Big backup generator

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  #1  
Old 05-31-19, 03:59 PM
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Big backup generator

Hi all,
I have a fair amount of experience doing wiring around the house... I did all the conduit and wiring in our 2000 soft shed and actually got compliments on it from the inspector.
Our latest project is a bit beyond my comfort zone without a little help
We are purchasing a used 50KW Koehler diesel generator for a backup on our house.
The home has 400 Amp service with 2 200 Amp main beakers... there are two 200 Amp transfer switches already installed. They have been used once but we had a rental generator and they hooked it up on short leads so I canít use that for much comparison.
It is going to be in the neighborhood of 150í from the transfer switches (yes... it really needs to be that far )
I havenít yet found a good source for voltage drop tables and I believe I am overthinking the 220 split phase thing... I take 50,000/220 and get 227 or so amps... call it 250 for safety... but do I really have 250A at 220v? Or do I have half that on 2-120V lines?
Needless to say, this wire is gonna cost me... I sure donít want to buy the wrong thing!

If I can afford it, I would prefer to do conduit rather than direct burial for safeties sake and am hoping I can stay at 2Ē or less since the cost goes crazy after that.

Thanks in advance for any advice offered!!
 
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Old 05-31-19, 05:26 PM
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You have 250 amps at 220 available. You should use 240 volts for your calcs. That is the standard voltage. 240 volts gives an amperage of 208 amps.

You don't have the size for the full 250 amps unless you actually expect to use that much power at the same time.
 
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Old 05-31-19, 05:53 PM
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Thanks Joe! Kind of what I was expecting but I just wasn’t sure and this is way too much power to be guessing with
 
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Old 05-31-19, 05:59 PM
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Calculations are based on 240 volts, not 220.
 
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Old 05-31-19, 06:42 PM
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Does your generator have dual lugs for the power cables ?
You want to maintain 2" conduit...... that's two of them....correct ?

You'd need to run at least two sets of three 3/0 cables plus a ground.
That would be tight in a 2" conduit. Might need 2-1/2".

You're talking about 400A. That's at least 500mcm cables if you used three wires plus a needed ground you'd be in a 3-1/2" - 4" conduit.

These figures are in copper wire. Aluminum needs to be upsized.

Online wire size calculator
 
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Old 05-31-19, 07:02 PM
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Running those numbers through a voltage drop calculator, it appears that 1/0 wire will give a 2.55% voltage drop... as I understand it, under 3% is acceptable. I plan to run Sch 40 PVC conduit and can stay in reasonable sizes even with heavier wire than that
 
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Old 05-31-19, 07:06 PM
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The transfer switches are capable of a total of 400 amps but the 50KW generator can only do about half of that... we have never drawn more power than that to my knowledge... even in the worst of our summer heat since when it is daytime and hot, we donít run the dryers.
 
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Old 05-31-19, 07:12 PM
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I haven’t seen the generator yet... my brother in law is a friend of the guy who is servicing and cleaning it up for the sale, so I don’t know the output connections at this point.
 
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Old 05-31-19, 07:16 PM
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The wiring you run between the generator and the transfer switches is protected by a circuit breaker on the generator so based on the size of that breaker,.... that would dictate the minimum wire size needed.
 
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Old 05-31-19, 07:36 PM
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This is scary... the calculator that Pete sent me to says I would need 4/0 wire... but this one gives a vastly underrated answer... or am I missing something dumb?
https://www.calculator.net/voltage-d...es=208&x=0&y=0

That isn’t even close enough to say that one is just being a little more conservative!
gotta say... price wise, I sure liked the 1/0 answer better! ��
 
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Old 05-31-19, 07:39 PM
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Good point... I will definitely be checking that as soon as the generator gets here on Monday... and I get home from work.
Thanks for all the input... like I say... I am great on 20 amp circuits and little sub panels... but this is a little more than I felt safe with on my own.
 
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Old 05-31-19, 07:52 PM
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There seems to be a problem with the calculator you used as 1/0 is not even rated for 208A at any distance. Look down that page at the ampacity chart.

1/0 is rated for maximum 170A.

Ahhhh..... it gives you the drop depending on the wire size chosen. You need to pick the wire based on the amperage from the chart below.
 
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Old 05-31-19, 09:59 PM
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Some voltage drop calculators don't make this clear but if you start with "copper wire resistance" (or "aluminum wire resistance") you need the round trip distance (transfer switch to the generator, etc.) to do the proper calculation.

If you use a cord and plug (and inlet box) to hook up the generator, the plugs and receptacles need to be sized for the maximum amperage that will be supplied. An amperage smaller than what the generator can possibly deliver can be used for the installation by having a hard wired breaker at the generator with the connections to the house downstream from that.
 
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Old 06-04-19, 09:54 AM
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You are probably aware, but in case you are not you will need more conduits. You'll are going to need 120 volts at the generator for both a battery charger and block heater and a GFCI protected 120 volt receptacle for servicing would be handy too. In addition, you will need a raceway for control wiring between the transfer switches and the generator.
 
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