208 Volts and 4160 Volt Conductors Pull Box

[dkidd]

There was an attempt to add handholes in 2011. It was unanimously rejected.

2011 ROP

3-16 Log #1244 NEC-P03 Final Action: Reject
(300.3(C)(2)(e))
_______________________________________________________________
Submitter: Dan Leaf, Seneca, SC
Recommendation: Revise text as follows:
In handholes and manholes if the conductors of each system are permanently
and effectively separated from the conductors of the other system. and Where
installed in manholes conductors shall be securely supported and fastened to
racks insulators, or other approved supports. Conductor of different voltage
rating shall be permitted.
Conductors having nonshielded insulation and operating at different voltage
levels shall not occupy the same enclosure, cable, or raceway unless they
comply with 300.3(C)(2)(e).
Substantiation: Handholes should be included. Racks, insulators and supports
should not be necessary for handholes. Proposed deletion is superfluous, this
subsection concerns circuit ratings, not conductor ratings. Section 300.3 should
be referenced.
Panel Meeting Action: Reject
Panel Statement: Providing permanent and effective separation of 600-volt or
less from over 600-volt conductors within a handhole would be very difficult,
if not impossible. The submitter should provide additional technical
substantiation and data showing handholes and the method of providing
permanent and effective separation.
Number Eligible to Vote: 14
Ballot Results: Affirmative: 14
_______________________________________________________________

 

[jap]

Lets try this from a different angle: say you had installed conductors from 4160 and 208 in the same box, coming in from 2 different conduits/exiting 2 different conduits... simply one box was used as a dual pull point. Inspector fails it citing 300.3(C)(2). Do you think you would win an argument with him or his supervisor, or put in another pull box?

jap, I understand they have different requirements for CCC and sizing, however a pull box installed in a complete raceway has to by definition be part of that raceway - box to panel and everything in between.

Surely this question has been proposed to the CMP and a ruling has already been issued?

I wouldn't install conductors from 4160 and 208 in the same box at all, but, If I did, I'd think I'd loose any argument I'd be silly enough to bring up.

I agree and feel the box is part of the raceway system but others seem to feel that it may be not due to wording.
There was a post earlier about what actually a "Roof" was and seems we don't actually know what that is either.
I thought I did before joining this forum.

But there has been indication that some may feel that just because a medium voltage cable may be in the same vicinity as a 600v cable and may accidently have a nick in the insulation that it would cause the 600v insulation next to it to fail. That's simply not the case.

Common sense would say not to mix the 2 in the same raceway but there should be a solid explanation of why not.

Same as service conductors mixed with feeder conductors that are the same voltage. No, were not supposed to, but is there really an epedimic of service conductors insulation breaking down and feeder conductor insulation breaking down by some miracle that it happens to be the same phase on both where the Feeder overcurrent device could be bypassed instead of shorting out in the raceway and blowing up?

There are tons of up and down pole services around the country that do this very thing that I don't hear of a lot of problems with.

Just would like to know that the arguments for keeping them seperated are being made for the right reasons.

JAP>

 

[Bluewyvern]

So your saying a 600v cable will fail by the exposed conductor of a medium voltage cable simply touching it ?

JAP>

Yes, assuming the medium voltage cable insulation has been damaged and the damaged portion is near or touching the low voltage cable insulation, it will fail as well. It is not rated for that level of potential.

 

[jap]

Yes, assuming the medium voltage cable insulation has been damaged and the damaged portion is near or touching the low voltage cable insulation, it will fail as well. It is not rated for that level of potential.

That is simply not true.
The lower voltage cable is not carrying the higher voltage in that scenario to be able to damage the lower voltage cable's insulation.

JAP>

 

[JFletcher]

I wouldn't install conductors from 4160 and 208 in the same box at all, but, If I did, I'd think I'd loose any argument I'd be silly enough to bring up.

I agree and feel the box is part of the raceway system but others seem to feel that it may be not due to wording.
There was a post earlier about what actually a "Roof" was and seems we don't actually know what that is either.
I thought I did before joining this forum.

But there has been indication that some may feel that just because a medium voltage cable may be in the same vicinity as a 600v cable and may accidently have a nick in the insulation that it would cause the 600v insulation next to it to fail. That's simply not the case.

Common sense would say not to mix the 2 in the same raceway but there should be a solid explanation of why not.

Same as service conductors mixed with feeder conductors that are the same voltage. No, were not supposed to, but is there really an epedimic of service conductors insulation breaking down and feeder conductor insulation breaking down by some miracle that it happens to be the same phase on both where the Feeder overcurrent device could be bypassed instead of shorting out in the raceway and blowing up?

There are tons of up and down pole services around the country that do this very thing that I don't hear of a lot of problems with.

Just would like to know that the arguments for keeping them seperated are being made for the right reasons.

JAP>

I thought it was for theft of power reasons rather than any electrical failure reasons, and if so that makes perfect sense from a POCO perspective. I agree common sense says not to mix 4160 with 208, tho the question infinity posed is "is it prevented in pull boxes?". If not, it should be, or will be the first time an electrician dies working on such an incredibly bad idea. I dont think anyone who has posted here would do that, more a hypothetical... I hope.

I'll have to find that "roof" thread.

 

[jap]

I thought it was for theft of power reasons rather than any electrical failure reasons, and if so that makes perfect sense from a POCO perspective. I agree common sense says not to mix 4160 with 208, tho the question infinity posed is "is it prevented in pull boxes?". If not, it should be, or will be the first time an electrician dies working on such an incredibly bad idea. I dont think anyone who has posted here would do that, more a hypothetical... I hope.

I'll have to find that "roof" thread.

The rule is not to defend against theft of power.
It's a NEC rule.

JAP>

 

[infinity]

No more than a 8 gang switch box containing 20 or so CCC needs derating, or a 42 circuit panel needs derating. Just because certain parts of a complete raceway have different requirements... I guess I'm having a problem with this as a pull box would be somewhere in the middle rather than at the end. You're not going to find 4160 mixed with 208 in a switch box, or a panel, or a conduit... why is a pull box exempted? If it truly is, then my answer is "by mistake".

I've said my peace, win or lose. and I hope I never come across a pull box with 4160 in it if I'm working on a 208 or 480 system. I can imagine MH making a video on this and saying "No! you can't mix 4160 with 208! Anywhere!"

eta: if the conductors were fixed and there was a barrier ala the manhole requirement, I'd be okay with it. iow if pull boxes were treated the same as manholes, or (e) mentioned them specifically along with manholes.

You're wrong in thinking that an 8 gang switch box requires derating just because it has 20 CCC's.

For the record anyone who thinks that a pullbox is a raceway is completely incorrect. :slaphead:

 

[kwired]

For the record anyone who thinks that a pullbox is a raceway is completely incorrect. :slaphead:

I agree.

some supporting code:

300.18 Raceway Installations.
(A) Complete Runs.


Raceways, other than busways or exposed raceways having hinged or removable covers, shall be installed complete between outlet, junction, or splicing points prior to the installation of conductors.

Article 314

Outlet, Device, Pull, and Junction Boxes; Conduit Bodies; Fittings; and Handhole Enclosures

I would think NEC has similar classification for all the items in the title of 314, and raceway is not it.

 

[JFletcher]

You're wrong in thinking that an 8 gang switch box requires derating just because it has 20 CCC's.

For the record anyone who thinks that a pullbox is a raceway is completely incorrect. :slaphead:

Actually you misread my post.

Your post:

"If a pullbox is a raceway then whenever you had more than 3 CCC's in it you would need to apply derating."

I agree a pull box isnt a raceway and that you do not need derating...

"No more than a 8 gang switch box containing 20 or so CCC needs derating, or a 42 circuit panel needs derating."

None of these need derating.

A pullbox is partof a raceway, not a raceway.

 

[infinity]

Actually you misread my post.

Your post:

"If a pullbox is a raceway then whenever you had more than 3 CCC's in it you would need to apply derating."

I agree a pull box isnt a raceway and that you do not need derating...

"No more than a 8 gang switch box containing 20 or so CCC needs derating, or a 42 circuit panel needs derating."

None of these need derating.

You're correct I misread your post. Sorry for the confusion. :ashamed1:

 

[Bluewyvern]

That is simply not true.
The lower voltage cable is not carrying the higher voltage in that scenario to be able to damage the lower voltage cable's insulation.

JAP>

It's the difference in potential between the low voltage conductor and the high voltage conductor that breaks the insulation down. The failure may not occur instantly, but as soon as that low voltage conductor provides some path to ground related to the high voltage circuit you will very likely get a failure of the low voltage insulation. When linemen bond to a high voltage conductor they are totally isolated from any grounds, if they were grounded or near a ground trying to bond to a 250kv line they would be toast.

 

[jap]

It's the difference in potential between the low voltage conductor and the high voltage conductor that breaks the insulation down. The failure may not occur instantly, but as soon as that low voltage conductor provides some path to ground related to the high voltage circuit you will very likely get a failure of the low voltage insulation. When linemen bond to a high voltage conductor they are totally isolated from any grounds, if they were grounded or near a ground trying to bond to a 250kv line they would be toast.

If that's true, then as soon as any low voltage conductor was installed in close proximity to a high voltage cable the high voltage cable immediately starts eating at the low voltage cable's insulation and the low voltage cable would always be 100% guaranteed to fail.

Is that the case?


JAP>

 

[Bluewyvern]

If that's true, then as soon as any low voltage conductor was installed in close proximity to a high voltage cable the high voltage cable immediately starts eating at the low voltage cable's insulation and the low voltage cable would always be 100% guaranteed to fail.

Is that the case?


JAP>

Not unless the high voltage cable is not insulated or has damaged insulation. Insulated high voltage conductors have sufficient insulation to prevent this; however, you don't want to install like this for the same safety reason. If the high voltage cable insulation becomes damaged, then the conductor would essentially be against the low voltage insulation and would/could result in a failure. It also depends on the extent of the insulation damage, my assumption is that it has totally failed for some reason. That is also what the NEC typically assumes.

 

[jap]

Not unless the high voltage cable is not insulated or has damaged insulation. Insulated high voltage conductors have sufficient insulation to prevent this; however, you don't want to install like this for the same safety reason. If the high voltage cable insulation becomes damaged, then the conductor would essentially be against the low voltage insulation and would/could result in a failure. It also depends on the extent of the insulation damage, my assumption is that it has totally failed for some reason. That is also what the NEC typically assumes.

That can be said about any cable's insulation.

What I want to know is what about the high voltage cable would cause damage to a low voltage's insulation by simply being against it or near it ?

To me, the high voltage cable does not contain any ill effects on the low voltage cable's insulation what so ever.

Unless there's something I'm not aware of.

Now, if over time the insulation deteriorated on both the high and low voltage cable and they happened to create a path between them, or they rubbed together through the insulation and the conductors got together somehow, I could see where damage could occur.

Otherwise to me, as long as the insulation is still on the low voltage cable, whether the high voltage cable was next to it or laying on it without insulation, I don't see how it would cause the low voltage cable's insulation to fail.

Maybe someone can explain this better.


JAP>

 

[Bluewyvern]

That can be said about any cable's insulation.

What I want to know is what about the high voltage cable would cause damage to a low voltage's insulation by simply being against it or near it ?

To me, the high voltage cable does not contain any ill effects on the low voltage cable's insulation what so ever.

Unless there's something I'm not aware of.

Now, if over time the insulation deteriorated on both the high and low voltage cable and they happened to create a path between them, or they rubbed together through the insulation and the conductors got together somehow, I could see where damage could occur.

Otherwise to me, as long as the insulation is still on the low voltage cable, whether the high voltage cable was next to it or laying on it without insulation, I don't see how it would cause the low voltage cable's insulation to fail.

Maybe someone can explain this better.


JAP>

Here is an analogy. You are standing on your carpet on a cool winter day and touch something grounded such as a grounded outlet box. Nothing typically will happen. If you shuffle across the floor, building up static, and move your hand close to the grounded box a spark will jump from your fingertip to the box before you touch it. Say the gap is a millimeter when the spark appears. If you repeat this experiment you will find that depending on how high your static voltage charge is the spark will be longer or shorter and in some cases no spark at all. Look at the air as an insulator that has higher insulating properties as it gets thicker (further from the box). In the low voltage state, before you shuffled across the floor, the millimeter gap has plenty of insulating capability and no current flows until you touch the grounded box. But when you shuffle across the floor and build up a large static charge that same millimeter of insulation is inadequate to prevent a spark (current) from jumping across the gap. Since we are just talking about static electricity with no continuous regeneration it discharges and goes away quickly. As with the small air gap, it's the physics of the insulating material (composition and thickness) that determines how much potential difference (voltage) it can withstand before it breaks down. A low voltage insulated cable is rated at 600 volts and may be able to withstand twice or more than that value, but it's not guaranteed. Say you have a low voltage insulated conductor and one end of the conductor is attached to ground. If a bare conductor is placed against the insulated conductor and the voltage increased on the bare conductor it will reach a point where the potential difference across the insulation is too high for it to withstand and the high voltage will jump right through it, just like the spark jumped through the air gap in the analogy. Since the bare conductor has a power supply feeding it the current will continue to flow until the overcurrent device trips or the insulated conductor breaks loose from ground. It's even possible that the ground is a high impedance ground and it will continue to flow and hold the entire low voltage circuit at an unexpectedly high voltage.

 

[jap]

Here is an analogy. You are standing on your carpet on a cool winter day and touch something grounded such as a grounded outlet box. Nothing typically will happen. If you shuffle across the floor, building up static, and move your hand close to the grounded box a spark will jump from your fingertip to the box before you touch it. Say the gap is a millimeter when the spark appears. If you repeat this experiment you will find that depending on how high your static voltage charge is the spark will be longer or shorter and in some cases no spark at all. Look at the air as an insulator that has higher insulating properties as it gets thicker (further from the box). In the low voltage state, before you shuffled across the floor, the millimeter gap has plenty of insulating capability and no current flows until you touch the grounded box. But when you shuffle across the floor and build up a large static charge that same millimeter of insulation is inadequate to prevent a spark (current) from jumping across the gap. Since we are just talking about static electricity with no continuous regeneration it discharges and goes away quickly. As with the small air gap, it's the physics of the insulating material (composition and thickness) that determines how much potential difference (voltage) it can withstand before it breaks down. A low voltage insulated cable is rated at 600 volts and may be able to withstand twice or more than that value, but it's not guaranteed. Say you have a low voltage insulated conductor and one end of the conductor is attached to ground. If a bare conductor is placed against the insulated conductor and the voltage increased on the bare conductor it will reach a point where the potential difference across the insulation is too high for it to withstand and the high voltage will jump right through it, just like the spark jumped through the air gap in the analogy. Since the bare conductor has a power supply feeding it the current will continue to flow until the overcurrent device trips or the insulated conductor breaks loose from ground. It's even possible that the ground is a high impedance ground and it will continue to flow and hold the entire low voltage circuit at an unexpectedly high voltage.

But the high voltage does not sit there as soon as it's energized with the sole purpose of deteriorating the low voltage cable's insulation so it can jump to the conductor inside of it.

If conditions are right, it will jump through the low voltage cable's insulation even if the insulation on the low voltage conductor is in absolute perfect condition.

JAP>