ECN Forum Forums General Topics of the Trade General Discussion Area Hot pipes for lighting circuits

>Wouldn't more heat be generated on each and every neutral conductor?
No. If you look at the numbers, two of the three neutrals (in multiwire) are carrying more than the most carried by any one UGC.

>Wouldn't each neutral be hotter carrying the c.15A loads?
No. Four would be hotter, three would be cooler. 5 out of 8 would be overloaded instead of 2 out of 3.


Circuit 7 & 9 is a fine example. That circuit is legal for the UGCs but the GC is overloaded. That's what can happen. That's why the neutral should be oversized.

>the "whopping 6 amps" that the 4 neuts carry?
You might want to re-read the numbers.
How did you determine that there are four neutrals carrying 6 amps each?

I'm not going to go another round of your dissect-word-by-word-"quote" & quip arguments.
You'd better call all of the switchgear manufacturers, maybe Lithonia or Cooper lighting (their modular systems don't have an upsized neut), and inform them of your brilliant findings.
Then go rewire all the 3Ø - 277/480 warehouses in the country. They really aren't done to your liking. I'm done on this thread.


(My apologies to the rest of you for my rudeness)


[This message has been edited by electure (edited 07-24-2001).]

>You'd better call all of the switchgear manufacturers, maybe Lithonia or Cooper lighting (their modular systems don't have an upsized neut), and inform them of your brilliant findings.
Perhaps so.

>Then go rewire all the 3Ø - 277/480 warehouses in the country. They really aren't done to your liking.
Correct.

Joe Tedesco provided this link http://www.copper.org/pq/primer/ a while back.

Scott35 wrote on 02-04-2001 03:07 AM:
... 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.

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].
...
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%.