two different potentials, such as the red and black, you do not overload the conductor.

The red goes to one leg in the panel, the black goes to the other.
7.2kv is the poco's input at a default of 7200 volts.
The double pole makes sure that the hots stay on two different legs, so you don't overload.
Single poles cannot be used because if you turned off only one breaker, half of the circuit will still be live.
Lets say a fault occured, like above, half of the circuit will still be live.
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Conduit Fill and Derating-xformer-copy.jpg  


Last edited by rgsgww; 11-16-2008 at 12:13 AM.
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Old 11-16-2008, 04:09 AM   #47
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Conduit Fill and Derating


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Originally Posted by jamiedolan View Post
So does this in effect mean that 2 -20A circuits only really have the capicity for 20A total and not 20A each for a total of 40A? I am confused about how the double pole breaker ensures that the neutral is not overloaded.

I must be missing something.

Jamie
Yes I think so. Lets call the two sides of the MWBC (a) and (b).

Lets say that side (a) is has 5 amps load, and side (b) has 15 amps load.

The current flowing through the neutral will be 10 amps.

This scenario is using 20 amps at 120v or 2400 watts.

Now consider a balanced scenario. 10 amps per side. Neutral carries zero current. You are using 20 amps at 120v or 2400 watts.

The tricky stuff come in if the neutral is broken for any reason. Then what was a MWBC becomes a series circuit, and you have what amounts to loads in series across 240v circuit. This is a bad situation and you'll have 120v devices with higher and lower voltages than normal. Or even worse, if the phases are perfectly balanced, you might not notice it for a while, then you turn on an appliance and your TV explodes. It can really look like a haunted house!



Oh I realized I didn't actually fully answer your question. Because a 2 pole breaker is adjacent slots, it is always pulling from both phases. If you were to use two single pole breakers, and you skipped a slot, then they would both be on the same phase. Instead of currently flowing from (a) to (b), it would add up. so 10 on (a) and 20 on (b) becomes 30 on the neutral if you accidentally pulled from the same side.

Last edited by Gigs; 11-16-2008 at 04:52 AM.
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Old 11-16-2008, 06:19 AM   #48
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Conduit Fill and Derating


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The tricky stuff come in if the neutral is broken for any reason. Then what was a MWBC becomes a series circuit, and you have what amounts to loads in series across 240v circuit. This is a bad situation and you'll have 120v devices with higher and lower voltages than normal. Or even worse, if the phases are perfectly balanced, you might not notice it for a while, then you turn on an appliance and your TV explodes. It can really look like a haunted house!
Thanks for the explaination. An open neutral on a MWBC isn't really any different than an open neutral on a regular circuit is it? Because at the receptacle of a MWBC you still only have one hot and one neutral, so you would never get a 240v connection, just a regular open neutral condition such as can happen with any circuit that has an open neutral.

Correct?

Jamie
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Old 11-16-2008, 07:12 AM   #49
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Conduit Fill and Derating


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Originally Posted by jamiedolan View Post
Thanks for the explaination. An open neutral on a MWBC isn't really any different than an open neutral on a regular circuit is it? Because at the receptacle of a MWBC you still only have one hot and one neutral, so you would never get a 240v connection, just a regular open neutral condition such as can happen with any circuit that has an open neutral.

Correct?

Jamie

Not correct, an open neutral on a MWBC will automatically create a seriese 240v circuit. This can only happen on multiwire branch circuits, and just so you know, your service coming into your home is basically a multi wire branch circuit.

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Old 11-16-2008, 07:16 AM   #50
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Conduit Fill and Derating


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Originally Posted by jamiedolan View Post
I am confused about how the double pole breaker ensures that the neutral is not overloaded.

I must be missing something.

Jamie
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Old 11-16-2008, 07:20 AM   #51
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Conduit Fill and Derating


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Not correct, an open neutral on a MWBC will automatically create a seriese 240v circuit. This can only happen on multiwire branch circuits, and just so you know, your service coming into your home is basically a multi wire branch circuit.
So the danger is if the neutral between the 2 circuits were to be intact, but the neutral to the panel gets disconnected. Then the neutral between the 2 devices bridges the 2 legs if it isn't connected to the panel.

But if the neutral were severed at a point where it cut the path between the 2 circuits, and you just had a hot wire each going directly to an outlet and there was no in series neutral, then there could be no series circuit created.?

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Old 11-16-2008, 07:25 AM   #52
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Conduit Fill and Derating


I used double pole breakers. I just made the change about 20 minutes ago. Thanks to my obsessive labeling of each and every wire, it only took about 5 minutes to change it over to 2 MWBC. I just put a meter on one of the circuits and read 120v.

So if I put a 10A load on each "leg" of my MWBC and out a clamp meter on the neutral wire, I should see a very low load, or technically none if the circuits were perfectly balanced with there load.

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Old 11-16-2008, 12:34 PM   #53
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Conduit Fill and Derating


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So if I put a 10A load on each "leg" of my MWBC and out a clamp meter on the neutral wire, I should see a very low load, or technically none if the circuits were perfectly balanced with there load.

Jamie
Yes, it will never overload the neutral.

Between the two "legs" you should get 240 and from leg to neutral 120. That will tell you if its done right.
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Old 11-16-2008, 06:42 PM   #54
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Conduit Fill and Derating


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So the danger is if the neutral between the 2 circuits were to be intact, but the neutral to the panel gets disconnected. Then the neutral between the 2 devices bridges the 2 legs if it isn't connected to the panel.
I think you got it now, it turns into a series circuit and the relative resistance of the two circuits determine what voltage each side "sees".

This can't happen with regular circuits unless the main service neutral at the panel became disconnected, which is less likely.

So these are the two main risks, a broken neutral leading a series circuit, and incorrect feeding from the same phase causing an overloaded neutral. Since you used double pole breakers you have eliminated the possibility of the latter risk, but the former risk is something to be careful of.
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Old 11-16-2008, 08:42 PM   #55
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Conduit Fill and Derating


Jamie

In your situation the only place your neutral is shared is from the junction box to the circuit breaker panel. At the junction box you essentially split it into four 120 volt circuits that do not share the neutral. A 240 volt circuit can only occur if you open the 'shared' neutral at the junction box or breaker panel. If the neutral opens on the two 120 volt loops you will only lose that branch of the split multi wire.. the other branch will work normally. For example one advantage this configuration allows is gfci receptacles to be installed where ever you want them because the neutral is not shared after the junction box. the simple diagram below shows your neutrals entering the JB and joined with the shared neutral to the circuit breakaer box. If a double pole breaker is used then two neutrals will be on leg A and two on Leg B and will cancel each other as shown 20A - 18A = 2A on the shared neutral. The neutrals after they leave the JB will carry the full return amperage of there particular hot wire just like any 2 wire (H-N)120 volt circuit.
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Conduit Fill and Derating-split-multiwire-no-gfci.jpg   Conduit Fill and Derating-mwbc-neutrals.jpg  

Last edited by Stubbie; 11-16-2008 at 08:48 PM.
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Old 11-16-2008, 09:01 PM   #56
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Conduit Fill and Derating


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Jamie

In your situation the only place your neutral is shared is from the junction box to the circuit breaker panel. At the junction box you essentially split it into four 120 volt circuits that do not share the neutral. A 240 volt circuit can only occur if you open the 'shared' neutral at the junction box or breaker panel. If the neutral opens on the two 120 volt loops you will only lose that branch of the split multi wire.. the other branch will work normally. For example one advantage this configuration allows is gfci receptacles to be installed where ever you want them because the neutral is not shared after the junction box. the simple diagram below shows your neutrals entering the JB and joined with the shared neutral to the circuit breakaer box. If a double pole breaker is used then two neutrals will be on leg A and two on Leg B and will cancel each other as shown 20A - 18A = 2A on the shared neutral. The neutrals after they leave the JB will carry the full return amperage of there particular hot wire just like any 2 wire (H-N)120 volt circuit.
Sounds like I got it right. Thank You for your help and for the diagrams. Here are the updated photos. I am going to pull some thhn for that 2nd box soon, I just have to figure out if one of the circuits in there is suppose to be 15 or 20A ( in this house the feeder cable into the panel is not necessarly the same gage as the rest of the circuit ).

Conduit Fill and Derating-img_4510.jpg

Conduit Fill and Derating-img_4512.jpg

Conduit Fill and Derating-img_4513.jpg

Conduit Fill and Derating-img_4514.jpg

Conduit Fill and Derating-img_4515.jpg

Conduit Fill and Derating-img_4517.jpg

Thanks again!
Jamie
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Old 11-16-2008, 09:05 PM   #57
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Conduit Fill and Derating


Blue = 15 Amp Circuits
Yellow = 20 Amp Circuits
Green = Ground
White = Neutral

All of the Blue and Yellow are MWBC, I may run in some different colors for regular circuits (non MWBC). Althought I plan to make most of my new runs MWBC to there junction boxes in the atic that EMT is going to run to. Lots of EMT to use up, I bought 35 pieces last night and already had over a dozen. Yikes!

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Old 11-16-2008, 09:37 PM   #58
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I didn't know you could use any color you want for hots, but I looked it up and it looks like it's OK. Don't let a european electrician in there though or he might zap himself!
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Old 11-16-2008, 10:02 PM   #59
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Conduit Fill and Derating


Here is a diagram more closely resembling what you have but I think you have it figured out.

Also I can't remember if you said you removed the metal strap connecting the neutral bars together. You need to do that then reinstall the bonding jumper to the ground bar that the feeder insulated green equipment ground is connected to.
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Last edited by Stubbie; 11-16-2008 at 10:18 PM.
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Old 11-16-2008, 10:03 PM   #60
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I didn't know you could use any color you want for hots, but I looked it up and it looks like it's OK. Don't let a european electrician in there though or he might zap himself!
I remember reading about the colors used in other countries, but don't remember what they do differently...jamie

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