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Discussion Starter #1
Hey Guys,

I need to get my service entrance in this weekend and I'm not sure what to do on wire sizing. I want to use aluminum due to the cost of copper.

I have a 400a combination meter main / load center that sits 20 feet from the transformer. It's fed by parallel 4/0.

I need to run my feeder from the meter main to the panel in the home. The cable distance is going to be 175ft. This will be 200a residential service. Nine people will occupy the home, so the washer, dryer, dishwasher, etc will stay busy. The tankless water heater is 112 amp, fed by dual 60a circuits.

I feel like when the tankless heater is active and under high demand, we could see usage approaching the panel capacity.

Do you think I can run this underground in conduit using 4/0-4/0-4/0-2/0, or will I need to upsize? I want to do 250kcmil, but nobody locally carries anything between 4/0 and 350. I don't believe the circuits in the home exceed 50 feet from the panel, and most of the high draw appliances are within 20 feet of the panel.

Thoughts?

Thanks!!
 

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Very Stable Genius
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4,164 Posts
Southwire voltage drop calculator is here:
https://www.southwire.com/support/voltage-drop-calculator.htm

Entering 240V, 175ft, Al, conduit and 200A's give the results below.

Results
1 conductors per phase utilizing a #250 Aluminum conductor will limit the voltage drop to 2.89% or less when supplying 200.0 amps for 175 feet on a 240 volt system.
For Engineering Information Only:
205.0 Amps Rated ampacity of selected conductor
0.09 Ohms Resistance (Ohms per 1000 feet)
0.041 Ohms Reactance (Ohms per 1000 feet)
7.199999999999999 volts maximum allowable voltage drop at 3%
6.921. Actual voltage drop loss at 2.89% for the circuit
0.9 Power Factor
 

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A "Handy Husband"
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11,829 Posts
Electric tankless is a huge mistake. Will never work for nine people. Go with gas or electric tanks.


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Licensed electrician
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Are you sure that a 200 amp service is enough ?
 

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Disrespectful to dirt
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Do you think I can run this underground in conduit using 4/0-4/0-4/0-2/0, or will I need to upsize? I want to do 250kcmil, but nobody locally carries anything between 4/0 and 350.
Your particular configuration doesn't allow you to use the 83% ampacity rule for this feeder unless you're bringing the full 400A to the house. 4/0 aluminum isn't sufficient for a 200A feeder to the house regardless of length. 250 MCM is the minimum.

I feel like when the tankless heater is active and under high demand, we could see usage approaching the panel capacity.
You need to run a load calculation to know. But frankly, most people greatly overestimate what their actual electrical load is. That 200A feeder probably has much more spare capacity than you think and the tankless heater may well be fine. Run the calcs or post here for help with it.
 

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Disrespectful to dirt
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I'm going to order 250-250-250-3/0 and run it underground in 2 1/2" sch40, 24" deep.
If you are still going with the tankless electric heater, did you run any load calculations? Can't stress that enough.

Based on the sizes you mentioned, I'm guessing you are going to order URD. Most (not all) is going to have insulation rated for only USE-2. This isn't permitted indoors. If you aren't using an outdoor disconnect for the house look for multi-rated URD (USE-2/RHH/RHW-2 rated).

The conduit only needs 18" of cover.
 

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Discussion Starter #8
If you are still going with the tankless electric heater, did you run any load calculations? Can't stress that enough.

Based on the sizes you mentioned, I'm guessing you are going to order URD. Most (not all) is going to have insulation rated for only USE-2. This isn't permitted indoors. If you aren't using an outdoor disconnect for the house look for multi-rated URD (USE-2/RHH/RHW-2 rated).

The conduit only needs 18" of cover.
We did plan to continue with the electric tankless water heater. We had already purchased it and had most of the install work complete.

I found a good price on 350-350-4/0, so I went that route and added a #4 ground wire. We got the wire pulled through the 180ft conduit last night.

Thanks!
 
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