Parellel service conductors (feeders)

[Pierre C Belarge]

pdesai pe,

You say that these conductors run from the Utility Transfomer to the Main Disconnect, so

you will only terminate the Main Disconnect side and the Utility will terminate the Trans.

side. I hope the Utility does not cut the wires so they are unequal.

In our jurisdictions, the POCO does not terminate the secondary side of their pad mount transformers, the contractor does.

 

[Pierre C Belarge]

This has been a very interesting thread to read. One thing it does show is how different people perform their installations in different parts of the country, so I will not add to the already much discussed differences other than to say I agree with what Bob had to say.


I will add a comment about the inspection portion of this thread. If an inspector has concerns about an installation that is not readily distinguishable by visual inspection, I see no reason why he/she should not ask for some other means for verification of the installation. It is the inspector's prerogative to make this type of decision.

 

[ohmhead]

Well how would you make these the same length?

http://i611.photobucket.com/albums/tt195/stringking/right-photos.jpg


We terminate ouc transformers Orlando Utilitys now Florida Power does not let the electrical contractor terminate they do it .

Florida power checks megging on wires and checks phasen OUC just hooks them up without question .

And you better agree with Bob hes the moderator !

 

[iwire]

Well how would you make these the same length?

http://i611.photobucket.com/albums/tt195/stringking/right-photos.jpg

Those are raceways. :grin:

You can't make the raceways the same length, the code does not require that. Can you point out anyplace in this thread where anyone said you could make the raceways the same length?


But we have not been talking about raceways, we have been talking about conductors which are required to be the same length. If you put aside 'neat' you can keep the conductors the same length or closer to the same length. If your focused on neat ahead of length than IMO your doing it wrong.

And you better agree with Bob hes the moderator !

Thats right, don't forget it. :grin:

 

[Pierre C Belarge]

And you better agree with Bob hes the moderator !

He takes gifts in leu of agreeing with him. :grin:

 

[ohmhead]

Well just thinking a little more on this topic power loss heat and inductance.



Proximity Effect in multiple layered conductors also meaning groups of same phased conductors .

Proimity effect is when current in nearby conductors causes a time varying magnetic field and induces a circulating current inside a conductor its similar to skin effect but is more so a major effect which includes near multiple identical phased conductors in bunched or layed groups meaning near common phased conductors .

This effects ac resistance ,heat , and its current carrying ability is reduced .

Just thinking a 500 mcm resistance is fairly low does it really matter if its a little longer ?

We understand that resistance in parallel is very important but take a clamp on amp meter on any one phase of a multiple phased array of conductors and there is no differance shown funny thing we did it today on a 5000 amp switchboard of different length NEC code violation switchgear .

Now conduit length has lots to do with conductor length if one conduit containing 4 A phases is longer than one conduit containing 4 A phases.
Then the conductors inside must be longer or shorter .


You have 11 runs you will not be able to put equal groups in each this will now be unbalanced groups.
you have 4 in some and three in the others .

Also it takes more conduit and more money to install isolated phased conductors with odd numbers this only works with even runs of conduits .

 

[texassparky]

I was wanting to do an isolated phase install on my project due to the very tight make up area in the switchboards. 8 sets of 600 MCM, 2 times in a 4000 amp MTM board......The POCO doesnt have a problem with it, but the engineer is saying NO...

He is saying with no EGC, and the neutrals in different conduits from the CCC's, there is no way to clear a fault...

Anybody got an answer ???:-?

 

[iwire]

I was wanting to do an isolated phase install on my project due to the very tight make up area in the switchboards. 8 sets of 600 MCM, 2 times in a 4000 amp MTM board......The POCO doesnt have a problem with it, but the engineer is saying NO...

He is saying with no EGC, and the neutrals in different conduits from the CCC's, there is no way to clear a fault...

Anybody got an answer ???:-?

If that is what the EE thinks then it is time for them to find a new job.

 

[iwire]

We understand that resistance in parallel is very important but take a clamp on amp meter on any one phase of a multiple phased array of conductors and there is no differance shown funny thing we did it today on a 5000 amp switchboard of different length NEC code violation switchgear .

That seems very odd as I have never seen equal loading on parallel conductors.

Me thinks you need a better meter or operator.

Now conduit length has lots to do with conductor length if one conduit containing 4 A phases is longer than one conduit containing 4 A phases.
Then the conductors inside must be longer or shorter

.

Yes and electrically and code wise it does not mater at all if phase A is longer than phase B.

You have 11 runs you will not be able to put equal groups in each this will now be unbalanced groups.
you have 4 in some and three in the others .

Also it takes more conduit and more money to install isolated phased conductors with odd numbers this only works with even runs of conduits .

True, but many times you have even numbers.

No one method is always the best.

 

[texassparky]

I was wanting to do an isolated phase install on my project due to the very tight make up area in the switchboards. 8 sets of 600 MCM, 2 times in a 4000 amp MTM board......The POCO doesnt have a problem with it, but the engineer is saying NO...

He is saying with no EGC, and the neutrals in different conduits from the CCC's, there is no way to clear a fault...

Anybody got an answer ???:-?

If that is what the EE thinks then it is time for them to find a new job.

Well I expected that you would be the one to help, so how is the fault cleared then ?

 

[roger]

so how is the fault cleared then ?

The neutral or an oposing phase would clear a fault. If there is simply a couple of nicked wires in the same conduit you wouldn't have a fault.

Roger

 

[ohmhead]

That seems very odd as I have never seen equal loading on parallel conductors.

Me thinks you need a better meter or operator.



.

Yes and electrically and code wise it does not mater at all if phase A is longer than phase B.





True, but many times you have even numbers.

No one method is always the best.

The meters fine so is the operator if you clamp on to one phase of a parallel group of feeders its kinda plus or minus one or two amps but divided up between the conductors of anyone phase . Meaning one phase same phase .

Ive never seen 4000 amps on one conductor before its why they parallel them .

Thats why the resistance needs to be the same so the current is balance as much as possible or you will have a unbalance so there ya go my point is now made you can not have equal length even if it just looks like that in you switchboard but underground its not .


Yes i do agree on a low number even you might get lucky .





I just said A PHASE only and that no one can get a equal length on multiple runs of one phase on most parallel underground stub ups into a switchboard .

 

[texassparky]

If that is what the EE thinks then it is time for them to find a new job.

The neutral or an oposing phase would clear a fault. If there is simply a couple of nicked wires in the same conduit you wouldn't have a fault.

Roger

Ok after seeing your responses I realized I didn't really ask the question right...I guess I should have said the lack of a fault...

Basically what he is worried about is if only 1 conduit was hit containing only A phase it will not fault, so say if it was hit with a backhoe, he could likely energize the backhoe...

Hopefully that is better...

 

[iwire]

Basically what he is worried about is if only 1 conduit was hit containing only A phase it will not fault, so say if it was hit with a backhoe, he could likely energize the backhoe...

That is possible but the NEC does not seem concerned about it.

I am interested in how he thinks a fault in service conductors can be cleared?

There is no OCPD so mixed or isolated it will not shut down.

 

[texassparky]

That is possible but the NEC does not seem concerned about it.

I am interested in how he thinks a fault in service conductors can be cleared?

There is no OCPD so mixed or isolated it will not shut down.

Exactly, ok so you and I are on the same page then :smile:

that is my contention, regardless of the install there is no OCPD so in either way of installation, the best you could hope for is to blow the bayonet fuses at the transformer or have to go all the way back to the fuses in the S&C feeding the transformer...

Either way it is going to be bad...

 

[iwire]

Either way it is going to be bad...

Can't put it any better than that.:grin:


If the engineer is truly concerned about that, concrete encasement or lots of marking ribbons can reduce the chances of them being dug up.

 

[ohmhead]

Well just a thought during a ground fault current must flow to open a brk to prevent shock or damage to equipment or person .

Even a few ohms in a grounding circuit will effect or prevent or slow the set time curve of a breaker to effectively do its job.

Isolated parallel conductors present a problem to this time a ground must be full sized in each raceway per 250.122 f .

If you have a ground fault in just one conductor in a conduit with three isolated other paralleled same phased conductors its level of safe trip time and current level of protection is now being compromised by a larger grounding conductor passing a larger current and more damage will be the result of this . Its resistance the faulted phase conductor is higher and the resistance of the grounding conductor is lower i would think they should be closer .

The trip time is extended passed the point of safe operation for that one phase conductor .

Now this is just what i think and we all know iam just a electrician so engineers tell me iam wrong and explain this to me comments ?

 

[iwire]

The ground fault current will remain close to the same for 'normal' or isolated phase installations.

Either way there will be a full size EGC in each raceway and either way all conductors connected to the effected phase will contribute to the fault current.

 

[ohmhead]

The ground fault current will remain close to the same for 'normal' or isolated phase installations.

Either way there will be a full size EGC in each raceway and either way all conductors connected to the effected phase will contribute to the fault current.

Well ok your saying in any case or condition isolated paralleled conductors will trip a differential current relay and resistance doesnt play a part .

What if i had a floating neutral ?

A delta secondary feeding a motor load 3 phase and it grounded out one phase to case of motor and the breaker held for weeks ?

And the motor control center was isolated parallel fed no neutral just a ground .

What iam saying is when you have a ground fault the grounding conductor doesnt always take the fault there are other paths and the neutral return is also one of them and worst condition would be a one conductor in a group of paralleled conductors in a isolated arrangement .

This would make contact in circuit a resistance loop is now from that point to the grounding point and never trip the differential relay until it was too late .
The grounding conductor is for a much high trip current the single conductor cant take that under a condition of time and trip the minor fault now becomes a major fault .

High resistance will not trip a breaker fault or no fault but isolated paralleled conductors can present lots of other conditions you dont have with a mixed group of conductors .

If you have ever work a data center or a power plant you are not going to install a isolated run of paralleled conductors and what i just stated above was from a Black & Vech electrical engineer he said dont put all you eggs in the same basket its a EMI nite mare in a data center .
And in a Power Plant its a down time under a critical outage if you can run your load with less but not with all conductors lost.

Thats why they dont like paralled isolated runs .