dslreports logo
site
 
    All Forums Hot Topics Gallery
spc

spacer




how-to block ads


Search Topic:
uniqs
2457
share rss forum feed


54067323

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

Re: Wire Size

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

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


54067323

join:2012-09-25
Tuscaloosa, AL

said by lutful:

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

I guess you also fail to understand most conductor “sheaths” are made of plastic and metallic conduit being “metallic” is made of steel or aluminum.

Metallic conduit does not need to be black to be UV resistant.

None the less you are still stuck on that and 90C so there is no sense going any further with this as is quite clear you don’t understand nor do you want to understand…


54067323

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

said by lutful:

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

Ha what a joke, if one was to run 100 feet of LT for a generator install one is not only going to have a flat wallet one will be the laughing stock of the building department.

That is a total mis-application for LT.

That single sentence has clearly demonstrated your total lack of knowledge in matters concerning electrical contracting and design.
Expand your moderator at work

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

Re: Wire Size

As I keep repeating, that flexible conduit is used in extreme environments. I talked to my EE friends who work and even supervise electricians in such environments. I was not aware of the astronomical cost but it does not show lack of "knowledge" of thermal analysis or electrical design.

I understand electricians use rigid galvanized steel pipe outdoors because it is much cheaper and does the job. But it has pathetic thermal properties which result in tremendous heat rise in the conduit. You can Google it since you seem to have no educational background or interest in physics.

For example »www.cmacn.org/energy/basics/mat_reflect.htm says:

Infrared Emittance is a parameter between 0 and 1 which measures the ability of a warm or hot material to shed some of its heat in the form of infrared radiation ... clean, untarnished galvanized steel has a very low emittance

A material with an emittance of unity ("black body"), emits about 6.1 watts per square meter, for each degree C above ambient temperature.


So you can deduce that a 75C black-body in 50C ambient will radiate 150 watts per square meter. Compared to 5 watts for galvanized steel. Touch that pipe in full sun.



54067323

join:2012-09-25
Tuscaloosa, AL

said by lutful:

I understand electricians use rigid galvanized steel pipe outdoors because it is much cheaper and does the job.

Who said anything about using rigid?

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

said by 54067323:

said by lutful:

I understand electricians use rigid galvanized steel pipe outdoors because it is much cheaper and does the job.

Who said anything about using rigid?

Does NEC allow non-UV coated flexible metal conduit for 100ft outdoor application? What section should I check?


SparkChaser
Premium
join:2000-06-06
Downingtown, PA
kudos:3
Reviews:
·Verizon FiOS

1 recommendation

reply to lutful

said by lutful:

Infrared Emittance is a parameter between 0 and 1 which measures the ability of a warm or hot material to shed some of its heat in the form of infrared radiation ... clean, untarnished galvanized steel has a very low emittance

Just make one point before I go back to watching the contest.

emittance is not used alone as a criteria.



--
"Facts do not cease to exist because they are ignored." - Aldous Huxley

"Omne ignotum pro magnifico."


54067323

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

said by lutful:

Does NEC allow non-UV coated flexible metal conduit for 100ft outdoor application? What section should I check?

The answer to that question is somewhere between chapter one and nine of the NEC, when you find it let me know.

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

said by 54067323:

said by lutful:

Does NEC allow non-UV coated flexible metal conduit for 100ft outdoor application? What section should I check?

The answer to that question is somewhere between chapter one and nine of the NEC, when you find it let me know.

Can one of the forum electricians just post some links to inexpensive flexible metal conduits with sunlight resistant cool white paint that can be used for 100ft/100A outdoor run? The thread needs closure and I really do not have motivation to look up all the answers.

This is my very last post on anything electrical in this forum. Good riddance to me. Enjoy the peace and quiet.


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

said by lutful:

Can one of the forum electricians just post some links to inexpensive flexible metal conduits with sunlight resistant cool white paint that can be used for 100ft/100A outdoor run?

Most jurisdictions don't allow the use of LFMC in that manner.


motoracer

join:2003-09-15
united state

1 recommendation

reply to Ole pluck

Wow...talk about a pissing contest! Great work gentlemen.



54067323

join:2012-09-25
Tuscaloosa, AL

said by motoracer:

Wow...talk about a pissing contest! Great work gentlemen.

Really, I thought it was raining.