Why can't different insulation ratings be used in same conduit

[GreenAcres]

I'm not planning on doing this, but I don't understand why the code doesn't allow mixing wires with different insulation ratings in the same conduit. For example I work with THHN which is 600v and PV wire which is 1K/2K rated. Each of them has insulation that is adequate to allow the wire from shorting out to to the metal conduit. So why isn't the insulation on 1K wire adequate to keep it from shorting out on the insulation of 600v wire? Or is there some other reason they can't be mixed? I would think the concern would be more about thermal ratings rather than voltage ratings.

 

[ramsy]

concern would be more about thermal ratings rather than voltage ratings.

Isn't Current transferred by inductance, and Voltage transferred by capacitance?

In attics I've seen sparks fly & felt shocks from 120 volts propagating 10+ feet along the outside of Romex cable.

There could be a pin hole, crack, or flying splice exposing energized wire somewhere in the cable run, and crawl spaces act like conductors, the same way condensation makes all raceways wet.

 

[petersonra]

You can have conductors with insulation of different ratings in the same conduit as long as the voltages do not exceed the lesser rating.

It is just a rule. Like many rules in the code it does not have to make sense.

 

[electrofelon]

I have always thought this was a dumb rule that most likely serves no purpose and was not the result of any issues.

 

[GreenAcres]

I have always thought this was a dumb rule that most likely serves no purpose and was not the result of any issues.

Good, I'm glad it doesn't make sense to anyone else either. If I had to take a wild guess, somebody ran two different voltage wires in the same conduit and then hooked the low voltage wires up to the high voltage equipment and caused a failure. So instead of requiring people to know what they're doing, they tried to solve the problem by requiring every wire in the conduit to support the highest voltage equipment.

 

[paullmullen]

Isn't Current transferred by inductance, and Voltage transferred by capacitance?

In attics I've seen sparks fly & felt shocks from 120 volts propagating 10+ feet along the outside of Romex cable.

There could be a pin hole, crack, or flying splice exposing energized wire somewhere in the cable run, and crawl spaces act like conductors, the same way condensation makes all raceways wet.

First... wow. only 120V to generate such an effect. OK. Just wow.

Your inductance/capacitance recollection may be related to the question of whether voltage leads current or current leads voltage under each situation. By "leading" or "lagging" we mean that if you could plot the waveform of the AC voltage and current you would see this:

Purely Resistive	Capacitive	Inductive
Voltage and Current will be aligned	Voltage will lead Current	Current will lead voltage
	I think of it like a bucket. The voltage goes up pouring water into the bucket, but no water flows out of the bucket until it is full. So the current comes after the voltage.	I think of it like a hose. The current flows into the hose but the pressure (voltage) does not go up until the hose is full. So voltage comes after the current.

 

[wwhitney]

FWIW, in the typical case that all the conductors have same voltage to ground and the same insulation level, you have 1 layer of insulation between each ungrounded conductor and ground (the conduit, or a bare EGC), and 2 layers of insulation between each pair of ungrounded conductors. It would take a double failure of the insulation to get a short between two ungrounded conductors.

Now boost the voltage of one of those ungrounded conductors above the insulation level but just increase the insulation level on that conductor. Then it only takes a single insulation failure to cause a problem. In that if that higher voltage conductor's insulation failed and the conductor is up against one of the other ungrounded conductors, that other conductor's insulation level is not rated for the higher voltage.

So requiring the higher insulation level for all the ungrounded conductors in the conduit is necessary to provide a level of safety that matches the typical case.

Cheers, Wayne

 

[electrofelon]

FWIW, in the typical case that all the conductors have same voltage to ground and the same insulation level, you have 1 layer of insulation between each ungrounded conductor and ground (the conduit, or a bare EGC), and 2 layers of insulation between each pair of ungrounded conductors. It would take a double failure of the insulation to get a short between two ungrounded conductors.

Now boost the voltage of one of those ungrounded conductors above the insulation level but just increase the insulation level on that conductor. Then it only takes a single insulation failure to cause a problem. In that if that higher voltage conductor's insulation failed and the conductor is up against one of the other ungrounded conductors, that other conductor's insulation level is not rated for the higher voltage.

So requiring the higher insulation level for all the ungrounded conductors in the conduit is necessary to provide a level of safety that matches the typical case.

Cheers, Wayne

Ok but how are you concluding this? I'd say if we have an insulation failure, we have a big problem. Very well could short out on the raceway and just destroy everything in the raceway regardless of insulation level. Maybe we shouldn't allow different voltages in the same raceway at all. I have a big problem with the "sounds like a good idea" or "it is possible this could happen" reasoning for code requirements.

 

[wwhitney]

Ok but how are you concluding this?

For an ungrounded-ungrounded fault, I think I laid out the logic fairly clearly, there's an obvious difference in the level of protection. No opinion on whether that difference is a problem or not, just pointing out the difference, which I would expect is the reasoning behind the requirement.

Agreed on ungrounded-ground faults, particularly for the case of metal conduit.

Cheers, Wayne

 

[wwhitney]

I mean, take the case of an insulated EGC (even if insulation is not required) and a non-metallic conduit. The maximum voltage that exists between any two conductors (including the EGC) in the conduit is a well-defined value. Requiring all conductor insulation (including the EGC) in the conduit to match that voltage level is a perfectly consistent approach. It requires a double insulation failure/damage to get a fault.

So I think of the above as a starting point. Obviously allowing some conductors at ground potential to be less well insulated (the EGC, or the conduit if metallic) makes the situation less consistent. But it's still consistent with respect to the ungrounded conductors.

Cheers, Wayne

 

[electrofelon]

For an ungrounded-ungrounded fault, I think I laid out the logic fairly clearly, there's an obvious difference in the level of protection. No opinion on whether that difference is a problem or not, just pointing out the difference, which I would expect is the reasoning behind the requirement.

Agreed on ungrounded-ground faults, particularly for the case of metal conduit.

Cheers, Wayne

Right, I get the logic. I am just saying it is not clear to me that that further redundancy is necessary. If this were code change being proposed, I would want to see evidence that there were cases of this happening and that it has been an issue which could be corrected with this further redundancy.

 

[electrofelon]

It requires a double insulation failure/damage to get a fault.

I propose we should have 2 OCPD 's protecting Branch circuits and feeders. A single ocpd could fail it's good to have a redundant one in case this happens.

 

[wwhitney]

Right, I get the logic. I am just saying it is not clear to me that that further redundancy is necessary.

OK, just to reiterate I'm not commenting on what redundancy level is appropriate or should be required. I'm just pointing out that the rule maintains the redundancy level that we have for the typical situation of all the conductors having the same L-L and L-G voltages. Getting rid of the rule would mean installations with different voltage levels would have a lower level of redundancy.

Maybe that's fine, as maybe the current level of redundancy we have for L-L faults is excessive, and it just arises as a necessary side effect of protecting against L-G faults. But that's something to think about and discuss, vs just saying the rule is stupid. The rule is not stupid; it just maintains a consistent level of redundancy between different configurations.

Cheers, Wayne

 

[electrofelon]

OK, just to reiterate I'm not commenting on what redundancy level is appropriate or should be required. I'm just pointing out that the rule maintains the redundancy level that we have for the typical situation of all the conductors having the same L-L and L-G voltages. Getting rid of the rule would mean installations with different voltage levels would have a lower level of redundancy.

Maybe that's fine, as maybe the current level of redundancy we have for L-L faults is excessive, and it just arises as a necessary side effect of protecting against L-G faults. But that's something to think about and discuss, vs just saying the rule is stupid. The rule is not stupid; it just maintains a consistent level of redundancy between different configurations.

Cheers, Wayne

Okay, sorry I don't mean to shoot the messenger, I understand you were just throwing out what could be an explanation for the rule. IMO that should not be sufficient substantiation to make a change (if this wasn't required and this was a proposal).

 

[jaggedben]

Considering once again that the voltage between two conductors of two separate ungrounded systems may be higher than either system's voltage to ground, I think Wayne's reasoning is apt. I forgot which code section it is that prohibits devices of such systems to be in the same box, but someone will remember.

 

[Jraef]

Nothing says the insulation values all have to match. What it says is that all wires must have insulation that is rated for the highest voltage present. So if you have 480V and 120V in the same raceway, everything must have insulation rated for AT LEAST480V. If some conductors have insulation rated 1000V and others rated 600V, that’s fine, because the highest voltage USED is 480V, so the criteria is met.

They don’t all need to be 1000V just because one of them exceeds the requirement by that much.

 

[wwhitney]

Considering once again that the voltage between two conductors of two separate ungrounded systems may be higher than either system's voltage to ground

I think you are driving at an important point I didn't consider.

I assume you didn't mean to specify that the systems are ungrounded, as then there would no defined voltage to ground for any circuit conductor, and most likely no defined voltage between conductors of the two separate systems. [Although as far as I know there could be, i.e. you could connect L1 of a 480V ungrounded delta to L1 of a 240V ungrounded delta

So if you meant two separate grounded systems, your statement is true, but unremarkable. As the voltage between two ungrounded conductors of a single system typically will be higher than the voltage to ground of either conductor. So I imagine what you meant is that the voltage between a conductor of one system and a conductor of the other system could be greater than the L-L voltage of either system individually.

E.g. you could have a 480V corner grounded delta and a 240V corner grounded delta. Depending on how you do the corner grounding, you could end up with 720V between two conductors if you aren't careful.

So that raises the question, when 300.3(C) says "All conductors shall have an insulation rating equal to at least the maximum circuit voltage applied to any conductor within the enclosure, cable, or raceway," for the above example, does it mean at least 480V? As that's the maximum voltage of any intentional circuit. It doesn't say "at least the maximum voltage between any two conductors in the raceway," which is what I assumed.

If so, all the redundancy that electrofelon and I were discussing is gone. The 720V between conductors is handled by each one being insulated to at least 480V.

Cheers, Wayne

 

[jaggedben]

Yes I meant two grounded systems of different voltages, sorry for the typo.

What I had in mind is the situation shown in the graphic here and discussed in other threads, which could be fairly common.

404.8 (b)

What is the intention of 404.8 (b)? Does this literally mean I can't have two switches from different circuits in the same box?

forums.mikeholt.com

And yes, the point is that in some situations the maximum voltage between two conductors in a raceway may be greater than the required insulation on either of those conductors, and so I believe we are relying on the combined insulation rating of both conductors. In the example in that graphic, all conductor insulation could be rated for 300V but there could be 381 volts between two conductors.

 

[GoldDigger]

Yes I meant two grounded systems of different voltages, sorry for the typo.

What I had in mind is the situation shown in the graphic here and discussed in other threads, which could be fairly common.

404.8 (b)

What is the intention of 404.8 (b)? Does this literally mean I can't have two switches from different circuits in the same box?

forums.mikeholt.com

And yes, the point is that in some situations the maximum voltage between two conductors in a raceway may be greater than the required insulation on either of those conductors, and so I believe we are relying on the combined insulation rating of both conductors. In the example in that graphic, all conductor insulation could be rated for 300V but there could be 381 volts between two conductors.

So the concern is that two 300 volt insulated wires might not be able to withstand 600V line to line? Agreed that might not be specifically tested, but I would expect it to be true for low voltages.

 

[jaggedben]

So the concern is that two 300 volt insulated wires might not be able to withstand 600V line to line? Agreed that might not be specifically tested, but I would expect it to be true for low voltages.

No the concern is that if the code rule were less stringent the combined insulation rating might be less than the actual voltage between some pair of conductors, or perhaps at least too close for comfort. As written the code rule is simple to follow while keeping us comfortably away from such a situation.