Why full size neutral single phase?

Status
Not open for further replies.
iwire said:
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.
Click to expand...
Nope.
It was cumulative third harmonic. The third from all three phases is in phase.
 
Besoeker said:
Incorrect. I specifically mentioned A phase, B phase and a neutral.
Click to expand...
B phase was open-circuited in your scenario. The only thing left were loads connected from phase A to the ungrounded conductor. That my friend is a two-wire circuit when you subsequently want to talk about the load currents.

Good morning in the UK by the way. It is still early here yet. Still got time for a wee nap.:)
 
Besoeker said:
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.
Click to expand...
All that is fine. There are scenarios where the ungrounded conductor may carry all the load, even in residential (like an out-building or shop for example).

But the OP had a residential dwelling scenario where he had dispersed the loads between the two busses. Plenty of 240 volt loads in the sizing calcs. Plenty of diversity. No need for a full-size neutral.
 
mivey said:
But the OP had a residential dwelling scenario where he had dispersed the loads between the two busses. Plenty of 240 volt loads in the sizing calcs. Plenty of diversity. No need for a full-size neutral.
Click to expand...
Might I gently suggest you read what the op stated at the start of this thread?
 
Besoeker said:
Nope.
It was cumulative third harmonic. The third from all three phases is in phase.
Click to expand...

If the circuits themselves are are not maxed out current in the neutral is not going to exceed the rating of the neutral. Pretty straight forward.

But I agree with mivey all of this is so far beyond the OPs situation it is just noise.
 
iwire said:
If the circuits themselves are are not maxed out current in the neutral is not going to exceed the rating of the neutral. Pretty straight forward.
Click to expand...
That's simply incorrect when you have non-linear loads on a three phase system.
 
iwire said:
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.
Click to expand...



I am betting that was the case. Under BS7671 wire and equipment is legally permitted to carry much more current than when compared to the NEC. For example there are no 80% rule, load calcs dictate less KVA per service/feeder and something like #14 copper can get away with carrying 30 amps continuously.


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.
Click to expand...


Id argue that the added safety factors in the NEC take care of it.
 
Besoeker said:
Might I gently suggest you read what the op stated at the start of this thread?
Click to expand...
The OP was talking about service residential. I know what that is here in the US as I have been working with that for about 40 years.

The OP knew to balance the 120 volt loads as he knew about reduced neutral loading. He even tried to explain it to the inspector. Obviously he knows how to size a service per the NEC and he was not gigged on the 4/0 ungrounded conductor.

Not much else to it.

Your what-if was that all the load was one leg to neutral, and so much load in fact that it would fully load the service's 4/0 conductor. A fantasy scenario that has no bearing on what the OP was talking about.
 
iwire said:
Actually I don't believe it is.

If each underground conductor is less loaded the current in the neutral will be less.

Are you saying otherwise?
Click to expand...
I am saying otherwise.
It was a problem at the Burj al Arab.
Not a theoretical problem. An actual real life problem. It happened.
I flew out there to investigate, take measurements, and come up with a solution.
 
ptonsparky said:
What caused this to happen? Operator error? Design?

This is anything 'but' a single family residence.
Click to expand...

But still an application where the NEC could allow a reduced neutral. I know the OP asked about resi, but IMHO I see no harm in extending the theory to 3 phase.
 
Besoeker said:
I am saying otherwise.
It was a problem at the Burj al Arab.
Not a theoretical problem. An actual real life problem. It happened.
I flew out there to investigate, take measurements, and come up with a solution.
Click to expand...

You are not making any sense here.

My statement was that if you reduce branch circuit current the neutral current will decrease.

You say that is false, please elaborate.
 
iwire said:
You are not making any sense here.

My statement was that if you reduce branch circuit current the neutral current will decrease.

You say that is false, please elaborate.
Click to expand...
The third harmonic content from each phase in a three phase system adds in the neutral. That's what happened.
 
mbrooke said:
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.
Click to expand...
I have upgraded services on some of those when they added AC.

peter d said:
All the schools I attended where I grew up were built prior to 1970 and had no a/c whatsoever.
Click to expand...
Same here, but most of those same schools now have AC in at least portions of the buildings, though most are seeing AC nearly everywhere.

mbrooke said:
Id argue that still holds true outside of the nurse's office.
Click to expand...
The small school districts around here share a school nurse. They only are there long enough to perform some basic physical exams once a year on each student, and the room they use for that is vacant most of the time otherwise. If a child gets sick or injured during school a teacher or other staff may stay with that child until parent arrives to pick them up, unless it is something serious enough then an ambulance may be called, and parent is directed to meet them at hospital. Many small schools do have someone that is a volunteer EMT on their staff, not that they seek them out, it just happens that way in small towns.
 
ptonsparky said:
I have reduced it to the absolute minimum when we have no neutral loads. A 4/0 AL could have a #2 AL neutral.
Click to expand...
Same here. I imagine we both have had three phase systems at grain storage facilities that maybe are supplied with 400, 600, 800 or even higher amp service or feeders, the bulk of the load being straight three phase motor loads. Line to neutral loads may not even be 5% of the service or feeder capacity even if they were all connected to the same leg. 1/0 neutrals are used because you can not parallel anything smaller then that though the calculated neutral load may only be 30 -50 amps max.
 
Status
Not open for further replies.
Top