ECN Forum Forums Code Related Discussion NEC & other Code issues MULTI WIRE CIRCUITS

Merry Christmas, Folks...

The two hots on the same phase would not be good. The neutral would have the possibility of having twice the current.. i.e. 40A if on two 20A breakers... just like paralleling the circuit. I suppose one could size the neutral for 40A (#8 Thhn Cu) and have 20A Breakers, but this would prove expensive, unusual, and probably a violation of code...(help me here guys...)

The ideal is to balance the loads on the phases to cancel each other out and provide no neutral current at all. The neutral only being there for unbalanced "line to ground" loads. Hope this helps...

Doc
Look in the begining of the NEC Article 100 and look up the definition of "Branch Circuit, Multiwire." It states that a multiwire circuit consist of two or more ungrounded (hot wires) with a potential between them. this means hots that are from diffrent phases, so a circuit with both hot wires from the same phase is against the code and unsafe as Sparkey said. The neutral will carry the sum of the two circuits rather than the imbalance. In a perfect world you could have a multiwire circuit where each phase had the exact same load on it and the neutral can be lifted (remember this is just for an example.) The 110V circuits will continue to operate as if nothing is wrong. The neutral is not being used because the hots are in balance. In reality there is almost always some load on the neutral. Say one hot has 10A and the other has 14A on it. The neutral will carry 4A, the imbalance.

Dave T.

Let me try this again.....I'm new to this BB.

The facility I work at is filled w/ multiwire branch circuits. It is and was very common years ago. What we are experiencing is massive harmonic distortion as dept's decide to add computer labs where their were once broom closets.

The work they are creating for us here is endless as the neutrals are burning up because of it.

Tough to teach some of these electricians about harmonic distortion and the dangers of it. They are so use to dragging a single neutral along with the three phase conductors.

Now if I could only get them to start isolating their grounds 8^)

Great forum for these types of discussions. Lots of great postings.

I had a similar experience in a 5 story office building, computers everywhere, all anyone did was sit and work at them, rows of 'em. And all fluoresant lighting too.

The problem we also ran into were the old VT220 terminal rooms. Rows and rows of dumb terminals connecting to terminal servers for text only email......

Once the funds and technology were available the rows and rows of dumb terminals were replaced w/ high speed pentiums and nobody bothered with the existing three wire distributed circuits they were fed from. Shortly thereafter the smell of smoke and burning wires was ever present and the labs all had to be rewired.

Same thing occured with the electronic ballast retrofits all over campus.

Keeps us busy.....no complaints here.

Electrican,
Were the circuits in your last post "3-wire" or "3-phase"? My understanding is that there is no neutral overloading or harmonic problems with nonlinear loads on 120/240 single phase systems.
Don(resqcapt19)

Don,

The PC's power supply, since it is a switching power supply, causes the load to be non-linear. That not only creates a power factor - less than unity - but also brings in conserns of Harmonic Distortion.

The thing is though, even with the older Reactor core ballasts and linear power supplies [with step down transformers, bridge rectifiers and filters all running at the frequency of the power system - 60 Hz -], the problem of Harmonics was still present. Things have evolved to the point that nearly all loads in Commercial are now non-linear and have a possible >30% THD. Non-linear loads would be Reactive loads, where as linear ones would be more like Resistive loads. This is the reasoning behind why it is not allowed to derate the grounded conductor [AKA "Neutral"] on 120/240 V 1 phase 3 wire systems' feeders anymore. This would apply to 1 phase 3 wire systems derived from a single transformer, or from the 1 phase 3 wire transformer on a 4 wire Delta.

Just a little techno babble FYI

Electrician02125:

How high are these branch circuits loaded? Have you metered the RMS load current on these circuits with the barbecued grounded conductors?

Some of the older Ballasts that were "Hybrids" had as much of a Harmonic Distortion problem that some of the High Frequency Ballasts used currently do. Also, older Ballasts might not have been PF corrected, so only 1/2 as many fixtures could be on a circuit, due to twice as much line current.

I know this sounds like a load of bull, but not every non-linear load has a tremendously inheret THD problem. Mix-matching lamps and ballasts would most likely increase the THD above the Manufacturers' listed calcs.

Make sure that no one has mistakenly put two circuits from the same phase in with these multiwire branch circuits. It's not too clear what type of power system is being used for the multiwire circuits - 1 phase 3 wire [120/240 volt], or 3 phase 4 wire [208Y/120 volt low voltage, 480Y/277 volt high voltage], so if 1 phase 3 wire, the possibility of an overloaded grounded conductor might be that someone mistakenly tried balancing three lines to the common.
On the other hand, if it's 3 phase 4 wire, someone might have tried to balance "Two Phase A" circuits to the common. I only say this because I have troubleshot this one a lot when barbequed commons in sub panels turned up. The circuits had been wired this way maybe since the project was new. The loads were so low before hand that there wasn't an excessive load on the common. After several remodels and upgrades, the circuits became highly loaded - which brought up the hidded problem.


If you can, give the most delicate Computer equipment Dedicated - Isolated ground circuits. Dedicated meaning Dedicated Line and Common [two wire], plus Dedicated IG conductor, per circuit. Only having the Computer and related equipment on the circuit. Most desktop PC's that are Commercially used for business can get away with a balanced 4 wire branch, but remember to calculate the KVA loads as to their MAX draw [don't forget to figure the Laser Printers!!].

Not too sure of how many LANs you have wired [both Power and Data], so I'll leave things at this point. Feel free to reply to this thread and add some comments. If you need a few suggestions, let me know. Been doing LAN power systems for Banks for a good 15 years and have seen things change over the years. Only been doing in depth com/data cable setups for 6 years, where as previously, we would install the cable [IBM Type 2 and Triplex for 10base-2], then an IBM certified vendor would terminate cables. We did a few 10B-2 triplex LAN terminations [with soldered BNC ends, made by Amphenol].

Just a little "Retrothinking"

Scott "S.E.T."

Scott,
I didn't say there wouldn't be any harmonic distortion of the wave form due to nonlinear loads, however it is my understanding that the harmonic wave forms do not add in the 120/240 single phase systems as they do in the 208y/120 volt systems, and therefore there isn't the neutral overheating problem. Even with harmonic loads, the code still permits me to say the neutral is not a current carrying conductor in single phase systems. See 310-15(b)(4)(a-c)
The copper and transformer people really do not want the industry to understand this because if we did we'd just use 120/240 volt distribution for these loads in place of oversized neutrals and expensive "k" rated transformers.
Don(resqcpt19)

Don,

I should re-word my reply message to you, as I was attempting to add to your message with a little more baloney. If I have somehow offended you, please accept my appologies as I did not intend to insult your intelligence.

Actually, what I was planning to write at the beginning was to point out in regards to older services / feeders and sub feeders; where in the past it was OK to derate the Grounded Conductor on 1 phase 3 wire systems, due to the fact that the currents would have balanced themselves out across the Ungrounded Conductors when the Transformer was delivering it's rated KVA for multiwire circuits. This has been common in older Commercial installations - prior to circa 1985 or so - when the derating thing changed, not allowing derating for non-linear and/or high harmonic loads being the majority loads. I am unsure of the exact year that this took place, nor if it applies to Residential installations.

I am sure that you have knowledge of this and can inform of the date.

The Three Phase 4 wire systems will have the dreaded "Triplen" Harmonic frequencies to deal with, where as the Single Phase systems will deal with "Beat" frequencies - much like how a Single Phase AC Motor's rotor [Secondary] is subjected to - Double frequeny pulsations.

There is a difference in the level of THD that causes trouble between the two systems.

For a Three Phase system, if the THD is at 33% or more per Line on a Line to Common 4 wire multiwire branch, this almost always doubles the current in the common conductor.

For a Single Phase system, if the THD is at 50% or more per line on a line to neutral 3 wire multiwire branch circuit, this almost always doubles the current in the neutral conductor.

Depending on what frequency is carrying the highest distortion, there may not be an equal frequency on the other Line to balance out, so it is an unbalanced current that flows in the neutral conductor - even if both the loads appear to be equal in current value and on opposite Lines [1 phase 3 wire - 120/240 VAC].

Anyhow, this is just a bunch of techno stuff and EE stuff, so once again, please do not feel that I am trying to insult you. Quite the opposite, I am trying to add to your message.

To sum it up: The THD [Total Harmonic Distortion] would need to exceed 50% on each Line where their is a common harmonic frequency, to cause double current flow in the neutral of the 120/240 VAC 1 phase 3 wire multiwire circuit.
If the harmonic is not common to each line, then the Double Frequency Pulsation problem will make the THD 25%, instead of 50%, to cause an overloaded neutral.

The 3 phase 4 wire multiwire circuit has the problem of "triplen" harmonics, which would be 3 times the fundamental, then 3 times the triplen frequencies after that [or Hz3]. So with a fundamental frequency of 60 Hz, these would fall into 180 Hz, 540 Hz, 1,620 Hz and on. These overload the common conductor when the THD is or exceeds 33%

If anyone feels that this is incorrect, please feel free to let me know where and why, along with the text reference you have studied that base this information, so I can review that particular reference and apply it accordingly to present references [books].


Scott "S.E.T."

Scott,
I don't take offense at your posts, and often learn a lot from them. I'm not an EE, and do not have the technical background that you have, but I have never read incident reports citing problems on the grounded conductor caused by nonlinear loads on single phase 3 wire systems, only on 3 phase 4 wire systems. To the contrary, I have read a number of articles over the years that support my opinion that the cheapest and easiest way to solve the overloaded neutral problem in systems with large nonlinear loads would be to use 120/240 single phase distribution in place of the commonly used N.Y.C./120 systems. This type of installation does not maximize the profits of the manufacturers so they have no interest in supporting it and do not write articles for the trade publications about it (articles in many trade publications are written by the publications advertisers)
Please cite the code reference that prohibits neutral derating on 3 wire single phase circuits.
Don(resqcapt19)