How come u can't run a single phase in each conduit in parallel feeds?

[LouisianaApprentice]

Seems like it'd actually be safer as there is no possibility of line to line short. I'm just thinking of make up in big mdp's. Is this just to keep the distance of each phase as equal as possible? It seems if u run all your pvc close enough you'd be within 3ft on makeup.

 

[LouisianaApprentice]

It would also be much easier to install ct's

 

[LarryFine]

You can't do it with metallic conduit, because induced magnetic fields will induce currents in the conduits that will make them get hot, not unlike an old-fashion transformer-based soldering gun.

You can do it with PVC, but you need to cut slots between every knockout involved with the group, at each end, in each metal enclosure, effectively creating one large opening in the metal.

 

[LouisianaApprentice]

You can't do it with metallic conduit, because induced magnetic fields will induce currents in the conduits that will make them get hot, not unlike an old-fashion transformer-based soldering gun.

You can do it with PVC, but you need to cut slots between every knockout involved with the group, at each end, in each metal enclosure, effectively creating one large opening in the metal.

Ok most of ours are pvc anyways so if u are coming straight up into it with just pvc cut off at the slab is that ok?

 

[winnie]

Ok most of ours are pvc anyways so if u are coming straight up into it with just pvc cut off at the slab is that ok?

There are other details to ensure you don't have unintentional transformers (eg being careful about rebar) but yes.

Take a look https://forums.mikeholt.com/threads/example-of-300-3-b-3.61833/post-1023713

Jon

 

[xformer]

You can't do it with metallic conduit, because induced magnetic fields will induce currents in the conduits that will make them get hot, not unlike an old-fashion transformer-based soldering gun.

You can do it with PVC, but you need to cut slots between every knockout involved with the group, at each end, in each metal enclosure, effectively creating one large opening in the metal.

You could also leave out the metal plate at the bottom of the enclosure.

 

[xformer]

What if this installation is in a concrete bank with ferrous rebar installed for support?

 

[don_resqcapt19]

What if this installation is in a concrete bank with ferrous rebar installed for support?

That is an issue if the rebar passes between the conduits, but not an issue if the rebar is all on the outside of all of the conduits.

 

[jim dungar]

And don't use any metallic elbows.

 

[winnie]

My understanding is that the big restriction is on ferromagnetic metal (steel and such).

You have to run an EGC in each conduit (if this is a feeder and an EGC is required), and presumably you will see circulating current in all of the parallel EGCs. But unless I am mistaken code _requires_ this, and I've not heard that it is a problem.

-Jon

 

[electrofelon]

Despite the niceties this method affords on makeup and CT installation, the disadvantage is derating will likely be more of a factor.

 

[LouisianaApprentice]

Despite the niceties this method affords on makeup and CT installation, the disadvantage is derating will likely be more of a factor.

How come? Isn't it the same amount of ccc's

 

[winnie]

In most situations, you do not count the neutral as a CCC. Say you have 4 conduits with a phase in each conduit:

HHHNG, HHHNG, HHHNG, HHHNG

You have 3 CCC in each of 4 conduits.

Now change to isolated phase:
HHHHG, HHHHG, HHHHG, NNNNG

Now you have 4CCC in each of 3 conduits, and 1 conduit with no CCC.

Depending upon the size of the neutral relative to the phase conductors, if you have oversized conductors for voltage drop, if you are using 75C or 90C terminations, this may or may not make a difference.

-Jon

 

[wwhitney]

Now change to isolated phase:
HHHHG, HHHHG, HHHHG, NNNNG

Now you have 4CCC in each of 3 conduits, and 1 conduit with no CCC.

That should be 4 conduits each with 4 CCCs.

Cheers, Wayne

 

[winnie]

That should be 4 conduits each with 4 CCCs.

Cheers, Wayne

Are you counting the neutral as a CCC?

Jon

 

[gadfly56]

Are you counting the neutral as a CCC?

Jon

Well, the 4 neutrals in the last conduit under your second scenario aren't there for decoration.

 

[winnie]

Well, the 4 neutrals in the last conduit under your second scenario aren't there for decoration.

Agreed, they actually have current flowing in them, but they don't _count_ as 'current carrying conductors' for purpose of derating.

-Jon

 

[gadfly56]

Agreed, they actually have current flowing in them, but they don't _count_ as 'current carrying conductors' for purpose of derating.

-Jon

Ahhhh, now I get you.

 

[wwhitney]

Agreed, they actually have current flowing in them, but they don't _count_ as 'current carrying conductors' for purpose of derating.

Physics-wise, that clearly only applies when one conduit has a full boat. I.e. L1, L2, L3, and N together can only be 3 simultaneous CCCs. Separate those out into separate conduits, and all 4 N can be carrying current at once, and that conduit has 4 CCCs, not 0 CCCs.

The NEC language on this point may be lacking, but that would be an oversight in the NEC.

Cheers, Wayne

 

[LarryFine]

Agreed, they actually have current flowing in them, but they don't _count_ as 'current carrying conductors' for purpose of derating.

They do when current they carry does not reduce current in other conductors in the same conduit.

Load up any one phase, and the neutrals will have to carry the same current in a different conduit.

Those currents are not subtractive, which is why this thread about phase isolation is taking place.