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Old 12-22-2011, 10:05 AM   #1
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Default Min size wire for a generac 20kw

Good morning all. Over the last several months many of us have installed many generators. Wire size for a 20kw has been the most popular. After reading many threads there seems to be a discrepancy on the correct wire size to use. I have stated that it's based on the output of the generator and also the type of fuel being used. The fuel plays a major factor simply because it then reduces or increase the generator output.
IE. 20kw LP will produce 83.5amps
Where it's counter part natural gas will generator 18kw which will equate to 75amps maximum output. Using these numbers I will state that a 20kw will need a min size wire of #4cu or #2al
When natural gas is used the total output is 75amps where still number #4 can be used and now #3 alum. Based on table 310.16
Additionally Generac could have chosen To use a 90amp OCPD but UL excepted installing a 100OCPD.
220.40
225.5
240.4(b)
This will be very debatable.
Please let's stick with topic. I know many of you are very intelligent. But, the code can be interpreted in many ways.
Please do your HW properly.
Rules no debating or quoting anyone, please state your facts and leave the forum.

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Old 12-22-2011, 10:09 AM   #2
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Quote:
Originally Posted by Salvatoreg02 View Post
Good morning all. Over the last several months many of us have installed many generators. Wire size for a 20kw has been the most popular. After reading many threads there seems to be a discrepancy on the correct wire size to use. I have stated that it's based on the output of the generator and also the type of fuel being used. The fuel plays a major factor simply because it then reduces or increase the generator output.
IE. 20kw LP will produce 83.5amps
Where it's counter part natural gas will generator 18kw which will equate to 75amps maximum output. Using these numbers I will state that a 20kw will need a min size wire of #4cu or #2al
When natural gas is used the total output is 75amps where still number #4 can be used and now #3 alum. Based on table 310.16
Additionally Generac could have chosen To use a 90amp OCPD but UL excepted installing a 100OCPD.
220.40
225.5
240.4(b)
This will be very debatable.
Please let's stick with topic. I know many of you are very intelligent. But, the code can be interpreted in many ways.
Please do your HW properly.
Rules no debating or quoting anyone, please state your facts and leave the forum.

Sent from my iPhone using ET Forum


There is nothing debatable about it. You cannot change the wire size due to a fuel difference. The changeover from LP to gas takes very little time, and may be done at anytime in the future. The fuelt has absolutley nothing to do with sizing the wire.

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Old 12-22-2011, 10:14 AM   #3
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Quote:
Originally Posted by Salvatoreg02
Good morning all. Over the last several months many of us have installed many generators. Wire size for a 20kw has been the most popular. After reading many threads there seems to be a discrepancy on the correct wire size to use. I have stated that it's based on the output of the generator and also the type of fuel being used. The fuel plays a major factor simply because it then reduces or increase the generator output.
IE. 20kw LP will produce 83.5amps
Where it's counter part natural gas will generator 18kw which will equate to 75amps maximum output. Using these numbers I will state that a 20kw will need a min size wire of #4cu or #2al
When natural gas is used the total output is 75amps where still number #4 can be used and now #3 alum. Based on table 310.16
Additionally Generac could have chosen To use a 90amp OCPD but UL excepted installing a 100OCPD.
220.40
225.5
240.4(b)
This will be very debatable.
Please let's stick with topic. I know many of you are very intelligent. But, the code can be interpreted in many ways.
Please do your HW properly.
Rules no debating or quoting anyone, please state your facts and leave the forum.

Sent from my iPhone using ET Forum
I give up. Please post your company name and contact information as well as that inspector. I want a conference call with us you him and the state licensing board so they can straighten you two out.
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Old 12-22-2011, 10:22 AM   #4
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Quote:
Originally Posted by mcclary's electrical

There is nothing debatable about it. You cannot change the wire size due to a fuel difference. The changeover from LP to gas takes very little time, and may be done at anytime in the future. The fuelt has absolutley nothing to do with sizing the wire.
I will make a exception this time but you don't follow rules very well.
In some parts of the country NG is available to many home automatically via a gas conduit from your utility company. In my area many home don't have that option because it's not available. So LP IS THE ONLY OPTION. And yes it plays a critical part because LP BURNS HOTTER the NG which the generator can produce more power. Read.

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Old 12-22-2011, 10:26 AM   #5
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Quote:
Originally Posted by Salvatoreg02

I will make a exception this time but you don't follow rules very well.
In some parts of the country NG is available to many home automatically via a gas conduit from your utility company. In my area many home don't have that option because it's not available. So LP IS THE ONLY OPTION. And yes it plays a critical part because LP BURNS HOTTER the NG which the generator can produce more power. Read.

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You do know how many people are reading your posts right? I would logout and never come back. Even harry doesn't say crazy things like you have. Fuel type may increase output but It is regulated with either fuel type.

Last edited by Bulldog1; 12-22-2011 at 10:29 AM.
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Old 12-22-2011, 10:49 AM   #6
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Originally Posted by Bulldog1

You do know how many people are reading your posts right? I would logout and never come back. Even harry doesn't say crazy things like you have. Fuel type may increase output but It is regulated with either fuel type.
What the hell are talking about!! Generac specifically states a 20kw model #5785 has an output of only 18kw when natural gas is used. Read the specs!!!
LP on the other hand will produce a full 20kw.

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Old 12-22-2011, 10:53 AM   #7
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What the hell are talking about!! Generac specifically states a 20kw model #5785 has an output of only 18kw when natural gas is used. Read the specs!!!
LP on the other hand will produce a full 20kw.

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Duh, everybody knows that. You still can't size your wire by the fuel.
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Old 12-22-2011, 11:01 AM   #8
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Just forget the fuel being used and run #3CU for any 20k genny.. problem solved..

I have never seen a FPN about conductor size and type of gas..
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Old 12-22-2011, 11:08 AM   #9
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Originally Posted by mcclary's electrical

Duh, everybody knows that. You still can't size your wire by the fuel.
In this instance there is no change in the copper size but with alum it will decrease the size to #3 alum.
I'm simply stating that wire size doesn't have to be greater then #4 cu.
Have you ever realized that the #2AWG doesn't not fit under the neutral and grd detail in the Generac 20kw generstors.

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Old 12-22-2011, 11:15 AM   #10
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If your generator has a 100 amp OCPD the fuel type and actual generator output has nothing to do with wire sizing as previously stated in all manner of different ways.

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Old 12-22-2011, 11:24 AM   #11
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I think I know this guy and why he gets all of the bids….. He sizes his wires like everything is a dwelling unit
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Old 12-22-2011, 11:38 AM   #12
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Quote:
Originally Posted by Salvatoreg02 View Post
In this instance there is no change in the copper size but with alum it will decrease the size to #3 alum.
I'm simply stating that wire size doesn't have to be greater then #4 cu.
Have you ever realized that the #2AWG doesn't not fit under the neutral and grd detail in the Generac 20kw generstors.

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The OCP is 100 amp.. #4CU. is rated @ 85 amp... I don't see how that works..
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Old 12-22-2011, 11:38 AM   #13
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Quote:
Originally Posted by Pete m.
If your generator has a 100 amp OCPD the fuel type and actual generator output has nothing to do with wire sizing as previously stated in all manner of different ways.

Pete
Sure it does thats the nameplate rating of the generator. They could have easily installed a 90amp breaker.

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Old 12-22-2011, 11:40 AM   #14
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Sure it does thats the nameplate rating of the generator. They could have easily installed a 90amp breaker.
Is there a 90 amp breaker installed or a 100?

Using your logic they "could have installed" a 2000 amp GFP BPS.

What matters is what breaker is actually there!

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Old 12-22-2011, 11:42 AM   #15
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Quote:
Originally Posted by Pete m.
If your generator has a 100 amp OCPD the fuel type and actual generator output has nothing to do with wire sizing as previously stated in all manner of different ways.

Pete
So let's say that the generator has an output of 95amps and there was a wire size for 95amps. 240.4b allows us to use the next breaker size up. You capesci now!!!!

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Old 12-22-2011, 11:48 AM   #16
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Table 310.15b6 allows #4 copper for a 100amp service
Table 310.16 allow us to use #4 copper for 83.5 amps the generator produces based on 240.4b and all the others references I have provided.

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Old 12-22-2011, 11:49 AM   #17
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So let's say that the generator has an output of 95amps and there was a wire size for 95amps. 240.4b allows us to use the next breaker size up. You capesci now!!!!
Thanks for the code lesson...

I know that very well but you are maintaining that you are permitted, in some other dimension of reality that I am unfamiliar with, to size your wire to the actual output of the generator based on the fuel type being employed and ignore whatever OCPD size is actually installed at the generator.

Can you back that with a code section that I could use to approve such an install?

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Old 12-22-2011, 11:54 AM   #18
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Originally Posted by Pete m.

Thanks for the code lesson...

I know that very well but you are maintaining that you are permitted, in some other dimension of reality that I am unfamiliar with, to size your wire to the actual output of the generator based on the fuel type being employed and ignore whatever OCPD size is actually installed at the generator.

Can you back that with a code section that I could use to approve such an install?

Pete
Check my opening statement as I created this thread. All code references are there and left for interrogation.

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Old 12-22-2011, 11:54 AM   #19
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Originally Posted by Salvatoreg02 View Post
Table 310.15b6 allows #4 copper for a 100amp service
Table 310.16 allow us to use #4 copper for 83.5 amps the generator produces based on 240.4b and all the others references I have provided.

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What size service is installed at the house (assuming residential since you chose T310.15(B)(6) as a reference)?

If it is a 200 amp service then you better go do some more homework. Table 310.15(B)(6) is only applicable where ALL loads for a dwelling are involved.

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Old 12-22-2011, 12:04 PM   #20
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One of the most basic rules of the trade is that you size your wires according to the size of the breaker.

Sure, there are various rules for changing the breaker size for various special loads, but generator output isn't one of the ten listed 'exceptions.' You need that wire to be able to handle whatever the breaker can feed it.

Otherwise, we might as well be choosing circuit wire based on what size light bulb we're using.

As an electrician, all I care about is what comes out of the generator. I care not what is inside. Gas, diesel, gerbils ... doesn't matter, all I see is the rating of that breaker.

Keep in mind that the generator also has a 'surge' rating, where it can be called upon to deliver much more than the 'rated' power, though for a very short time before stalling out. You NEED the bigger wire.

As for "following the rules," the OP can go chase cows. He wants my opinion, he takes it on my terms. He can feel free to ignore it, but I won't let him dictate it.

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