Circuit routing

Status
Not open for further replies.
Couple of questions. The equipment is a 200HP (skid mounted) compressor that will be installed routinely as a temporary supply for about one week duration and 6-8 weeks intervals. The compressor is 480V/3phase.

From the engineering standpoint I would want a 3/c+G single flex cable, but the manufacturer insists that single conductors are fine. I am not going into the list of problems of what unbundled single conductors can cause, but would like to know if the Code says it either way, namely that the individual conductors of a three phase circuit should be routed together in close proximity of each other. When it is routed in a conduit, raceway or common cable it is obviously not a problem. When routed as parallel feeders the A-B-C's of each circuit part should be bundled together, not A-A, B-B etc. and be equal length and routed on the same path. In the cable tray instructions the talk about single conductors side-by-side and triplexed, but is there a general paragraph addressing this issue about 3 phase circuits?
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector
I am of the opinion that 300.3 would not allow "open conductors" other than as prescribed in Art 398. (Single Type "W" cables, or others listed as "cords" would fall under Art 400 rules)
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Single conductor Type W cables are often used for these types of applications because of the size and weight of a multicondutor cable. You would be permitted to use 3/0 Type W as long as you run them so they are not touching each other, but you would need 250kcmil for 3 conductor type G assuming a 75?C rating of the terminations.
 
Single conductor Type W cables are often used for these types of applications because of the size and weight of a multicondutor cable. You would be permitted to use 3/0 Type W as long as you run them so they are not touching each other, but you would need 250kcmil for 3 conductor type G assuming a 75?C rating of the terminations.
Don, thank you, I understand your reasoning, but the specific provision of 300.3(B) is for the purpose so that the three phase conductors of the same ciruit SHOULD be in close proximity AND equal in lenght and the specifically talk about to be 'contained within the same ....cable"? There don't seem to be an exception to your stipulation, eg. we can manually bundle them, although the provisions of cable tray installation - and only there - refers to running individual conductors, but they don't spell it out that it is for three phase connections (but certainly can't think of what else would they would be thinking of)?!
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Laszlo,
You bring up a good point about 300.3(B), but if you install the single conductor Type W with the conuctors touching each other the ampacity would go down. Take a look at Note 1 to Table 400.5(B).

I don't see anything in Article 400 that specifically permits the use of single conductors for this application and even 590.4 appears to prohibit the use of single conductor cables for this application.

I think a code change is needed and the substantiation would be "to make the code conform to standard industry practices".

 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Doesn't 300.3(B) simply says that they must be in the same trench, or cable tray. It does not require the conductors to be 'touching' each other, nor does it proscribe how far away they may be.

300.3(B)(3) would allow single conductor type W conductors to be run no where near each other.

For cable tray installations,
392.8(D) requires "paralleled" single conductors to be grouped
392.8(E) allows single conductors to be installed in a single layer, which means grouping is not required.
 
Laszlo,
You bring up a good point about 300.3(B), but if you install the single conductor Type W with the conuctors touching each other the ampacity would go down. Take a look at Note 1 to Table 400.5(B).

I don't see anything in Article 400 that specifically permits the use of single conductors for this application and even 590.4 appears to prohibit the use of single conductor cables for this application.

I think a code change is needed and the substantiation would be "to make the code conform to standard industry practices".
I understand your comment about the ampacity going down, but I also think that from the safety standpoint 300.3(B) has primacy over the cost savings a smaller conductor would provide.

I don't think a 'standard industry practice' that is TECHNICALY wrong and potentially DANGEROUS should be continued, not to mention supported and allowed by the NEC, or that cost consdiderations over safet should drive the NEC. There were electrcial installations before the Code existed and one of the impetus of developing the Code was - and continues to be - to make sure that the installations are safe and technically sound.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector
Since 300.3 references conductor specifies in 310.13, would it even apply to Type W Cable from Art 400 ?
 
Doesn't 300.3(B) simply says that they must be in the same trench, or cable tray. It does not require the conductors to be 'touching' each other, nor does it proscribe how far away they may be.

300.3(B)(3) would allow single conductor type W conductors to be run no where near each other.

For cable tray installations,
392.8(D) requires "paralleled" single conductors to be grouped
392.8(E) allows single conductors to be installed in a single layer, which means grouping is not required.
Jim,

(B) Conductors of the Same Circuit.​
All conductors of the same circuit and, where used, the grounded conductor and all equipment grounding conductors and bonding conductors shall be contained within the same raceway, auxiliary gutter, cable tray, cablebus assembly, trench, cable, or cord, unless otherwise permitted in accordance with 300.3(B)(1) through (B)(4).

Single conductors in a single tray are still grouped and installation practice is that either they are installed side-by-side or a specific space from each other and that space and the position of the cables in the tray is maintained by Ty-Wrapping them at regular intervals to the rungs. Have not seen an installation when the individual conductors would cross each other, except perhaps at the ends, or randomly change places. (Perhaps this will become a more controlled point in the future, bu the NEC.)

After major faults we survey the conductors to see that they are still in place and did not got thrown and cut as the result. with three phase cables, this is never an issue.


 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Jim,


(B) Conductors of the Same Circuit.
All conductors of the same circuit and, where used, the grounded conductor and all equipment grounding conductors and bonding conductors shall be contained within the same raceway, auxiliary gutter, cable tray, cablebus assembly, trench, cable, or cord, unless otherwise permitted in accordance with 300.3(B)(1) through (B)(4).

I see nothing in 300.3(B) that requires a specific grouping or 'lay' of non-paralleled single conductors, it simply requires them to be 'in close proximity'. 300.(3)(B)(3) specifically allows single nonmetallic or nonmagnetic sheathed conductors (i.e. type W cord) to run 'differently' even though they are part of the same circuit.

And yes, during fault conditions there can be extreme movement of individual conductors.

 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
xxx There were electrcial installations before the Code existed and one of the impetus of developing the Code was - and continues to be - to make sure that the installations are safe and technically sound.
"Creo que no, Timoteo":roll:

There is little in the code requiring 'technically sound". In fact one might think 90.1.B (2008) suggests one may be about as moron as one wishes - just so long as the installation is essentally free from hazards

xxx I don't think a 'standard industry practice' that is TECHNICAL(L)Y wrong and potentially DANGEROUS should be continued, not to mention supported and allowed by the NEC, xxx
Assuming this comment applies to using adequately protected single conductor type W: How is this "TECHNICAL(L)Y wrong and and potentially DANGEROUS"?

JFTR I would go with your recomendation, 3C - w/G, type G, 4/0. It is fairly common and not too bad to handle - as long as it was direct connected and the installation did not require a cord and plug connection.

ice
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
"...
JFTR I would go with your recomendation, 3C - w/G, type G, 4/0. It is fairly common and not too bad to handle - as long as it was direct connected and the installation did not require a cord and plug connection.

ice
You need 300 amp wire at a temperature that is suitable for the terminations. Assuming 75?C terminations, you need 250kcmil 3c type G.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Given that this is a temporary installation, I think we need to look to Article 590. 590.4 does not permit single conductor cables for this application.
 

jusme123

Senior Member
Location
NY
Occupation
JW
Couple of questions. The equipment is a 200HP (skid mounted) compressor that will be installed routinely as a temporary supply for about one week duration and 6-8 weeks intervals. The compressor is 480V/3phase.

From the engineering standpoint I would want a 3/c+G single flex cable, but the manufacturer insists that single conductors are fine. I am not going into the list of problems of what unbundled single conductors can cause, but would like to know if the Code says it either way, namely that the individual conductors of a three phase circuit should be routed together in close proximity of each other. When it is routed in a conduit, raceway or common cable it is obviously not a problem. When routed as parallel feeders the A-B-C's of each circuit part should be bundled together, not A-A, B-B etc. and be equal length and routed on the same path. In the cable tray instructions the talk about single conductors side-by-side and triplexed, but is there a general paragraph addressing this issue about 3 phase circuits?
590.4 states that it has to be a multiconductor cord or cable because it is a temporary installation
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
xxx The equipment is a 200HP (skid mounted) compressor that will be installed routinely as a temporary supply for about one week duration and 6-8 weeks intervals. The compressor is 480V/3phase.

From the engineering standpoint I would want a 3/c+G single flex cable, but the manufacturer insists that single conductors are fine. xxx
I've got a couple of questions that really don't apply, but I am curious:

Is this compressor and cables a rental unit or are you purchasing?

You say it is a "manufacturer" and every 6 - 8 weeks - forever I guess. That makes me think purchased. But if purchased, why aren't you telling them what you want?

Now if it is a rental, did you happen specify cord and plug connectors?

Just curious.

ice
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
You need 300 amp wire at a temperature that is suitable for the terminations. Assuming 75?C terminations, you need 250kcmil 3c type G.
Yes that is true.

Most of the installations I deal with (and I deal with a lot*) of this type, have a J-box with a PDB rated for fine-strand conductors, right next to the controller. And that makes the 90C column effective.

Since few of the controllers are rated for fine-strand about the only other option is to use 250, as you correctly noted - and then crimp on an adapter - such as a Shoo-pin.

Additionally, 250 Type G is not usually readily available - 4/0 is. Unless one is feeding a PDC, and a neutral and equipment ground is required - then one must go to type W, 5C, and 4/0 is about as big as is available. AND, the electricians tell me they really prefer 4/0 over 250

The Worm (is suposed to be working - but is goofing off :cool:)

* "a lot" is defined as several temporary power installations of this ilk per year.
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
Given that this is a temporary installation, I think we need to look to Article 590. 590.4 does not permit single conductor cables for this application.
Yes, that is also my understanding of 590.4.

What would you (and others) think of installing cable tray on treated wood sleepers and putting the single conductor type W in the tray?

ice (still goofing off)
 

iwire

Moderator
Staff member
Location
Massachusetts
I don't think a 'standard industry practice' that is TECHNICALY wrong and potentially DANGEROUS should be continued, not to mention supported and allowed by the NEC,
You have an uphill battle as there are likely thousands of these installations daily.

That is how all higher current temporary power is provided. From theaters, to mines, to construction sites, to fairs, to portable power generation etc.

It seems you would have to prove that the existing practices are hazardous.

It is not practical to require portable power cables of 5 conductor 4/0 or 5 conductor 500 Kcmil, they would no longer be portable.













Just because it works it doesn't mean its right!

Just because something is perceived to be a danger does not mean that it is.
 
Status
Not open for further replies.
Top