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54067323

join:2012-09-25
Tuscaloosa, AL
reply to lutful

Re: Wire Size

said by lutful:

OK, let's make the temperature 49C or whatever max temp does not trigger that 25% derating.

Anything over 86 degrees must be de-rated.
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John Galt
Forward, March
Premium
join:2004-09-30
Happy Camp
kudos:8
reply to 54067323

Re: Wire Size

How hot does conduit get in the direct sun...??



54067323

join:2012-09-25
Tuscaloosa, AL
reply to lutful

said by lutful:

We can also verify that any better choice of cable (75C rated #1 or 90C rated #2) will actually keep the insulation below 60C ...

Also keep in mind it is quite possible the lugs of the generator and the transfer switch are not rated for 90C precluding the use of 90C rated cable.


54067323

join:2012-09-25
Tuscaloosa, AL
reply to John Galt

said by John Galt:

How hot does conduit get in the direct sun...??

On a sunny day grey PVC can hot enough you wouldn't want to touch it.

rich3236

join:2006-08-22
Fort Lauderdale, FL

1 recommendation

reply to 54067323

I have never seen any equipment lugs rated at 90c, they are rated at 75c. The 90c table is only used for derating purposes



John Galt
Forward, March
Premium
join:2004-09-30
Happy Camp
kudos:8
reply to 54067323

said by 54067323:

said by John Galt:

How hot does conduit get in the direct sun...??

On a sunny day grey PVC can hot enough you wouldn't want to touch it.

Believe me, I know that. I have worked in the SoCal deserts before.

It gets so hot you can't touch anything after 10 AM...

We were pulling 750MCM copper to get 300A capacity.

lutful
... of ideas
Premium
join:2005-06-16
Ottawa, ON
kudos:1

1 edit

said by John Galt:

said by 54067323:

said by John Galt:

How hot does conduit get in the direct sun...??

On a sunny day grey PVC can hot enough you wouldn't want to touch it.

Believe me, I know that. I have worked in the SoCal deserts before.

Suppose transfer switch user manual dictates maximum wire size ... leaving a single choice of 90C rated 1/0 copper (170A*0.82=139A) ... and we only have to suggest insulated conduit and generator shelter suitable for "SoCal desert" or Death Valley.

*** OK, there are outdoor metal conduits with plastic coating ... just in case someone "dreams" of installing a 20kW generator in SoCal desert despite all the fear-and-doubt.


54067323

join:2012-09-25
Tuscaloosa, AL

said by lutful:

Suppose transfer switch user manual dictates maximum wire size ... leaving a single choice of 90C rated 1/0 copper

Maybe 75C.


mackey
Premium
join:2007-08-20
kudos:12

1 recommendation

reply to lutful

said by lutful:

said by John Galt:

If one is willing to base design decisions on wild supposition, then we can make pretty much anything work.

No. You could not make #14 AWG work.

Actually, I remember a (now deleted) thread of some guy asking how many #14's he needed to parallel for something like a 80A circuit...

/M


54067323

join:2012-09-25
Tuscaloosa, AL

said by mackey:

Actually, I remember a (now deleted) thread of some guy asking how many #14's he needed to parallel for something like a 80A circuit...

That was something I did and it worked well for the few days we needed it. The wiring being European, turned out to be some metric gauge some where around 13AWG tinned and stranded copper and since there where many more conductors then I thought was present to the locations we needed it we almost doubled the capacity for the amperage we where running.

In the beginning I wanted to pull out the existing wiring and pull in new but the wiring refused to budge, after the fact I discovered whatever shipyard built that tub filled the ends of the pipes with some form of what appeared to be blue epoxy.


54067323

join:2012-09-25
Tuscaloosa, AL

2 edits
reply to lutful

said by lutful:

Suppose transfer switch user manual dictates maximum wire size ... leaving a single choice of 90C rated 1/0 copper (170A*0.82=139A) ... and we only have to suggest insulated conduit and generator shelter suitable for "SoCal desert" or Death Valley.

Actually per the NEC, unless the both the generator and the transfer switch’s connections in the example you posted are rated for 75C, then they shall be utilized with conductors sized at the 60C rating which eliminates number 2 as a candidate.


54067323

join:2012-09-25
Tuscaloosa, AL
reply to lutful

said by lutful:

Suppose transfer switch user manual dictates maximum wire size ... leaving a single choice of 90C rated

You cannot use a 90C rating in this application.


John Galt
Forward, March
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join:2004-09-30
Happy Camp
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said by 54067323:

You cannot use a 90C rating in this application.

For those who not deal with this issue on a regular basis, you can use 90C conductors, but the ampacity must be calculated based on the termination device temperature. If the termination temp is not specified explicitly, then the 60C column must be used for the conductor ampacity rating.

Using the 90C wire generally allows for a reduction in the conduit size, hence there is an advantage in using those insulation types.
--
No amount of force can control a free man, a man whose mind is free. You can't conquer a free man; the most you can do is kill him. Well, you can try to...



Red_Menace
poking around since 1978

join:2001-11-03
Littleton, CO
reply to Anon

Code compliance in many cases (such as this) can be a life-safety issue. I find this thread interesting, as it demonstrates the difference between just looking stuff up and the proper application, which requires details about said application. Of course, I'm probably biased because I hold a JW license in a couple of states, but whatever...



54067323

join:2012-09-25
Tuscaloosa, AL

1 edit

said by Red_Menace:

Of course, I'm probably biased because I hold a JW license in a couple of states, but whatever...

How many years before you move up to master.

lutful
... of ideas
Premium
join:2005-06-16
Ottawa, ON
kudos:1
reply to Red_Menace

said by Red_Menace:

difference between just looking stuff up and the proper application, which requires details about said application.

Can one of you forum electricians post the "details" you need?

Perhaps we can make that a mandatory requirement for anyone who wants to connect a generator to a transfer switch.


54067323

join:2012-09-25
Tuscaloosa, AL

1 edit

1 recommendation

said by lutful:

Can one of you forum electricians post the "details" you need?

Ask Nunya, when it comes to electrical he is the AHJ around here.
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John Galt
Forward, March
Premium
join:2004-09-30
Happy Camp
kudos:8

1 recommendation

Re: Wire Size

More than likely has an enclosure with a CB in it...no plug.



54067323

join:2012-09-25
Tuscaloosa, AL
reply to Anon

said by lutful:

Thankfully our forum's electrical guru needs just one detail from OP ... where the first over-current protection device is located.

That’s a beginning.

Most likely the generator has over-current protection right at the 100A outlet. The socket is most likely 4 round hole type rated for 75C with matching plug (from Hubbell/Bryant/etc.) specifying upto 1/0 wire size.

Unless it is trailer mounted genset for temporary use probably not, if it a standard pad mounted in place genset it will have lugs such as the example "you" already posted.

Either way I would suggest you study the NEC, chapters one through nine and get an idea of all the variables involved with specifying a conductor for any application...


John Galt
Forward, March
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join:2004-09-30
Happy Camp
kudos:8
reply to lutful

said by lutful:

*** OK, there are outdoor metal conduits with plastic coating ... just in case someone "dreams" of installing a 20kW generator in SoCal desert despite all the fear-and-doubt.

That is not "insulating"...it does nothing to eliminate heat transfer. Nor does it do anything to eliminate the need to derate the conductors.


54067323

join:2012-09-25
Tuscaloosa, AL

said by John Galt:

That is not "insulating"...it does nothing to eliminate heat transfer.

It actually "insulates" the air flow around the metallic conduit exacerbating the problem which de-rating of conducters is based upon.


John Galt
Forward, March
Premium
join:2004-09-30
Happy Camp
kudos:8

said by 54067323:

said by John Galt:

That is not "insulating"...it does nothing to eliminate heat transfer.

It actually "insulates" the air flow around the metallic conduit exacerbating the problem which de-rating of conducters is based upon.

Yes, that.

I was referring to the presumed insulative qualities with respect to the environment that lutful See Profile was suggesting.
--
No amount of force can control a free man, a man whose mind is free. You can't conquer a free man; the most you can do is kill him. Well, you can try to...


lutful
... of ideas
Premium
join:2005-06-16
Ottawa, ON
kudos:1
reply to 54067323

said by 54067323:

said by John Galt:

That is not "insulating"...it does nothing to eliminate heat transfer.

It actually "insulates" the air flow around the metallic conduit exacerbating the problem which de-rating of conducters is based upon.

That particular conduit is used for (max 90C rated) AC cables in extreme environments like Libyan oil/gas installations and the Australian outback.

Maybe you two can suggest something else with actual specs.


John Galt
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join:2004-09-30
Happy Camp
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1 edit

You're conflating two separate issues. One is material construction rating and the other is temperature rise and dissipation for the conductors at the rated ampacity.

As 54067323 See Profile pointed out, insulation works both ways, and it is necessary for the conductors to dissipate the I2R losses in some manner.

Another limitation is the termination temperature ratings, which are probably the most difficult to deal with since the conductors transfer the heat to the equipment.

It is possible to use conductors like Type Z, FEP, FEPB or PFA, but they are not generally available and are very costly.

In environments where high temperatures are encountered and general use 90C conductors are used, every effort is made to avoid the high temperatures by routing of the conduits. In instances where routing cannot be avoided...break out the checkbook. A 500 ft. roll of 12GA TPFE insulated wire will cost nearly $900 vs. $70 for roll of 12GA THHN of similar length.

The ambient temperature correction factor for PTFE is 0.97 vs. 0.82 for THHN at 50C, but it is still necessary to deal with the termination temp limitation, which is usually lower.
--
No amount of force can control a free man, a man whose mind is free. You can't conquer a free man; the most you can do is kill him. Well, you can try to...



Red_Menace
poking around since 1978

join:2001-11-03
Littleton, CO
reply to 54067323

I was just referring to my interest in this topic - I have no desire to "move up" to master.


lutful
... of ideas
Premium
join:2005-06-16
Ottawa, ON
kudos:1
reply to John Galt

said by John Galt:

You're conflating two separate issues.

I had a chat with a former classmate who told me that a 100ft/100A run in exposed conduit should really be simulated using the Neher-McGrath equation by a licensed engineer. NEC apparently allows it and all the derating tables are trying to approximate that but it gets too complex for outdoor conduits in full or partial sun.

Anyway, that equation takes into account "many separate issues" including the effective thermal resistance from copper to ambient through sheath, conduit metal and final rubber coating.

said by John Galt:

As 54067323 See Profile pointed out, insulation works both ways, and it is necessary for the conductors to dissipate the I2R losses in some manner.

That conduit is designed for extreme temperature outdoor applications no matter what you two point out. The black coloring, which may be concerning you, only comes from adding robust UV protection which is required by code.

The effective thermal resistance is dominated by the thicker steel since the outer rubber coating is quite thin. There is also more black-body surface radiating heat away than the surface facing the sun directly at any moment. Cable in conduit simulation takes into account all of those factors.


54067323

join:2012-09-25
Tuscaloosa, AL

1 edit

1 recommendation

said by lutful:

The black coloring, which may be concerning you, only comes from adding robust UV protection which is required by code.

There is no such requirement in the NEC for metallic conduit.

The effective thermal resistance is dominated by the thicker steel since the outer rubber coating is quite thin.

Thicker steel would require further derating of the conductors contained within.

There is also more black-body surface radiating heat away than the surface facing the sun directly at any moment. Cable in conduit simulation takes into account all of those factors.

False, metallic conduit is silver and being silver reflects considerably more energy than black conduit.

I had a chat with a former classmate who told me that a 100ft/100A run in exposed conduit should really be simulated using the Neher-McGrath equation by a licensed engineer.

Or one could merely follow the code and pass the inspection.

NEC apparently allows it and all the derating tables are trying to approximate that but it gets too complex for outdoor conduits in full or partial sun.

They may be too complex for an engineer to figure out, but licensed electricians and plan examiners do it on a daily basis without a problem.

You see there is no need to over complicate the issue, just read and understand the book like they do, start at the beginning chapter one and go until you hit nine and then the charts that follow will make more sense and or work a few years in the industry as an apprentice.

Until you are willing to understand the code or get some real world experience there is no point in belaboring the issue with your attempts to bend the code to fit your ideas….

lutful
... of ideas
Premium
join:2005-06-16
Ottawa, ON
kudos:1

2 edits

said by 54067323:

Thicker steel would require further derating of the conductors contained within.
...
False, metallic conduit is silver and being silver reflects considerably more energy than black conduit.

Curiously outdoor conductor sheaths are usually black and Thermos internal surfaces are usually shiny.

We are trying to limit insulation temp (to 75C or 90C) by radiating internally generated heat out to the ambient. That is a bit more important than reflecting sunlight.

NEC tables provide worst-case temperature rise for rooftop conduit in full sun if some electrician is unwilling to do proper thermal analysis "under engineering supervision". Using those tables will certainly lead to much larger conductor diameter than necessary and hence larger diameter conduit.

Anyway, if you are proposing (indirectly) that rigid metal conduit can be used for this application ... of course I will be flexible.

*** OK, here are the relevant code sections:
310.15(C) Neher-McGrath equation to be used under engineering supervision

Tc = conductor temperature in degrees Celsius (°C)
Ta = ambient temperature in degrees Celsius (°C)
Rdc = dc resistance of conductor at temperature Tc
Yc= component ac resistance resulting from skin effect and proximity effect
Rca = effective thermal resistance between conductor and surrounding ambient

350.2 ... sunlight-resistant jacket over an inner flexible metal core