4000 amp 480v 3ph switch gear

[electrofelon]

I think the reason you dont see more isophase installations where they would be permitted are:

1. most electricians dont know about his method or have some irrational fear of it
2. most electricians dont follow the same conductor length rule for parallel runs so there is significantly less advantage to isophasing
3. many people dont seek to improve or find better ways

The 3 lugs will typically be arranged in a line.

With isophase, each lug will have 9 conductors going to just 3 different conduits, and those conduits carry nothing else. You can arrange those conduits so that one is under the lug, the others are neighboring and spread perpendicular to the spread of the lugs. The variation in lug to conduit distance is low, and it is not much trouble to keep the conductors all the same length. The layout of conduits is a separate issue for each lug, no interference.


With mixed phase, each lug will have 9 conductors, 1 going to each of the 9 different conduit. The spread of those 9 conduits will be much greater. Also, at most one lug can be centered over the centroid of the conduit pattern; for the other lugs, the spread of distances from lug to conduit will be quite a bit higher. It will be harder to keep all the conductors the same length.

As someone who has wrested with polyphase grouping, keeps conductors the same length, and tries real hard to make it look neat, I fully agree. Some may think it is "no big deal" to have the conduits a little further apart, but the hassle increases rapidy. Say two of the pipes are 18 inches apart. That is 18 inches of conductor that has to go somewhere. I would probably isophase each and every time I had the opportunity. So you can only have 3 conductors per conduit without derating, that probably bumps you down to a smaller pipe size.

 

[Ingenieur]

...the pic in this thread could have just as easily (easier?) been done as follows

upsize conductor if required, probaby not since it uses 4/conduit=20% derate

Use 6 sets of 3 with ground vs 6 x 4 plus 2 grounds: 2 less conduits

eliminates 2 conduit, 2 pulls and 16 terminations (8 ea end)
6 phase x 2 + 2 gnd x 2

and less copper, 6 cond plus 2 ground eliminated since no derating is required...

the reason isophase is not used is not fear, lack of knowledge or lack of desire to improve
it is pointless and is more expensive with no benefit, and some disadvantages
the conventional way is better >95% of the time
electricians aren't stupid

as I pointed out the isophase pic in this thread could have been done easier and cheaper conventionally
they may have had a reason but it doesn't seem like a good one
perhaps someone was trying to be clever and reinvent the wheel lol

 

[electrofelon]

the reason isophase is not used is not fear, lack of knowledge or lack of desire to improve
it is pointless and is more expensive with no benefit, and some disadvantages
the conventional way is better >95% of the time
electricians aren't stupid

as I pointed out the isophase pic in this thread could have been done easier and cheaper conventionally
they may have had a reason but it doesn't seem like a good one
perhaps someone was trying to be clever and reinvent the wheel lol

I stand by my three points and belief that it can be much easier to isophase. May I ask how much hands on field experience you have? Again, those who don't obey the same length rule and aren't real picky about neatness may find little advantage.

 

[iwire]

I stand by my three points and belief that it can be much easier to isophase.

Sure it can be, that is why I chose to do it the two times I did.

But in general I think it is not easier, the numbers of pipes, conductors and circuit type plays into this a great deal.

Also the fact it is limited to underground limits it 90% of the time.

 

[dionysius]

Executive Summary of Isophasing where Permitted.......

Executive Summary of Isophasing where Permitted.......

I think the reason you dont see more isophase installations where they would be permitted are:

1. most electricians dont know about his method or have some irrational fear of it
2. most electricians dont follow the same conductor length rule for parallel runs so there is significantly less advantage to isophasing
3. many people dont seek to improve or find better ways

As someone who has wrested with polyphase grouping, keeps conductors the same length, and tries real hard to make it look neat, I fully agree. Some may think it is "no big deal" to have the conduits a little further apart, but the hassle increases rapidy. Say two of the pipes are 18 inches apart. That is 18 inches of conductor that has to go somewhere. I would probably isophase each and every time I had the opportunity. So you can only have 3 conductors per conduit without derating, that probably bumps you down to a smaller pipe size.

I wish I had stated it so well. This captures the essence of the thread. I challenge anyone, repeat anyone, to find fault with this summary.

 

[iwire]

I wish I had stated it so well. This captures the essence of the thread. I challenge anyone, repeat anyone, to find fault with this summary.

You need to wire a 1,000 amp service underground.

Please describe how you will accomplish this as an NEC compliant isophase installation.

 

[electrofelon]

You need to wire a 1,000 amp service underground.

Please describe how you will accomplish this as an NEC compliant isophase installation.

Are you getting at that this is not likely to be a switchboard due to its size? FYI, I have been lumping in "its a switchboard" as a characteristic of "an installation where isophase is permitted."

 

[iwire]

Are you getting at that this is not likely to be a switchboard due to its size? FYI, I have been lumping in "its a switchboard" as a characteristic of "an installation where isophase is permitted."

No, not at all and keep in mind I have chosen to install isophase.

I am just interested in the wire size, quantity of conductors and conduits dionysius comes up with in an isophase arrangement that is not more trouble or money than doing it in the conventional way.

 

[roger]

Are you getting at that this is not likely to be a switchboard due to its size? FYI, I have been lumping in "its a switchboard" as a characteristic of "an installation where isophase is permitted."

A Switchboard, ATS, Safety Switch, etc.... is not the issue. You do not need an open bottom enclosure to do an isophase installation.

Roger

 

[electrofelon]

No, not at all and keep in mind I have chosen to install isophase.

I am just interested in the wire size, quantity of conductors and conduits dionysius comes up with in an isophase arrangement that is not more trouble or money than doing it in the conventional way.

OK

A Switchboard, ATS, Safety Switch, etc.... is not the issue. You do not need an open bottom enclosure to do an isophase installation.

I do concur, see post #100. Personally, I would likely not employ those methods and would pass on isophase in those cases. Besides then you have the locknut issue.

 

[electrofelon]

Actually, upon further cogitation, I may not have a problem with a narrow slot between the KO's and using a PVC box adapter or end bell to terminate the PVC in the cabinet.......

 

[wwhitney]

I am just interested in the wire size, quantity of conductors and conduits dionysius comes up with in an isophase arrangement that is not more trouble or money than doing it in the conventional way.

For a service with a neutral, the isophase arrangement is going to require around 4/3 as many conduits as a mixed phase arrangement (although possibly the conduits will be smaller). Plus it would behoove one to pick a number of sets that is a multiple of 3.

So for 1000 amps isophase, one could choose 3 sets of 400 kcmil (4 conduits), 6 sets of 2/0 (8 conduits), or 6 sets of 3/0 (4 conduits, 90C conductors). That's just from a quick glance at Table 310.15(B)(16), I haven't calculated conduit sizes or considered that the neutral could be reduced in size. For example, you might end up with 7 conduits, where the ungrounded conductors are split among 2 conduits each, but the grounded conductors will work all in one conduit.

Cheers, Wayne

 

[iwire]

So for 1000 amps isophase, one could choose 3 sets of 400 kcmil (4 conduits),

Will you indulge me and walk me through this?

 

[wwhitney]

Will you indulge me and walk me through this?

I'm unexperienced at this so I'll probably make a mistake, but I'll give it a try.

First, let me ask for a couple parameters: 3 ungrounded conductors with 1000 amps ampacity, 1 grounded conductor with how many amps ampacity? When sizing conduit, is 40% conduit fill acceptable, or do you want a lower conduit fill for ease of pulling?

Cheers, Wayne

 

[iwire]

I'm unexperienced at this so I'll probably make a mistake, but I'll give it a try.

Well three sets of 400 copper works as well isophase as it does conventionally so it would only be a question of how the layout is.


But my point has been that due to the restrictions found in 310.10 / 300.3 etc isophase does not always work as well from a cost standpoint as conventional methods. There is no one method fits all answer, each situation needs to be examined.

For some here to say it is just a case of ignorance or fear is ridiculous.

 

[Ingenieur]

I stand by my three points and belief that it can be much easier to isophase. May I ask how much hands on field experience you have? Again, those who don't obey the same length rule and aren't real picky about neatness may find little advantage.

35 years
all heavy commercial/industrial
oil/gas
defense infrastructure
bio/pharma
manufacturing
mining
IBEW member for my first 4 years
most of it field work and design build

As I have proven even the example used in this thread would have been easier using the conventional method
2 less conduit
2 less gounds
6 less conductors
16 less terminations
2 less pulls
absolutely no disadvantage vs isophase, in fact a better more cost effective method

 

[Ingenieur]

I wish I had stated it so well. This captures the essence of the thread. I challenge anyone, repeat anyone, to find fault with this summary.

I picked it apart
even the iso example pic in this thread would have been better using the conventional method

2 less conduit
2 less gounds
6 less conductors
16 less terminations
2 less pulls

refute this fact
it can't be

 

[Ingenieur]

Using isophase I have seen it so few times and therefore not up to speed
for a wye ckt
must the N be in seperate conduit or combined with the phase(s)?
must the EGC be in a seperate conduit? Induced ground current?
can EGC and N be combined in the same conduit? Parallel sizing constraints?

I think I can come up with answers and reasons but not sure

 

[electrofelon]

Will you indulge me and walk me through this?

I think there are too many possibilities to try to make a case with an example, but I have been working on some hypotheticals for a bit and it does look like POLY works out better more of the time due to more freedom in number of sets, plus with three conductors, which is a frequent candidate for iso, you get in to jamming which may cause you to bump up a pipe size.

Ok 1000 amps three phase, assume full size neutral, compact aluminum THWN conductors, neutral not a CCC, PVC sched 40. This doesn't work well with ISO

Standard polyphase: I would likely go with 4 sets of 350. I could get 4 conductors in 2.5", so its either 2.5 or 3 depending on the specifics.

Isophase: 4 conductors obviously doesnt work well, assuming we dont want to get into derating (In this case going up 1 size ALMOST works and could be used if we didnt quite need the full 1000 amps). A few possibilities:
1. 3 350 in 2", and a single conductor in 1 1/4", so a total of 4 2" and 4 1 1/4". kinda goofy.
2. 3 600's per pipe: 3" we risk jamming, 3.5 maybe but could be hassle to get, maybe we just go 4"

so #2 could work out real nice, although we are paying a bit more for wire due to less ampacity per unit area, and more for pipe.

Copper works better:

POLY: 3 sets of 400 or 4 sets of 250, 3 inch and 2.5 respectively.
ISO: 3 400 per pipe, 3" pipe, 4 total pipes
ISO: 2X3X2/0, 2" pipe, 8 total pipes (note 3" costs twice as much as 2")

 

[electrofelon]

Using isophase I have seen it so few times and therefore not up to speed
for a wye ckt
must the N be in seperate conduit or combined with the phase(s)?
must the EGC be in a seperate conduit? Induced ground current?
can EGC and N be combined in the same conduit? Parallel sizing constraints?

I think I can come up with answers and reasons but not sure

Interesting question. Could we get the best of both worlds and put each phase in a pipe along with 1 neutral? I dont see any wording prohibiting that.