Why full size neutral single phase?

[mbrooke]

I did hear of an incident in a school building one time where they ran three banks of lights in all the classrooms, Line A supplied the front of the room, B the middle and C the back (or something similar to that). Problem was before /after hours maintenance would come in and turn on the front row in all rooms and nothing else. These lights maybe even remained on during the day when a room wasn't in use. This put all of that load on line A and limited loads on other two lines. Over time this apparently did cause neutral issues, but was not a situation ever planned for. The plan was load balancing when all the lights were in use

Now I can't believe that lighting was that significant of a load to cause this problem when there was probably more HVAC load then lighting, and that HVAC load was more likely to be more balanced, but this is a story I was told and not something I personally witnessed.

Schools back in the day had very limited HVAC because school would be out for the summer, and oil fired boilers used in the winter. There used to be some elementary schools that were fed by nothing more than a 50kva pole pig.

 

[peter d]

Schools back in the day had very limited HVAC because school would be out for the summer, and oil fired boilers used in the winter. There used to be some elementary schools that were fed by nothing more than a 50kva pole pig.

All the schools I attended where I grew up were built prior to 1970 and had no a/c whatsoever.

 

[mbrooke]

OK.
Suppose there are no 240V loads.
Half the 120V loads are on one of the lives, say A and Half on B. Balanced and presumably compliant? Now turn off, and/or unplug all the loads on B and leave on all the loads on A.
Wouldn't the current in the neutral be that same as that flowing in A?

It would, but I doubt it would be over 30 amps. Even if you had the room in the panel to deliberately put all the 120 volt loads on leg "A", I still doubt it would top 50amps peak. Unlike BS7671, the NEC is very conservative when it comes to sizing services. Residential services are often grossly over sized and commercial services are on average loaded 1/2 to 1/3 of the NEC's load calcs.

 

[mbrooke]

All the schools I attended where I grew up were built prior to 1970 and had no a/c whatsoever.

Id argue that still holds true outside of the nurse's office.

 

[iwire]

Id argue that still holds true outside of the nurse's office.

And I would say you don't have any idea. :happyno:

 

[mbrooke]

And I would say you don't have any idea. :happyno:

For the board of education in your town, you are spot on :thumbsup:

 

[GoldDigger]

Well the contrapositive (switch and negate) would be logically equivalent: "if I am not doing it, the code doesn't allow it." :blink:

Actually, it would be "if I am not doing it, the code allows it."

Sent from my XT1585 using Tapatalk

 

[mbrooke]

And I would say you don't have any idea. :happyno:

And to clarify; yes I am well aware that some middle schools and high schools are now air conditioning the libraries, special education rooms, faculty/support offices, library, auditorium, drama theater, or some combination thereof. However the overall square footage is still un-aircontioned for most public schools, and many existing schools still have cooling contained to the nurses office/spec ed room.

 

[iwire]

And to clarify; yes I am well aware that some middle schools and high schools are now air conditioning the libraries, special education rooms, faculty/support offices, library, auditorium, drama theater, or some combination thereof. However the overall square footage is still un-aircontioned for most public schools, and many existing schools still have cooling contained to the nurses office/spec ed room.

I can't dispute that one way or another just like you can't back up what you are saying. You are simply guessing.

 

[mbrooke]

I can't dispute that one way or another just like you can't back up what you are saying. You are simply guessing.

I've went to enough US schools and researched enough board of ed projects to know that there are still schools in existence which have next to no AC. But in reality the ultimate proof are the tax payers. Whose is willing to pay for even partly air conditioned schools?

 

[iwire]

I've went to enough US schools and researched enough board of ed projects to know that there are still schools in existence which have next to no AC.

No one has said otherwise.

You had claimed schools don't have AC outside the nurses office.

Moving on.

 

[mbrooke]

No one has said otherwise.

You had claimed schools don't have AC outside the nurses office.

Moving on.

Nope:

Id argue that still holds true outside of the nurse's office.

I never specified what ratio-or that it was 100% of schools. But in any case I still stand by the fact most public schools have limited AC.

 

[Besoeker]

This thread seems to have wandered off topic. I have posts deleted in another thread for allegedly doing so.

 

[mbrooke]

Ok, Ill behave. I'm back on track :thumbsup:



Besoker, where your from, are 3 phase services allowed to have undersized neutrals? And if not, why so?

 

[iwire]

This thread seems to have wandered off topic. I have posts deleted in another thread for allegedly doing so.

The posts you keep bringing up where between you and mbrooke about nuclear bombs and immigration in a thread about AC or DC distribution.

That was too far removed from the thread topic and had it been the topic of a thread I would have closed it for being outside the scope of this forum.

 

[Besoeker]

Ok, Ill behave. I'm back on track :thumbsup:



Besoker, where your from, are 3 phase services allowed to have undersized neutrals? And if not, why so?

TBH, I don't know whether the regs (BS 7671) permit it.
Normally, we use 4-core (3-ph + neutral SWA). All cores are the same size so the question doesn't arise.

And there are cases where a higher rated neutral is required.
That luxury hotel, the Burj al Arab is a case in point. The company I worked for put in 3 MW of controlled lighting, all single phase, and there were lots of other single phase non-linear loads.
This resulted in a high third harmonic component and that all flows in the neutral of a three phase system. And it overloaded the neutral even though it was the same rating as the phase conductors.

It was a different division of the company, mine was industrial rather than commercial, but I got asked to look at the issue. After the event.

 

[mbrooke]

TBH, I don't know whether the regs (BS 7671) permit it.
Normally, we use 4-core (3-ph + neutral SWA). All cores are the same size so the question doesn't arise.

And there are cases where a higher rated neutral is required.
That luxury hotel, the Burj al Arab is a case in point. The company I worked for put in 3 MW of controlled lighting, all single phase, and there were lots of other single phase non-linear loads.
This resulted in a high third harmonic component and that all flows in the neutral of a three phase system. And it overloaded the neutral even though it was the same rating as the phase conductors.

It was a different division of the company, mine was industrial rather than commercial, but I got asked to look at the issue. After the event.

I would imagine the regs should- at least in theory? Picture an industrial building or supermarket where the 3 phase load far exceeds to the single phase load. In such a case a reduced size neutral should be ok from an electrical theory stand point. But for residential in the UK an undersized neutral would be an issue due to everything being L-N single phase.

 

[Tony S]

Distribution doesn’t come under BS7671. There’s nothing in the regulations specifically against ½ sized neutral conductors.

 

[iwire]

And there are cases where a higher rated neutral is required.
That luxury hotel, the Burj al Arab is a case in point. The company I worked for put in 3 MW of controlled lighting, all single phase, and there were lots of other single phase non-linear loads. This resulted in a high third harmonic component and that all flows in the neutral of a three phase system. And it overloaded the neutral even though it was the same rating as the phase conductors.

Sounds like a bad design from the start, they must have loaded the circuits to the upper end to actually cause an overload on the neutral.

Oh surprise, we do know about non-linear loads here too and almost 100% of the info suggesting overloaded neutrals are a concern are coming from those with a stake in selling conductors.

For some time in the 80s and 90s we were running a lot of what were called 'super neutrals' in many cases the engineers had us run two neutrals to every panel. So a 200 amp panel would have at least a 400 amp neutral.

A better solution seems to be just not maxing out the circuits when dealing with non-linear loads.

The installations of super neutrals seems to have subsided but there is at least one area (and perhaps more) where the NEC requires an up sized neutral and that is for stage dimming equipment.

I suspect the requirement has as much to do with the non-linear loads as it does with the likelihood of the workers overloading the circuits.

520.53(O) Neutral Conductor.

(1) Neutral Terminal. In portable switchboard equipment
designed for use with 3-phase, 4-wire with ground supply,
the current rating of the supply neutral terminal, and the
ampacity of its associated busbar or equivalent wiring, or
both, shall have an ampacity equal to at least twice the
ampacity of the largest ungrounded supply terminal.

Exception: Where portable switchboard equipment is specifically
constructed and identified to be internally converted
in the field, in an approved manner, from use with a
balanced 3-phase, 4-wire with ground supply to a balanced
single-phase, 3-wire with ground supply, the supply neutral
terminal and its associated busbar, equivalent wiring, or
both, shall have an ampacity equal to at least that of the
largest ungrounded single-phase supply terminal.

(2) Supply Neutral Conductor. The power supply conductors
for portable switchboards utilizing solid-state phasecontrol
dimmers shall be sized considering the neutral conductor
as a current-carrying conductor for ampacity adjustment
purposes. The power supply conductors for portable switchboards
utilizing only solid-state sine wave dimmers shall be
sized considering the neutral conductor as a non–currentcarrying
conductor for ampacity adjustment purposes. Where
single-conductor feeder cables, not installed in raceways, are
used on multiphase circuits feeding portable switchboards
containing solid-state phase-control dimmers, the neutral conductor
shall have an ampacity of at least 130 percent of the
ungrounded circuit conductors feeding the portable switchboard.
Where such feeders are supplying only solid-state sine
wave dimmers, the neutral conductor shall have an ampacity
of at least 100 percent of the ungrounded circuit conductors
feeding the portable switchboard.

 

[Besoeker]

The posts you keep bringing up where between you and mbrooke about nuclear bombs and immigration in a thread about AC or DC distribution.

I don't recall that I mentioned nuclear bombs.
Shall we get back on topic?