lutful... of ideas
Premium Member
join:2005-06-16
Ottawa, ON |
to John Galt6
Re: Wire Sizesaid by John Galt6: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?  |
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 John Galt6Forward, March
Premium Member
join:2004-09-30
Happy Camp |
said by lutful:These are reasonable assumptions...
How do you think I arrived at the answer I gave based on the information given...? |
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54067323 (banned)
join:2012-09-25
Tuscaloosa, AL |
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. |
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lutful... of ideas
Premium Member
join:2005-06-16
Ottawa, ON |
lutful
Premium Member
2013-Apr-20 1:37 pm
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. |
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John Galt6Forward, March
Premium Member
join:2004-09-30
Happy Camp |
You left out some steps... |
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54067323 (banned)
join:2012-09-25
Tuscaloosa, AL |
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. |
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lutful... of ideas
Premium Member
join:2005-06-16
Ottawa, ON
1 edit |
lutful
Premium Member
2013-Apr-20 1:53 pm
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? |
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 SparkChaser
Premium Member
join:2000-06-06
Downingtown, PA |
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 |
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lutful... of ideas
Premium Member
join:2005-06-16
Ottawa, ON |
lutful
Premium Member
2013-Apr-20 2:00 pm
OK, let's make the temperature 49C or whatever max temp does not trigger that 25% derating.  |
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54067323 (banned)
join:2012-09-25
Tuscaloosa, AL |
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. |
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| 54067323 |
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. |
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 your moderator at work
 hidden : Trolling
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John Galt6Forward, March
Premium Member
join:2004-09-30
Happy Camp |
to 54067323
Re: Wire SizeHow hot does conduit get in the direct sun...?? |
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54067323 (banned)
join:2012-09-25
Tuscaloosa, AL |
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. |
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| 54067323 |
to John Galt6
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. |
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rich3236
join:2006-08-22
Fort Lauderdale, FL
1 recommendation |
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 |
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John Galt6Forward, March
Premium Member
join:2004-09-30
Happy Camp |
to 54067323
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. |
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lutful... of ideas
Premium Member
join:2005-06-16
Ottawa, ON
1 edit |
lutful
Premium Member
2013-Apr-20 2:34 pm
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. |
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54067323 (banned)
join:2012-09-25
Tuscaloosa, AL |
54067323 (banned)
Member
2013-Apr-20 2:35 pm
said by lutful:Suppose transfer switch user manual dictates maximum wire size ... leaving a single choice of 90C rated 1/0 copper Maybe 75C. |
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 mackey
Premium Member
join:2007-08-20
1 recommendation |
to lutful
said by lutful:said by John Galt6: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 |
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54067323 (banned)
join:2012-09-25
Tuscaloosa, AL |
54067323 (banned)
Member
2013-Apr-20 2:45 pm
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. |
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54067323
2 edits |
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. |
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| 54067323 |
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. |
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John Galt6Forward, March
Premium Member
join:2004-09-30
Happy Camp |
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. |
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 Red_Menacepoking around since 1978
join:2001-11-03
Fruita, CO |
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... |
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54067323 (banned)
join:2012-09-25
Tuscaloosa, AL
1 edit |
54067323 (banned)
Member
2013-Apr-20 3:58 pm
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. |
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lutful... of ideas
Premium Member
join:2005-06-16
Ottawa, ON |
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. |
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54067323 (banned)
join:2012-09-25
Tuscaloosa, AL
1 edit
1 recommendation |
54067323 (banned)
Member
2013-Apr-20 5:05 pm
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 Galt6Forward, March
Premium Member
join:2004-09-30
Happy Camp |
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. |
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54067323 (banned)
join:2012-09-25
Tuscaloosa, AL |
54067323 (banned)
Member
2013-Apr-20 8:21 pm
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. |
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