What Size Conductor

[mpd]

This is something I have to say is very hard for me to determine as a plans examiner. We do not get to see the end result and 100% of the engineers we deal with will not break this factor down and since I know very clearly what MY paycheck covers versus what an engineers paycheck covers....I will always ask them the question and 100% of the time their response back to me is.....No Mr. Abernathy, the nonlinear loads are less than 50%. So I have to take them on face value since they put their licensed professional engineering seal on it...and I am no engineer.

i know what you mean paul, they are the design expert all you can do as a plan examiner is ask as many questions and make them show it on plans

 

[radiopet]

i know what you mean paul, they are the design expert all you can do as a plan examiner is ask as many questions and make them show it on plans

Yep...what gets to me is just last week I did one set for review and found over 100 errors. Wrong conductor sizes, many EGC's grossly undersized and you name it....wrong size GEC's from XFMR's and so on. I get this almost on a daily basis and would say my plan review passing rate is about 10% pass on the first review.

When I send my comments to the engineers I always get the same thing.."Thanks Paul for the comments, we will correct them right away and re-submit".....I honestly feel like a "PUNCH LIST" provider most of the time....very frustrating.

Just so you know.....My building official says I am overly critical and anal on my reviews. But also know he points fingers alot if someone gets something wrong also so go figure. You would think he knew this when he hired a code guy to do their plan review...lol

 

[ohmhead]

Ohmhead,

Well 3/0 CU is good for 200A and (2) sets would give you 400A capacity so I don't see him as wrong. Just decided to parallel them versus running a 600 KCMIL.

Unless I am missing something on that statement....it has been a rather long day for me...lol

Well yes i know the two runs of 3/0 is fine at 400 amps but read thur the top transformer is 500 mcm one run you see the load on that transformer can not be more than it can put out in watts .

So your brk size is not what you size the load of that transformer to.

The 112.5 can only give you 312.5 amps of output so at 125 percent of that we have 390amps the 500 mcm is only good for 380 amps but if your using dual rated wire then its fine . thwn-2 is good for 75 or 90 degree and it is passing the nec code rule on ampacity .

 

[mpd]

yes paul its amazing, i get the same thing from wrong size conductors, 600 amp breaker with a calculated load that exceeds the breaker, just did a fitout review that i knew what the available fault current from the shell permit, the engineer showed all 22k equipment, not even close. but if it got out of the my office with mistakes i would never hear the end of it from both ends.

 

[radiopet]

Well yes i know the two runs of 3/0 is fine at 400 amps but read thur the top transformer is 500 mcm one run you see the load on that transformer can not be more than it can put out in watts .

So your brk size is not what you size the load of that transformer to.

The 112.5 can only give you 312.5 amps of output so at 125 percent of that we have 390amps the 500 mcm is only good for 380 amps but if your using dual rated wire then its fine . thwn-2 is good for 75 or 90 degree and it is passing the nec code rule on ampacity .

lol...I could not see anything in the image posted...so I stuck with the 400A OCPD portion of the question....lol...but you can't use the 90 degree unless doing adjustments because I am sure those lugs are not rated for 90 degrees. So the 500kcmil is no good because it does not have a rating of 400A where as the 600kcmil does....and if ran with (2) 3/0 CU under the 75 degree ratings in two seperate raceways....and lets say less than 50% Nonlinear loads.....600Kcmil or (2) sets of 3/0CU would by the choice.

 

[ohmhead]

lol...I could not see anything in the image posted...so I stuck with the 400A OCPD portion of the question....lol...but you can't use the 90 degree unless doing adjustments because I am sure those lugs are not rated for 90 degrees. So the 500kcmil is no good because it does not have a rating of 400A where as the 600kcmil does....and if ran with (2) 3/0 CU under the 75 degree ratings in two seperate raceways....and lets say less than 50% Nonlinear loads.....600Kcmil or (2) sets of 3/0CU would by the choice.

Well so what your saying is dual rated wire -( 2) is only good for 75 deg rated terminals or termination ?

That insulation on that wire dual rated for say 400 amps is going to be unsafe ?

But if we load that same terminal with a larger conductor its ok ?

Dual rated wire THW-2 /THWN-2
is use on 75 and 90 deg so what good is it no one has 90 deg equipment never have i ever seen 90 deg gear or equipment .

Explain please in detail show the code section on this ?

 

[mpd]

Well so what your saying is dual rated wire -( 2) is only good for 75 deg rated terminals or termination ?

That insulation on that wire dual rated for say 400 amps is going to be unsafe ?

But if we load that same terminal with a larger conductor its ok ?

Dual rated wire THW-2 /THWN-2
is use on 75 and 90 deg so what good is it no one has 90 deg equipment never have i ever seen 90 deg gear or equipment .

Explain please in detail show the code section on this ?

take a look at 110.14 (C)

 

[ohmhead]

take a look at 110.14 (C)

Well read that it doesnt tell the theory behind the facts just tells you that its not acceptable .

Question insulation higher in rating the conductor runs at a higher ampacity its physical circular mill area is smaller but the same size as the circular mill area of the lower ampacity insulation conductor which will burn up its insulation at the high temp of ampacity the only difference is the insulation so if the wire can go to the level of ampacity of that load output of 312 amps even though the 125 % breaker was used what is the difference the transformer can not exceed its kva rating so why do they limit the 90 deg wire to just 75 deg ?

 

[chris kennedy]

Question insulation higher in rating the conductor runs at a higher ampacity its physical circular mill area is smaller but the same size as the circular mill area of the lower ampacity insulation conductor which will burn up its insulation at the high temp of ampacity the only difference is the insulation so if the wire can go to the level of ampacity of that load output of 312 amps even though the 125 % breaker was used what is the difference the transformer can not exceed its kva rating so why do they limit the 90 deg wire to just 75 deg ?

Thats got to be sentence of the day, well done Phil.

 

[joebell]

Why is that grounding electrode conductor marked with brown tape?

 

[joebell]

Well read that it doesnt tell the theory behind the facts just tells you that its not acceptable .

Question insulation higher in rating the conductor runs at a higher ampacity its physical circular mill area is smaller but the same size as the circular mill area of the lower ampacity insulation conductor which will burn up its insulation at the high temp of ampacity the only difference is the insulation so if the wire can go to the level of ampacity of that load output of 312 amps even though the 125 % breaker was used what is the difference the transformer can not exceed its kva rating so why do they limit the 90 deg wire to just 75 deg ?

I think it has more to do with the temp at the terminal than the insulation. If you load the 500 thhn up to its full load of 430 amps then the temp of the insulation at the terminal is suppose to be 90 degrees C. There are not many terminations out there listed above 75 degree C so you could have a problem at the termination of overheating.


I get what you are saying about the 312 amps and the transformer not putting out more than it's rating but the whole reason for my Op is we have a job were a 112.5 KVA was supposed to be installed and it is thought that the transformer actually is a 75. Alledgedly, according to the manufacturer, the same housing is used for both sizes, and it is believed that we have a housing marked 112.5 with 75 kva coils. We have had a thermal scan done and found temps over 200 degrees C on the coils.with load readings well over 200.

So in actuallity a transformer will just keep giving until it burns itself , kinda like a motor,up or a OCPD does its job. Why has the primary cb not tripped? IDK just trying to figure out how to correctly size and install conductors and OCPD's for the new 112.5 unit.

 

[infinity]

Why is that grounding electrode conductor marked with brown tape?

Probably because the GEC can be any color.

 

[Smart $]

Well read that it doesnt tell the theory behind the facts just tells you that its not acceptable .

Question insulation higher in rating the conductor runs at a higher ampacity its physical circular mill area is smaller but the same size as the circular mill area of the lower ampacity insulation conductor which will burn up its insulation at the high temp of ampacity the only difference is the insulation so if the wire can go to the level of ampacity of that load output of 312 amps even though the 125 % breaker was used what is the difference the transformer can not exceed its kva rating so why do they limit the 90 deg wire to just 75 deg ?

It's not about the wire insulation. It's about the rating of the terminal and connected equipment not being rated for 90?C operation. The temperature of current conductors within the equipment, the terminations, and the wire due to heat generated by current flowing through them is approximately the same... and, in general, excellent electrical conductors are also excellent heat conductors. While your wire's insulation, perhaps even the compression lug connected it, is rated for 90?C operation, the other connected equipment is only rated for 75? operation, so the current on the circuit cannot exceed that which makes any part in the general proximity of the 75?-rated equipment heat up to greater than 75?C.

 

[ohmhead]

Well i agree but were not exceeding 391 amps on a 400 amp breaker so why would this be a problem the wire is actually rated higher but the load of that transformer will not be or can not exceed that base amperage .

 

[joebell]

Probably because the GEC can be any color.

The man is correct. I always assummed it had to be marked green but I can not find anything in my code book saying it does, just equipment grounding/bonding conductors.


Never Assume

 

[Smart $]

...but the load of that transformer will not be or can not exceed that base amperage .

Actually the transformer can and very well may... for inrush and similar "spurts" of current... just shouldn't continuously as that would substantially shorten the useful life of the transformer.

Yet, I agree. If the max load is calculated at 125% continuous plus 100% of non-continuous loads and correlated to less than or equal to the transformer's nominal power rating, I see no reason why the secondary conductors cannot utilize the full 90?C rating of wire if the secondary OCPD is based on the 125% of the nominal power rating of the transformer.

 

[ohmhead]

Actually the transformer can and very well may... for inrush and similar "spurts" of current... just shouldn't continuously as that would substantially shorten the useful life of the transformer.

Yet, I agree. If the max load is calculated at 125% continuous plus 100% of non-continuous loads and correlated to less than or equal to the transformer's nominal power rating, I see no reason why the secondary conductors cannot utilize the full 90?C rating of wire if the secondary OCPD is based on the 125% of the nominal power rating of the transformer.

Well yes that is what i kinda went thur on our project that was the purpose of posting it i was kinda trying to help .

The riser that i posted the 112.5 kva trans secondary 500 mcm was acceptable do to the load and at 400 amps ocpd the transformer had a load lower than the 400 amps it was 310 amps plus or minus so that was my point .

I know there are current inrush and times of higher start and stop amp increases but most breakers can take it along with the terminals and i dont see a problem with normal daily loads if we had to calculate inrush on every load on /off time and loading and unloading i guess we would all be installing 750 mcm in every conduit .

I do however understand the higher ampacity of a insulated dual rated wire and that dual rated wire can be used but only if it does not exceed its 75 deg terminal rating and limiting by its maxium total current rating by terminal rating .

good post !!!



Take care be safe Smart $