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

lutful to 54067323

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Re: Wire Size

said by 54067323:

said by lutful:

ampacity rules do not change based on brand/model of generator and transfer switch

No, but they do change based upon how the conductors are run, and the temperatures they will be exposed to and so far that information is a big unknown...

Instead of propagating such fear-and-doubt, let's see if we can provide helpful suggestions to remove the unknowns.

Let's assume worst-case location in USA ( Death Valley, CA for example) and let's also assume NEC guidelines and common sense will be used to determine how the "conductors are run" so the insulation temperature rise will be mainly due to I2R losses.

Let's assume lower cost cable option is chosen (75C rated #2 copper) which can still carry worst case 100A+15%=115A. We can easily verify that the insulation will not exceed 75C even when ambient temperature is 50C on a record breaking summer day.

We can also verify that any better choice of cable (75C rated #1 or 90C rated #2) will actually keep the insulation below 60C ... which will allow some compromises to be made on how the "conductors are run" without violating code.

I will wait to see first response by OP to many helpful suggestions to his clearly stated problem.

John Galt6
Forward, March
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Happy Camp

John Galt6

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You left out some steps...
54067323 (banned)
join:2012-09-25
Tuscaloosa, AL

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said by lutful:

Let's assume lower cost cable option is chosen (75C rated #2 copper) which can still carry worst case 100A+15%=115A. We can easily verify that the insulation will not exceed 75C even when ambient temperature is 50C on a record breaking summer day.

Then per the NEC the cable is only rated for carrying 78.75 amps and would be quite overloaded at 100 amps.

We can also verify that any better choice of cable (75C rated #1 or 90C rated #2) will actually keep the insulation below 60C ... which will allow some compromises to be made on how the "conductors are run" without violating code.

Then again per the NEC the cable is only rated for carrying 97.58 amps and is still overloaded.

Either way it's a "code" violation.
lutful
... of ideas
Premium Member
join:2005-06-16
Ottawa, ON

1 edit

lutful

Premium Member

said by 54067323:

Then per the NEC the cable is only rated for carrying 78.75 amps and would be quite overloaded at 100 amps.

I am puzzled because NEC table (see earlier screenshot) shows 115A in the 75C/copper column for #2 wire size and full text says that rating is good for up to 3 phase conductors in the cable.

Are you claiming now that even 90C rated #1 copper will violate code?

SparkChaser
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join:2000-06-06
Downingtown, PA

SparkChaser

Premium Member

said by lutful:

said by 54067323:

Then per the NEC the cable is only rated for carrying 78.75 amps and would be quite overloaded at 100 amps.

I am puzzled because NEC table (see earlier screenshot) shows 115A in the 75C/copper column for #2 wire size and full text says that rating is good for up to 3 phase conductors in the cable.

It's derated by 25% for ambient temp of 50C
lutful
... of ideas
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join:2005-06-16
Ottawa, ON

lutful

Premium Member

OK, let's make the temperature 49C or whatever max temp does not trigger that 25% derating.
54067323 (banned)
join:2012-09-25
Tuscaloosa, AL

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said by lutful:

I am puzzled because NEC table (see earlier screenshot) shows 115A in the 75C/copper column for #2 wire size and full text says that rating is good for up to 3 phase conductors in the cable.

It's called de-rating and conduit.
54067323

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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.
Expand your moderator at work

John Galt6
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Re: Wire Size

How hot does conduit get in the direct sun...??
54067323 (banned)
join:2012-09-25
Tuscaloosa, AL

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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

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said by John Galt6:

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

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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 Galt6
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said by 54067323:

said by John Galt6:

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 Member
join:2005-06-16
Ottawa, ON

1 edit

lutful

Premium Member

said by John Galt6:

said by 54067323:

said by John Galt6:

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 (banned)
join:2012-09-25
Tuscaloosa, AL

54067323 (banned)

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said by lutful:

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

Maybe 75C.
54067323

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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

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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 Galt6
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John Galt6

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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.

Red_Menace
poking around since 1978
join:2001-11-03
Fruita, CO

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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 (banned)
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54067323 (banned)

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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
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Ottawa, ON

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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 (banned)
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Tuscaloosa, AL

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54067323 (banned)

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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.

John Galt6
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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 (banned)
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54067323 (banned)

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said by John Galt6:

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 Galt6
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John Galt6

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said by 54067323:

said by John Galt6:

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

lutful to 54067323

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said by 54067323:

said by John Galt6:

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 Galt6
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John Galt6

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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.

Red_Menace
poking around since 1978
join:2001-11-03
Fruita, CO

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I was just referring to my interest in this topic - I have no desire to "move up" to master.
lutful
... of ideas
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Ottawa, ON

lutful to John Galt6

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said by John Galt6:

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 Galt6:

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.