Conductor protection

[mikehughes8]

From transfomer secondary (3 phase 4 -wire) contractor has installed 90c rated 500mcm which connects to a 400A Main in the service disconnect. Article 110.14 says "Conductors with higher temperature ratings, provided the ampacity of such conductors does not exceed the 75?C (167?F) ampacity of the conductor size used" After deration the conductor meets this requirement. What I am not sure of is whether or not I am covered by Article 240.4 when applying a breaker size that is above the ampacity of the conductors. Specifically, I am having difficulty understanding exactly what is required by 240.4(B)2

500mcm @ 374A (after deration for ambient temp) and a 400A breaker
Is this allowed?

 

[iwire]

Specifically, I am having difficulty understanding exactly what is required by 240.4(B)2

500mcm @ 374A (after deration for ambient temp) and a 400A breaker
Is this allowed?

That is a violation, we can not roll up to the next size with transformer secondary conductors.

See 240.21(C) last sentence.

They should have used 600 Kcmil

 

[mikehughes8]

Thanks for that. that was the piece I was missing.

But wouldn't have to be 700 for for 45 ambient temp???

 

[mikehughes8]

nevermind.....600mcm is correct

 

[iwire]

But wouldn't have to be 700 for for 45 ambient temp???

Why would you call it 45 C Ambient?

Is that the entire room or just inside the transformer?

If it is just the temp inside the transformer take a look at 310.15(A)(2) Exception.

 

[mikehughes8]

This is an outdoor install (Iraq) where ambient temp often meets or exceeds 45c ambient.

 

[iwire]

This is an outdoor install (Iraq) where ambient temp often meets or exceeds 45c ambient.

Ahh I see, I spontaneously combust at temperatures well below that.

 

[mikehughes8]

Oh but it's a dry heat :grin:

 

[Twoskinsoneman]

This is an outdoor install (Iraq) where ambient temp often meets or exceeds 45c ambient.

aren't you supposed to take an average of ambient temp, not max possible?

 

[Pierre C Belarge]

From transfomer secondary (3 phase 4 -wire) contractor has installed 90c rated 500mcm which connects to a 400A Main in the service disconnect. Article 110.14 says "Conductors with higher temperature ratings, provided the ampacity of such conductors does not exceed the 75?C (167?F) ampacity of the conductor size used" After deration the conductor meets this requirement. What I am not sure of is whether or not I am covered by Article 240.4 when applying a breaker size that is above the ampacity of the conductors. Specifically, I am having difficulty understanding exactly what is required by 240.4(B)2

500mcm @ 374A (after deration for ambient temp) and a 400A breaker
Is this allowed?

If the transformer is supplied as a utility, and this is a SERVICE, your installation is fine. (I do not know if they call it a utility in Iraq, hopefully you get the gist if what I am writing)




If the transformer is not utility supplied, and is on the secondary side of a service installation, then you have the issue that Bob mentioned.

 

[iwire]

aren't you supposed to take an average of ambient temp

IMO no, but it is certainly up for debate.


Do you size a conductor for average loading or max continuous loading?

 

[weressl]

aren't you supposed to take an average of ambient temp, not max possible?

Well, this is an another ambiguity in the NEC. There are some standards that define the time window of average, but the NEC stays silent on both if it is supposed to be averaged and if so what the value should be.

 

[Twoskinsoneman]

IMO no, but it is certainly up for debate.


Do you size a conductor for average loading or max continuous loading?

So you derate every circuit you install exposed to ambient temp based on record highs for that area?

I don't really have an opinion about it yet. I just happen to remember on a MH DVD he said it was an average but did not provide anything to explain or back up the statement.

 

[iwire]

So you derate every circuit you install exposed to ambient temp based on record highs for that area?

Yes, every single one and I use the highest recorded temp. :wink:

I don't really have an opinion about it yet.

It is tough to form a rock solid opinion with so little info to base it on.

I say we use the worst case temp only because that is the normal way the NEC works.

As far as I know the ampacities in 310.16 are based on conductor temp and insulation damage, few of us (outside of the utility guys) would think of sizing a conductor based on the average loading if they new that 4 hours a day for 30 times a year it would exceed the ampacity of the conductor and damage the insulation. Knowing we will exceed the temperature used for engineering the job a large number of times a year seems to me to be poor design.


That said Charlie Beck has a good argument for the other side, I believe he used the term 'insulation integrity bank' or something like that to basically say 'You can run the insulation over temp a certain number of times before damaging the insulation.' I can't say that he is wrong, I am sure we have all seen it happen in field first hand with no insulation damage.