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Ole pluck

@comcast.net

Wire Size

I'm installing a 20 KW genset that has 100 amp out put, my transfer switch is 100 ft away, what size wire do I use?



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

Since you don't seem overly concerned with details you could probably use #1 Cu THHN/THWN.

There is more to consider, but that would require more details about the installation.


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

1 edit
reply to Ole pluck

The generator user manual will suggest conductor specs for 100 amps but you may want even larger diameter which still fit the transfer switch since you are going 100ft.



54067323

join:2012-09-25
Tuscaloosa, AL
reply to Ole pluck

As already mentioned number 1 copper should do the job, if you are concerned about excessive voltage drop in the the run, then consider using 1/0, that is if the lugs on both the generator and the transfer switch are rated for that large of a conductor.


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

I see that NEC rules allow 1 AWG copper conductor rated above 60C for 100 amps and voltage drop at 100ft will be less than 1.3V.



nunya
Premium,MVM
join:2000-12-23
O Fallon, MO
kudos:12

1 recommendation

reply to Ole pluck

Can't answer without knowing where the first OCP device is.
Have you contacted someone qualified to do this work?
--
If someone refers to herself / himself as a "guru", they probably aren't.



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

1 recommendation

said by nunya:

Can't answer without knowing where the first OCP device is.

Yes...tiny details like that.

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

said by Ole pluck :

20 KW genset ... 100 amp out put ... transfer switch is 100 ft away, what size wire do I use?

I am assuming you already purchased both the generator and transfer switch. Can you kindly post the brand/model for both?

Anyway, your choices for wire size will be very limited by code requirements and maximum wire size allowed by the transfer switch and/or generator. See above example.

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

said by John Galt:

said by nunya:

Can't answer without knowing where the first OCP device is.

Yes...tiny details like that.

The generator almost certainly has a 100A circuit breaker. NEC over current protection rules only seem to demand 115A rated conductor.

»ecmweb.com/print/content/using-o···nductors
»forums.mikeholt.com/showthread.php?t=146097


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

The generator almost certainly has a 100A circuit breaker. NEC over current protection rules only seem to demand 115A rated conductor.

As I stated previously, there is not enough detail with regard to the installation to make an informed choice.

If one is willing to base design decisions on wild supposition, then we can make pretty much anything work.
--
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
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join:2005-06-16
Ottawa, ON
kudos:1

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.

OP wanted to know what size wire will carry 100A from a 20kW generator to a transfer switch located 100ft away.

This is a public discussion, not an RFQ sent to an electrician. These are reasonable assumptions:
a) code compliant generator with 100A circuit breaker
b) code compliant transfer switch which accepts at least 100A
c) 240V/100A matches 20kW nameplate rating
d) ampacity rules do not change based on brand/model of generator and transfer switch which means at least #2.
e) user manuals of generator and transfer switch provide maximum wire size
f) it is better to use bigger diameter wire, so consider #1 if it fits.
... and so on

However, because of other recent anonymous electrical threads, I am curious to see how this pans out. Probably OP will come back with a separate OCP just after the generator?


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

said by lutful:

These are reasonable assumptions...

How do you think I arrived at the answer I gave based on the information given...?


54067323

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

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...

So until the OP posts that information everything else is speculation.

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

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

You left out some steps...



54067323

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

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
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join:2005-06-16
Ottawa, ON
kudos:1

1 edit

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
kudos:3
Reviews:
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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
--
"Facts do not cease to exist because they are ignored." - Aldous Huxley

"Omne ignotum pro magnifico."

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

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



54067323

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

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

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

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 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