> >
>

# multiwire circut question

#1
12-15-10, 01:29 PM
Member
Join Date: Dec 2010
Location: Cressona, Pa, USA
Posts: 2,546
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
#2
12-15-10, 01:42 PM
Forum Topic Moderator
Join Date: Feb 2005
Location: Near Lansing, Michigan
Posts: 10,296
Originally Posted by Justin Smith
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.

#3
12-15-10, 03:29 PM
Group Moderator
Join Date: Nov 2007
Location: Twin Cities, MN
Posts: 11,987
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: (A²+B²+C²)-(AB)-(BC)-(AC)

The letters = the current on each phase.

#4
12-15-10, 04:55 PM
Member
Join Date: Dec 2010
Location: Cressona, Pa, USA
Posts: 2,546
thanks, guys! will the total balanced load still be lower than the load on a phase wire?

#5
12-15-10, 06:23 PM
Group Moderator
Join Date: Nov 2007
Location: Twin Cities, MN
Posts: 11,987
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:
(10²+0²+10²)-(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:
(8²+8²+8²)-(8x8)-(8x8)-(8x8)
(64+64+64)-64-64-64
192-64-64-64
128-64-64
64-64=0

#6
12-15-10, 09:05 PM
Forum Topic Moderator
Join Date: Feb 2005
Location: Near Lansing, Michigan
Posts: 10,296
...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.

#7
12-15-10, 10:14 PM
Member
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 18,374
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.

#8
12-16-10, 01:43 PM
Member
Join Date: Dec 2010
Location: Cressona, Pa, USA
Posts: 2,546
so, i am guessing multiwire circuts are not a good idea on 3 phase.

#9
12-16-10, 01:48 PM
Forum Topic Moderator
Join Date: Feb 2005
Location: Near Lansing, Michigan
Posts: 10,296
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.

#10
12-16-10, 02:43 PM
Member
Join Date: Dec 2010
Location: Cressona, Pa, USA
Posts: 2,546

#11
12-16-10, 06:20 PM
Member
Join Date: Jan 2010
Location: USA
Posts: 9,221
Originally Posted by Justin Smith
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.

#12
12-16-10, 07:22 PM
Group Moderator
Join Date: Nov 2007
Location: Twin Cities, MN
Posts: 11,987
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.

#13
12-17-10, 05:13 PM
Member
Join Date: Jan 2010
Location: USA
Posts: 9,221
Originally Posted by Tolyn Ironhand
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.

#14
12-17-10, 06:36 PM
Member
Join Date: Dec 2010
Location: Cressona, Pa, USA
Posts: 2,546
I typed in the equation using 50amps on each phase, and they cancelled.

#15
12-18-10, 02:50 PM
Group Moderator
Join Date: Nov 2007
Location: Twin Cities, MN
Posts: 11,987
Originally Posted by Justin Smith
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.

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off