3 phase multiwire branch circuits

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All mostly very true in respect as to what portion of the neutral you are planning on oversizing 200 or 300 % to correct this issue.

JAP>
 
Andy Delle said:
You have to consider harmonic current if said machine is a large computer or uses VFDs or large switch mode power supplies like for DC motors. It is well known in technical installations that harmonic currents can easily exceed a properly rated neutral. A neutral per phase certainly prevents this. Another allowed option is to oversize the neutral 200 or 300%.

I would consult with the machine manufacture to see if separate neutrals are advised.
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Are the harmonics likely to be that additive on an individual multiwire branch circuit that it causes much of a problem?

With feeders and services you potentially have much more additive harmonics from multiple branch circuits to cause such problems with the neutral, but still is typically only a problem in environments with a lot of non linear loads like a data center.
 
kwired said:
Are the harmonics likely to be that additive on an individual multiwire branch circuit that it causes much of a problem?

With feeders and services you potentially have much more additive harmonics from multiple branch circuits to cause such problems with the neutral, but still is typically only a problem in environments with a lot of non linear loads like a data center.
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I would say exactly the opposite. With a branch circuit serving only one non-linear load which is close to the full circuit capacity you would be more likely to have harmonic problems than for a feeder which sums up a variety of linear and less linear loads.

In partial support of what you say, I think that what often saves you on branch circuits is the "small wire rule". That results in most branch circuits having excess real world ampacity and therefore a better chance of handling additive neutral harmonic current.
 
GoldDigger said:
I would say exactly the opposite. With a branch circuit serving only one non-linear load which is close to the full circuit capacity you would be more likely to have harmonic problems than for a feeder which sums up a variety of linear and less linear loads.

In partial support of what you say, I think that what often saves you on branch circuits is the "small wire rule". That results in most branch circuits having excess real world ampacity and therefore a better chance of handling additive neutral harmonic current.
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I question how often, if ever this is actually an issue anymore, with modern power supplies and such. I believe it takes a very "dirty" power supply to result in equal or greater current on the neutral as the phase conductors (assuming equal balance and neutral same size as ungrounded).
 
electrofelon said:
I question how often, if ever this is actually an issue anymore, with modern power supplies and such. I believe it takes a very "dirty" power supply to result in equal or greater current on the neutral as the phase conductors (assuming equal balance and neutral same size as ungrounded).
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It's not a matter of the power supply being dirty as you say. It has to do with pwm and scr switching power supplies. The electronic power supplies modify the original sine wave and create odd harmonic currents i e 3rd 5th 7th Etc that do not swing in line with the original input waveform

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DrSparks said:
It's not a matter of the power supply being dirty as you say. It has to do with pwm and scr switching power supplies. The electronic power supplies modify the original sine wave and create odd harmonic currents i e 3rd 5th 7th Etc that do not swing in line with the original input waveform

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Correct, but the waveform can and is mitigated. If you look at THD figures from modern electronic ballasts, driver, power supplies, etc, they have rather low THD.
 
Jamesco said:

...a dedicated circuit as required by a manufacture of a piece of equipment that required a dedicated branch circuit?

By dedicated, I mean, a branch circuit from an electrical panel with a, dedicated, Hot conductor, neutral conductor, and insulated equipment grounding conductor. (NO shared neutral) I even had a few manufactures spec the dedicated branch circuit must be installed in it's own conduit. The dedicated branch circuit cannot share a conduit with other branch circuits. Best regards,
Jim
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I, as a simple electrician that is not a mind reader, like to think of it more like the customer/manufacturer wants a dedicated neutral and dedicated disconnecting means for each piece of equipment. Rather than think of it as a dedicated circuit, because what a non-electrician calls something isn't what I call something. I like to think that a 3 pole 20 amp breaker give 3 separate 20 amp circuits with or with out a dedicated neutral, but as long as the neutral is in play of course.

Edit should be individual branch circuit to begin with...
 
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jap said:
The only thing that would require you to pull a dedicated neutral for a dedicated circuit would be the rule that you have to follow the manufacturer's instructions.

If the Manufacturer's instruction said to pull 4 dedicated circuits to their machine, but, didn't say "with dedicated neutrals or no shared neutrals" anywhere in the literature, there would be nothing that would require dedicated neutrals or no shared neutral because a dedicated circuit is not defined.

It's simply someone's interpretation.


JAP>
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Agree!:):):)
Key words, Manufacturer, and the power requirements, specs, that is required for the piece of equipment.

In my neck of the woods a separate circuit is understood to mean it may or may not share a neutral with other circuits. The other circuit/s may or may not be separate circuits. They could be general purpose branch circuits.
The separate circuit is installed for a specified purpose that requires a separate circuit. Example, a nothing special appliance or piece of equipment that requires a separate circuit.

How about this explanation/definition for a dedicated branch circuit.

Dedicated branch circuit: A branch circuit where the conductors of the branch circuit are specifically dedicated for the purpose.
Example, the manufacturer of medical equipment that is used in a medical health care facility. Example, a piece of equipment used in the Lab of a hospital.

Example: A $650K Blood Analyzer that is leased by the Hospital from the manufacturer.

The manufacturer's power requirements.
120Vac 20 amp dedicated branch circuit.
Receptacle outlet, NEMA L5-20R

So how would you have wired the power for this equipment?
 
Jamesco said:
Agree!:):):)
Key words, Manufacturer, and the power requirements, specs, that is required for the piece of equipment.

In my neck of the woods a separate circuit is understood to mean it may or may not share a neutral with other circuits. The other circuit/s may or may not be separate circuits. They could be general purpose branch circuits.
The separate circuit is installed for a specified purpose that requires a separate circuit. Example, a nothing special appliance or piece of equipment that requires a separate circuit.

How about this explanation/definition for a dedicated branch circuit.

Dedicated branch circuit: A branch circuit where the conductors of the branch circuit are specifically dedicated for the purpose.
Example, the manufacturer of medical equipment that is used in a medical health care facility. Example, a piece of equipment used in the Lab of a hospital.

Example: A $650K Blood Analyzer that is leased by the Hospital from the manufacturer.

The manufacturer's power requirements.
120Vac 20 amp dedicated branch circuit.
Receptacle outlet, NEMA L5-20R

So how would you have wired the power for this equipment?
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When you think about it the entire service or separately derived system is one big multiwire branch circuit. Really the only issue with multiwire Branch circuits is harmonic currents

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Jamesco said:
How about this explanation/definition for a dedicated branch circuit.

Dedicated branch circuit: A branch circuit where the conductors of the branch circuit are specifically dedicated for the purpose.
Example, the manufacturer of medical equipment that is used in a medical health care facility. Example, a piece of equipment used in the Lab of a hospital.

Example: A $650K Blood Analyzer that is leased by the Hospital from the manufacturer.

The manufacturer's power requirements.
120Vac 20 amp dedicated branch circuit.
Receptacle outlet, NEMA L5-20R

So how would you have wired the power for this equipment?
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Since the term is undefined by the NEC you're free to make up your own definition. If the manufacturer wants a separate neutral then they should say so. Being that it's undefined by Article 100 of the NEC then the closest thing you're going to get is to require a Branch Circuit, Individual which can be part of a MWBC.
 
infinity said:
Since the term is undefined by the NEC you're free to make up your own definition. If the manufacturer wants a separate neutral then they should say so. Being that it's undefined by Article 100 of the NEC then the closest thing you're going to get is to require a Branch Circuit, Individual which can be part of a MWBC.
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"This code is not intended as a design specification or an instruction manual"


.
 
infinity said:
I'm confused, what does that have to do with the post that you quoted?
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I think his point is that not everything is defined or prescribed in the NEC. Designers and installers need to have some basic understanding of electrical engineering principles.

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GoldDigger said:
I would say exactly the opposite. With a branch circuit serving only one non-linear load which is close to the full circuit capacity you would be more likely to have harmonic problems than for a feeder which sums up a variety of linear and less linear loads.

In partial support of what you say, I think that what often saves you on branch circuits is the "small wire rule". That results in most branch circuits having excess real world ampacity and therefore a better chance of handling additive neutral harmonic current.
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Small wire rule probably is a reason there isn't so much troubles on MWBC's.

Those harmonics are on the ungrounded conductors as well, they just have additive effects in the neutral conductor and are worse in a wye system with added effects from three ungrounded conductors then they are in say 120/240 single phase. 120 instead of 180 phase angle might not help either, not fully certain though.

I still believe that places like data centers are the kind of places where there is the most concern for services and feeders, because of the majority of the load being non linear. More industrial loading like large VFD's are not operating line to neutral, so even though they still have similar harmonics, they are not typically adding up on the neutral like a large data center with most loads being 120 volt and non linear.
 
kwired said:
Those harmonics are on the ungrounded conductors as well, they just have additive effects in the neutral conductor and are worse in a wye system with added effects from three ungrounded conductors then they are in say 120/240 single phase. 120 instead of 180 phase angle might not help either, not fully certain though.
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Unless I'm mistake, the neutral harmonics are only an issue with 3ph wye systems.
 
LarryFine said:
Unless I'm mistake, the neutral harmonics are only an issue with 3ph wye systems.
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My understanding is they exist on all non linear loads - but are only a problem with conductor ampacity issues in neutrals of 3 phase wye systems because they have additive effects on the neutral in those systems, but only when there are line to neutral loads, if you had nothing but line to line loads (like VFD's) there is no neutral currents and no additive effects on what isn't there to begin with.
 
kwired said:
I still believe that places like data centers are the kind of places where there is the most concern for services and feeders, because of the majority of the load being non linear.
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For the service, I wonder if the Air conditioning load would make the non linear load not the majority of load?

More industrial loading like large VFD's are not operating line to neutral, so even though they still have similar harmonics, they are not typically adding up on the neutral like a large data center with most loads being 120 volt and non linear.
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I admit I know nothing about large data centers and the equipment that supplies the servers, but I question whether they are supplied line to neutral. I recently ran some new circuits for a few servers racks, and all the equipment power supplies ran on 208. I suppose the large scale stuff could take 277. Still though, I question whether this is an issue anymore. Every time I have measured electronic power supplies on a MWBC, The neutral current was only a few amps above expected. I havent seen anything come anywhere close to causing neutyral current to equal phase current, let alone exceed it, and I have never heard of anyone that has.
 
kwired said:
My understanding is they exist on all non linear loads - but are only a problem with conductor ampacity issues in neutrals of 3 phase wye systems because they have additive effects on the neutral in those systems, but only when there are line to neutral loads, if you had nothing but line to line loads (like VFD's) there is no neutral currents and no additive effects on what isn't there to begin with.
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Correct. Third-order (odd-numbered) harmonic currents add on a shared 3ph neutral.
 
electrofelon said:
For the service, I wonder if the Air conditioning load would make the non linear load not the majority of load?



I admit I know nothing about large data centers and the equipment that supplies the servers, but I question whether they are supplied line to neutral. I recently ran some new circuits for a few servers racks, and all the equipment power supplies ran on 208. I suppose the large scale stuff could take 277. Still though, I question whether this is an issue anymore. Every time I have measured electronic power supplies on a MWBC, The neutral current was only a few amps above expected. I havent seen anything come anywhere close to causing neutyral current to equal phase current, let alone exceed it, and I have never heard of anyone that has.
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I have no experience in a real data center either. I can see such equipment easily being rated for 200 - 277 or even 120-277 volts though. 120 volt only equipment may have been common several years ago though.
 
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