Separate Neutrals

[Alwayslearningelec]

Had someone recently tell me whether or not you can share neutrals doesn't make much of a difference in terms of amount of wire/cable needed.

To me that makes no sense. If you must run separate neutrals for all your branch, you wind up with a lot more copper. At least in my experience.

 

[roger]

You are correct.

 

[ptonsparky]

Total cost may not be much different if you include the common trip issue, identifying the circuit neutrals, etc.

 

[roger]

Total cost may not be much different if you include the common trip issue, identifying the circuit neutrals, etc.

I know you know this but, common trip is not required, only simultaneous disconnecting.

 

[infinity]

Had someone recently tell me whether or not you can share neutrals doesn't make much of a difference in terms of amount of wire/cable needed.

Hmm,
Two extra wires for every 3 circuits = 50% more copper and
50% more wire markers.
Larger conduits for extra neutrals.
Two extra terminations at each end for the separate neutrals.
Longer set up time for pulling extra neutral conductors.
Multiple conductors in raceways would require numbering so no savings there.

 

[don_resqcapt19]

And additional I²R losses over the life of the installation because all of the neutral current has to run back to the panel when you have neutrals for every circuit. With a multiwire circuit, there is less current on the common neutral because it sums at the field junction point and not back at the panel.

 

[letgomywago]

And additional I²R losses over the life of the installation because all of the neutral current has to run back to the panel when you have neutrals for every circuit. With a multiwire circuit, there is less current on the common neutral because it sums at the field junction point and not back at the panel.

I like multiwires for sheds for this reason.

 

[tthh]

In most cases, the MWBC neutral gets split in two directions at some point. So, you may save a neutral, but only up to the point of it getting split into two and sent around to wherever those are going.

 

[roger]

An example of Don's post.

 

[Fred B]

An example of Don's post.

View attachment 2568801

Nice illustration.
What was assumption to get .1ohm resistance over the length of the circuit?
What was the math on diagram B for load wattages? I see the per line loss calculation but not seeing how the total watt per load is calculated?
I must be missing some piece, I get Load1 4600W, Load2 6800W.

 

[xformer]

In most cases, the MWBC neutral gets split in two directions at some point. So, you may save a neutral, but only up to the point of it getting split into two and sent around to wherever those are going.

IMHO... I also think that MWBC are frowned upon due to the types of loads they supply and due to the inexperience of the workforce repairing or augmenting them.

 

[ptonsparky]

Every FHA and custom home I wired had MWBC, until the code changed. It was SOP for all electricians in the area.

Dumbing down of the work force is the term, IIRC.

 

[infinity]

Every FHA and custom home I wired had MWBC, until the code changed. It was SOP for all electricians in the area.

Dumbing down of the work force is the term, IIRC.

We always used them for all types of construction because they saved money. The new handle-tie rules are about enhancing safety due to electricians getting hurt or killed especially with commercial buildings that have 277 volt lighting.

 

[wwhitney]

What was assumption to get .1ohm resistance over the length of the circuit?

Just a value selected to make the math easy and so that the voltage drops would end up large to show the effect. The 60A 120V circuit in diagram A has a 12V VD, which would not be a sound choice in practice.

What was the math on diagram B for load wattages? I see the per line loss calculation but not seeing how the total watt per load is calculated?
I must be missing some piece, I get Load1 4600W, Load2 6800W.

It's just P = I * V, where the loads are considered constant current loads, and the voltages are as shown. So 118V * 40 A = 4720W, etc.

There is a little subtlety in determining that 118V figure. Because the current on the lower ungrounded leg of the MWBC is higher than the current on the upper ungrounded leg, the voltage "drop" on the neutral is away from the lower ungrounded leg and thus towards the upper ungrounded leg. So Load #2 see 6V + 2V of voltage drop, but Load #1 sees 4V - 2V of voltage drop.

Cheers, Wayne

 

[ActionDave]

Every FHA and custom home I wired had MWBC, until the code changed. It was SOP for all electricians in the area.

Dumbing down of the work force is the term, IIRC.

I never knew a day of wiring without them doing both commercial work and homes till the handle tie rule.

 

[jaggedben]

Had someone recently tell me whether or not you can share neutrals doesn't make much of a difference in terms of amount of wire/cable needed.

To me that makes no sense. If you must run separate neutrals for all your branch, you wind up with a lot more copper. At least in my experience.

The difference becomes more and more real the longer the wire run.

 

[infinity]

I never knew a day of wiring without them doing both commercial work and homes till the handle tie rule.

Same here but now on commercial work the spec usually prohibits them. As much as I dislike the handle-tie rule MWBC's are dangerous when you're not thinking about the shock potential from the neutral. We had an seasoned electrician with 25 years experience get seriously hurt when he de-energized only one of the three 277 volt lighting circuits so that he could work on it. The shock from the neutral almost killed him, he ended up with burns and permanent nerve damage to his hands and arms. He never worked another day after the accident.

 

[Krusscher]

I don't really see the big deal with the handle tie rule. You are not working on these circuits enough that it should be that big of a deal if you have to shut off 1 or 2 extra circuits to work on it. I still see MWBC as a valuable option and the handle tie rule as a necessary precaution.

 

[electrofelon]

I know you know this but, common trip is not required, only simultaneous disconnecting.

And for 3 phase full boats, three single poles with a handle tie is quite a bit cheaper than a three pole breaker.

 

[Rick 0920]

These circuits were safe when "qualified" persons worked on them. The difference is educating our apprentices and journeymen. We need to take the time to teach them to recognize the obvious signs of these circuits. It only took me one time of installing a lighting circuit on "B" phase of a 120/240 open delta. The fluorescent fixtures worked great for a very short time. Then it was followed by an awful smell of fried ballasts! The biggest danger I see of MWBC's is the open neutral. Then you have a 240v series circuit. Depending on the loads and where the open occurs, you could have 50v on 1 circuit, which is not a problem. Equipment will simply not work. But the voltages in series are dropped across each load. So you'll have 190v on the other circuit. OUCH! I agree that the handle ties makes things safer for our lesser educated tradesmen, but nothing beats good education. Experience is a horrible teacher. It gives the test first before it delivers the lesson!