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Old 01-15-2009, 05:38 AM   #1
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"There may be instances where multiwire branch circuits should not be used. The Fine Print Note (FPN) of 210.4 warns that a "shared neutral" or multiwire branch circuit may not be advisable when nonlinear loads such as computer loads are encountered. Two-wire branch circuits would be better than multiwire branch circuits where harmonic currents are present."

Found that on the iaei site. What kind of concerns are there with computers on MWBC?

Jamie

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Old 01-15-2009, 07:20 AM   #2
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The differences between the equipment can be a problem when they are connected together.

I don't know the "exact" details.

For instance, you connect your pc to leg one and your crt monitor to leg 2, your monitor may have interference.

I see it worse with sound systems, where the audio is terrible.

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Old 01-15-2009, 08:23 AM   #3
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That particular note concerns nonlinear loads. A linear load has the voltage and current wave forms in phase with each other. Capacitive and inductive loads cause a phase shift and drive the two waves apart. Smaller higher frequency waves can be generated on top of the regular current. These are called harmonics. The odd harmonics are additive, which means that on a MWBC there can be a larger current flowing in the neutral than in either of the two hots. There is some debate about whether this actually is a problem.

One study I read showed how a balanced load of HID lights on a 3-phase MWBC could overload the neutral. There was approximately 100 A on each hot, but 140 A on the neutral, even though they were balanced. This has led some designers to spec "super neutrals", which are oversized compared to the hots, or to call for each circuit to have it's own neutral.

Bottom line Jamie, unless you are running a server bank or illegal casino, don't worry about it.

p.s.: this is also the reason a neutral must be counted as a current carrying conductor on discharge lighting MWBCs.
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Old 01-15-2009, 12:19 PM   #4
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Originally Posted by InPhase277 View Post
That particular note concerns nonlinear loads. A linear load has the voltage and current wave forms in phase with each other. Capacitive and inductive loads cause a phase shift and drive the two waves apart. Smaller higher frequency waves can be generated on top of the regular current. These are called harmonics. The odd harmonics are additive, which means that on a MWBC there can be a larger current flowing in the neutral than in either of the two hots. There is some debate about whether this actually is a problem.

One study I read showed how a balanced load of HID lights on a 3-phase MWBC could overload the neutral. There was approximately 100 A on each hot, but 140 A on the neutral, even though they were balanced. This has led some designers to spec "super neutrals", which are oversized compared to the hots, or to call for each circuit to have it's own neutral.

Bottom line Jamie, unless you are running a server bank or illegal casino, don't worry about it.

p.s.: this is also the reason a neutral must be counted as a current carrying conductor on discharge lighting MWBCs.
Thanks for the information. No big server banks or casinos at my parents (Just a couple of macs). I'll make sure the computers don't share a MWBC with the microwave or anything else with a heavy draw like that.

Thanks
Jamie
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Old 01-15-2009, 02:58 PM   #5
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Hmmm... you've got me thinking about a situation I have.
My washer receptacle in the laundry rm is on an MWBC with a receptacle in the dinette where we normally plug a 4-slice toaster, which can draw up to 17A!
The two circuits are on opposite legs, but I'm wondering whether the washer could be considered a non-linear load, due to the heavy motor.

This would be very easy to rewire, and I was thinking about doing so anyway.

p.s. Jamie;
I apologize if I have hijacked your thread. It's just that my situation seems realted to your original post.
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Old 01-15-2009, 07:48 PM   #6
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Originally Posted by KE2KB View Post
Hmmm... you've got me thinking about a situation I have.
My washer receptacle in the laundry rm is on an MWBC with a receptacle in the dinette where we normally plug a 4-slice toaster, which can draw up to 17A!
The two circuits are on opposite legs, but I'm wondering whether the washer could be considered a non-linear load, due to the heavy motor.

This would be very easy to rewire, and I was thinking about doing so anyway.

p.s. Jamie;
I apologize if I have hijacked your thread. It's just that my situation seems realted to your original post.

You shouldn't have any problems with this set up. I am by no means an expert on harmonic loads, but briefly stating, the only times you need to consider them is when you have several motors (such as in a computer) where several different frequencies come in to play. These different frequencies cause the grounded neutral conductor to work harder causing excessive heat within the conductor. A toaster, a laundry machine - don't sweat it.
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Old 01-15-2009, 08:08 PM   #7
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I thought the problem with computers is the switching power supply, which pulls pulses of current and so the computer somehow looks a load with Power Factor not equal to, or even close to, 1.0.

http://en.wikipedia.org/wiki/Switched-mode_power_supply

The non-linear refers to the diodes used in the supply.


Last edited by Yoyizit; 01-15-2009 at 08:19 PM.
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