4000 amp 480v 3ph switch gear

[kwired]

That is a clean installation but could have been done by sets too
the conductors were derated since >3, correct?
does the cond area < 20% cnd area? How long was the run? In feet
short no big difference, long Cu and cnd add up fast
you have 8 cnd = 8 sets, could have rotated cnd 90 deg and been pretty clean
could have possibly used 6 smaller sets (conductor and cnd at 40% fill) without derating?
easier manipulating into position then

there is no right or wrong, these are preferences and judgement
as long as code compliant doesn't matter

But look at how much easier it was to get all conductors of same phase the same length by doing it that way. Now you have to justify cost of that vs, being able to go with smaller conductors if you put all three phases and neutral in each raceway, if it is about a wash then what was pictured will likely win out.

 

[Phil Corso]

And why do you think that? If the non metallic-raceways are run in close proximity to one another it doesn't matter anyways.Roger

Roger...

Because the phases are separated there is a greater impact of inductivity caused by phase-separation, thereby increasing the "effective resistance"! The result is higher losses than when the 3-phase conductors are bundled in groups of three (tre-foil).

The added "resistance" increase varies as conductor-size increases! For example, up to say #4 AWG (cu), the increase is nil! But for 250 kcmil, the increase is appreciable. If configuration is tre-foil, and operated near rated-current, resistance is r-Ohm/k-ft, but if arranged with separation of say 5-diameters shown in photo (my guesstimate) effective-resistance will increase by 1.05-1.15 x r-Oms/k-ft (an increase of 5-15%!

Phil

 

[iwire]

That is a clean installation but could have been done by sets too
the conductors were derated since >3, correct?

That particular instillation was not mine.

I have done two 3,000 amp services using 8 sets of 600 CU in isolated phases. In my case I had far less room between the floor and the lugs so crisscrossing 8 sets would have been a pain.

On the other end where pad mounts sitting on vaults so plenty of room to crisscross the cables to the lugs.

there is no right or wrong, these are preferences and judgement
as long as code compliant doesn't matter

That was pretty much my point to Phil.

 

[roger]

Roger...

Because the phases are separated there is a greater impact of inductivity caused by phase-separation, thereby increasing the "effective resistance"! The result is higher losses than when the 3-phase conductors are bundled in groups of three (tre-foil).

The added "resistance" increase varies as conductor-size increases! For example, up to say #4 AWG (cu), the increase is nil! But for 250 kcmil, the increase is appreciable. If configuration is tre-foil, and operated near rated-current, resistance is r-Ohm/k-ft, but if arranged with separation of say 5-diameters shown in photo (my guesstimate) effective-resistance will increase by 1.05-1.15 x r-Oms/k-ft (an increase of 5-15%!

Phil

Phil, the groups might as well be in one large conduit if they are routed correctly, they are in non ferrous raceways.

Now addressing the grounded conductors being run with each phase in their respective conduits in lieu of their own conduits.

Let's use a hypothetical situation where at a given moment the only current flow is on "A" phase and the grounded conductor. With current division on the grounded conductors the only current flow in the "B" and "C" conduits is on the grounded conductors, why is this better than having the grounded conductors routed in their own conduits?

Roger

 

[Ingenieur]

But look at how much easier it was to get all conductors of same phase the same length by doing it that way. Now you have to justify cost of that vs, being able to go with smaller conductors if you put all three phases and neutral in each raceway, if it is about a wash then what was pictured will likely win out.

That is why the length is important
what ampacity?

less wires = less terminations
and cutting is cutting
although this easier to get a 'clean' looking result
but with a skilled electrician the labor difference is small imo

 

[Ingenieur]

Assume 2500 A and use 8 sets
no neutral ignore egc
if grouped by phase 3 runs x 8 conductors with 30% derating and 20% fill
700 mcm
5600 mcm / lf
3 pvc runs
too lazy to look up the conduit lol

if balanced 6 x 3 no derating
600 mcm
3600 mcm / lf
6 smaller pvc runs

depending on length could be significant
again, an educated judgement based on cost

 

[Phil Corso]

Phil, the groups might as well be in one large conduit if they are routed correctly, they are in non ferrous raceways.

Roger

The problem of separation increasing cable losses is exacerbated because the ducts are non-magnetic!

Phil

 

[Ingenieur]

imho no technical reason to select one method over the other
as long as both are code compliant
comes down to cost

 

[roger]

The problem of separation increasing cable losses is exacerbated because the ducts are non-magnetic!

Phil

That still doesn't answer the question.

Using your theory of non ferrous raceways it would be the same problem even if the raceways contained all phases and grounded conductor.

Roger

 

[Phil Corso]

That still doesn't answer the question.
Using your theory of non ferrous raceways it would be the same problem even if the raceways contained all phases and grounded conductor.
Roger

It has to do with separation between phases. In the case you illustrated the distances between phases are not symmetrical, but instead asymmetrical, hence the problem!

If each group of a three-phase bundle were installed in one conduit, their conductors would be more equidistant, hence effective-resistance in each bundle is minimum! Consider the extreme case, i.e., installing each of the 12 phases in its own conduit!

Phil

 

[electrofelon]

But look at how much easier it was to get all conductors of same phase the same length by doing it that way.

I agree. If one obeys the same length rule, it is really hard to get a balanced set installation to not look like a rats nest. I always try real hard and I am never happy with it.

 

[Phil Corso]

I agree. If one obeys the same length rule, it is really hard to get a balanced set installation to not look like a rats nest. I always try real hard and I am never happy with it.

Obviously a key-factor is circuit-length! There is a saving grace, operating-current is seldom rated-current! But, never use it as an excuse in design!

Phil

 

[roger]

It has to do with separation between phases. In the case you illustrated the distances between phases are not symmetrical, but instead asymmetrical, hence the problem!

If each group of a three-phase bundle were installed in one conduit, their conductors would be more equidistant, hence effective-resistance in each bundle is minimum! Consider the extreme case, i.e., installing each of the 12 phases in its own conduit!

Phil

Phil there is no guarantee that the conductors would lay symmetrical in a conduit and although not practical, given a few hot box benders I could weave 8 raceways like a rope.

The bottom line is, there is no real reason not to do individual phase installations if it is feasible and all the reasons that Ingenieur brought to the table are considered.

Roger

 

[Ingenieur]

Inductance is proportional to the log of seperation
So if less than 10 times as much it is moot
even at 100 times only doubles
at an increase of 10 times dist it is ignored

and the equation has multiple factors, some of which do not change, which further reduces the significance of seperation

d is dist between conductors
GMR geometric mean radius is a property of the conductor radius x e^-0.25

 

[GoldDigger]

The problem of separation increasing cable losses is exacerbated because the ducts are non-magnetic!

Phil

The code language addresses putting sets of phases plus grounded (not necessarily neutral) together as sets in raceway, and isophase installation with each phase or neutral in its own conduit(s).
It does not specifically mention mixing phase and neutral as a variation on isophase.

If you have a balanced load, putting returns in one conduit or distributing them across all phase conduits will have exactly the same inductance. (Nothing to cancel).
If you look at a single phase fault to ground or neutral, then putting full sized neutrals in each phase conduit would be ideal. Splitting the neutrals evenly across the phase conduits less helpful and might lead to a (momentary) factor of three or even ten overload of some neutrals.

 

[jgt480v]

4000 amp 480v 3ph switch gear

500 kcmil at 90 degC 430 amps
12 * 430 = 5160 derate 20% 5160 - 1032 = 4128

So 12/phase

A set is defined as A, B, C, N

75degC is mostly common used due to your lugs are rated at 75 degC.

 

[dionysius]

grouping phases is seldom done
usually on pads where there are bending radius constraints

This makes little sense. In fact the opposite is the case and is a cogent reason for grouping phases.

 

[don_resqcapt19]

The problem of separation increasing cable losses is exacerbated because the ducts are non-magnetic!

Phil

If you tried an isolated phase installation with ferrous raceways, you will have a much bigger problem than increased cable losses.

 

[Smart $]

The problem of separation increasing cable losses is exacerbated because the ducts are non-magnetic!

Phil

If you tried an isolated phase installation with ferrous raceways, you will have a much bigger problem than increased cable losses.

Additionally, Chapter 9, Table 9 indicates inductance is greater using steel conduit. I believe the values are derived from balanced-current circuits within the conduit.

 

[Ingenieur]

This makes little sense. In fact the opposite is the case and is a cogent reason for grouping phases.

Absurd

isophase is probably less than 5% of all installations if that
the required derating and oversized cnd make it less cost effective

using words like 'cogent' does not impress nor make your case any more valid