multiwire circut question

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Old 12-15-10, 01:29 PM
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multiwire circut question

if it is 208 volt, and you are only using 2 phases for a multiwire circut, do they still cancel?
 

Last edited by Justin Smith; 12-15-10 at 01:31 PM. Reason: misworded
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  #2  
Old 12-15-10, 01:42 PM
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Originally Posted by Justin Smith View Post
if it is 208 volt, and you are only using 2 phases for a multiwire circut, do they still cancel?
Not completely; the math can get a little tricky with three-phase shared neutral circuits.
 
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Old 12-15-10, 03:29 PM
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If I recall correctly I believe this is the formula to figure out neutral current on a three phase multiwire circuit:

Take the square root of: (A+B+C)-(AB)-(BC)-(AC)

The letters = the current on each phase.
 
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Old 12-15-10, 04:55 PM
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thanks, guys! will the total balanced load still be lower than the load on a phase wire?
 
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Old 12-15-10, 06:23 PM
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I created this into it own thread so as not to get too off topic on the other thread.

If you have the same load on two phases of a three phase multiwire circuit the neutral current will still be the same as the load on one of the phases. See below:

Phase A and phase C both have 10 amps on it. Using the above formula:
(10+0+10)-(10x0)-(0x10)-(10x10)=
(100+0+100)-(0)-(0)-(100)=
200-100=
100
square root of 100 = 10 amps on the neutral wire

I did the same calculation using 10 for A and 5 for B and you would have 8.66 amps on the neutral.

Just for fun here is a totally balanced three phase load:
(8+8+8)-(8x8)-(8x8)-(8x8)
(64+64+64)-64-64-64
192-64-64-64
128-64-64
64-64=0
 
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Old 12-15-10, 09:05 PM
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...unless the three-phase circuit powers some combination of harmonic loads like computer power supplies or some types of industrial equipment, then the neutral current can actually exceed the hot currents. So it's hard to give a rule that holds true for all shared neutral three-phase circuits.
 
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Old 12-15-10, 10:14 PM
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Yep, harmonics generated by switching-type power supplies can cause neutral currents to soar. That is why special transformers (K rated) are often specified when a significant percent of the load will be switching power supplies or often discharge-type lighting. These can allow up to 200% neutral currents. K-rated panelboards are also available.
 
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Old 12-16-10, 01:43 PM
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so, i am guessing multiwire circuts are not a good idea on 3 phase.
 
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Old 12-16-10, 01:48 PM
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They're still a good idea, just require some additional design considerations. MWBC saves conduit fill space and copper installation expense. Pretty much every circuit in a three-phase building is going to share the neutral to some degree.
 
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Old 12-16-10, 02:43 PM
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what about receptacle circuts?
 
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Old 12-16-10, 06:20 PM
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Originally Posted by Justin Smith View Post
what about receptacle circuts?
10 to 12 years ago it was pretty common to pick up a set of plans where the engineer would specify K rated transformers, 200% neutral bars and super neutrals on multi wire branch circuits. That would generally mean using a #10 shared neutral along with 3 #12 phase conductors on receptacle circuits. I haven't seen it so much in the last 7 or 8 years.
 
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Old 12-16-10, 07:22 PM
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Over the 10 years I have been in the trade I have yet seen any evidence of overloaded neutrals on multiwire circuits. The only things I have seen is furniture systems (cubes) with #10 neutrals or individual neutrals for each phase conductors. We, of course, just hook them up to a 4 wire #12 multiwire circuit we pulled in. Again, no issues. Makes you wonder.
 
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Old 12-17-10, 05:13 PM
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Originally Posted by Tolyn Ironhand View Post
Over the 10 years I have been in the trade I have yet seen any evidence of overloaded neutrals on multiwire circuits. The only things I have seen is furniture systems (cubes) with #10 neutrals or individual neutrals for each phase conductors. We, of course, just hook them up to a 4 wire #12 multiwire circuit we pulled in. Again, no issues. Makes you wonder.
Yep, dedicated neutral conductors was another favorite of some engineers and furniture manufacturers. The furniture sales people would argue long and hard that the customer HAD TO HAVE dedicated neutrals, but they wouldn't know what to say when you quietly inform them, in front of the customer in the same project meeting, that the neutrals all go back to the same neutral bar in the panel.
 
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Old 12-17-10, 06:36 PM
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I typed in the equation using 50amps on each phase, and they cancelled.
 
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Old 12-18-10, 02:50 PM
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Originally Posted by Justin Smith View Post
I typed in the equation using 50amps on each phase, and they cancelled.
Yup. When you have the same load on each phase the neutral current is zero.
 
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