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

[wwhitney]

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.

The question is if those two wires are one each from two different grounded voltage systems (from separate transformers), each of which is 277V to ground, and they are in the same conduit, and the voltage between them happens to be 554V, what insulation level does 300.3(C) require? Again two separate voltage systems but each referenced to ground, loads are segregated to one or the other, but the conductors are in a common conduit.

If that scenario only requires 277V insulation, then 300.3(C) is relying on "the concern" you mentioned not being an issue. And that's why 300.3(C) requires each conductor to have an insulation level at least the highest "circuit voltage" present, as the highest conductor-to-conductor voltage may be twice the highest "circuit voltage".

Whereas if the highest "circuit voltage" would be 554V in the above, then 300.3(C) does seem redundant, and it seems like it would suffice to insulate each conductor at a level of at least its voltage to ground. As the only bare metal it may be exposed to (e.g. metal conduit) is at ground potential.

But I don't really see how you can say the "circuit voltage" is 554V in the above, as any circuit will be entirely on one voltage system or the other. Only a fault could give you a 554V potential difference.

Cheers, Wayne

 

[sgtsparky]

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
View attachment 2572522	View attachment 2572520	View attachment 2572521
	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.

I always use ELI the ICE man mad a PIE
Or voltage leads current in an inductive circuit
Current leads voltage in a capacitive circuit
And P=I x E
Just one of the silly sayings I was taught in the 70’s I still remember.
But the bucket & hose explanations give a good visual.