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Discussion Starter · #1 ·
We're constructing a new house on a new lot without any electric in place. The utility company has its line at the start of our property and we're placing our meter down there (320Amp panel) and bringing up 320Amps to the main panel in the house where it will branch off 100Amps to the garage. From the meter to the panel in the house it is a 400 ft run.

Using an online calculator for voltage drop, it seems like 2 parallel runs of 600kcmil (aluminum) will bring up 320Amps over 400ft and stay under a 3% voltage drop.

654273

  • Does 2 parallel runs mean 2 hot wires and 1 neutral or 4 hot wires and 2 neutral?
  • Is 600kcmil good?
  • Is there any reason to upsize to 750kcmil?
We're looking to do aluminum but might need to do a connection just before the house to a smaller copper wire to fit our conduits (there are two 2" conduits). We understand there is a special connector to transition from aluminum to copper. (We're working with 600 or 750 kcmil sized aluminum wire.)
  • What is this connector?
  • Does it fit in the conduit, or does the connection have to happen it a christy box or something else?
Lastly, I know conductors are pulled through the conduit, is there a downside (besides time and labor) to "threading" the conduit with the wire and then gluing them together?
 

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I think you're taking voltage drop too seriously. You aren't required to hold it at 3%, that's just a wire salesman's dream. And you're not required to calculate it based on service size, You need to do a Load Calculation for the house based on the actual major appliances present, and then, base your voltage drop calc on those amps and the voltage drop you consider tolerable for yourself. Which can be, e.g. 5%.

Mind you, the voltage drop occurs based on the load that is actually present in that moment, not breaker trip or even load calc. Most of the time, a big house is only pulling maybe 20A, so you have some trivial fraction of a percent drop at that point.

Or heck, most people think AC power is 110V, and that's already 8% drop off what it really is.
Or your power company may have jumped the transformers a little hot so you're getting 246V, well that buys you 2-1/2% right there!


Also you can calculate load for voltage drop based on whatever you please, but you must also use the minimum wire size for your application, with an 83% favorable derate for the whole-house run because of 310.15(b)(7) - so 332A.



As far as paralleling, you can't just stick some wires together and call it paralleling. You need certified special equipment on the supply side, and that is nosebleed expensive industrial tier stuff. I own some. Right off the bat, each wire is individually fused. So paralleling is probably going to be totally impracticable. If you are absolutely bullheaded in taking all "320A"/400A over in one feeder, consider non-parallel 1000 kcmil. It'll give 3.74% voltage drop @ 320A actual and @90% PF, but there's nothing wrong with that. In the U.S.

However here's the other thing about "400A" service. There's no such thing as a 400A breaker or panel in the consumer-affordable space. Class 320 aka 400A service is actually wired as two 200A main breakers feeding two 200A panels. Most 400A meters are sold with double lugs intended for two 250 kcmil wires. Most 400A meter-mains are sold as dual 200A breakers. I have never seen one with a 400A breaker.

So most likely, you don't even want to carry the power over as one fat 400A main. You want to wire it as dual independent 200A feeders - going either off dual 200A main breakers at the meter, or into two 200A main panels at the house. Depending on what your AHJ requires. Those two 200A are thus independent, and not paralleled at all. 600 kcmil (non-parallel) is a perfectly fine size for those given your distance and voltage drop.
 

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We're constructing a new house on a new lot without any electric in place. The utility company has its line at the start of our property and we're placing our meter down there (320Amp panel) and bringing up 320Amps to the main panel in the house where it will branch off 100Amps to the garage. From the meter to the panel in the house it is a 400 ft run.

Using an online calculator for voltage drop, it seems like 2 parallel runs of 600kcmil (aluminum) will bring up 320Amps over 400ft and stay under a 3% voltage drop.

View attachment 654273
  • Does 2 parallel runs mean 2 hot wires and 1 neutral or 4 hot wires and 2 neutral?
  • Is 600kcmil good?
  • Is there any reason to upsize to 750kcmil?
We're looking to do aluminum but might need to do a connection just before the house to a smaller copper wire to fit our conduits (there are two 2" conduits). We understand there is a special connector to transition from aluminum to copper. (We're working with 600 or 750 kcmil sized aluminum wire.)
  • What is this connector?
  • Does it fit in the conduit, or does the connection have to happen it a christy box or something else?
Lastly, I know conductors are pulled through the conduit, is there a downside (besides time and labor) to "threading" the conduit with the wire and then gluing them together?
Most important answer first. Assembling conduit around the conductors is a specifically forbidden practice for a very good reason. No matter how carefully it is done the conductor insulation is likely to get damaged. That is especially true of PVC conduit because it is connected by a process called "Solvent Welding." PVC cement is actually a rather potent solvent. It partially dissolves the surfaces to be welded and when the solvent has evaporated it leaves behind a chemically welded joint. Doing that around the wire inevitably results in portions of the insulation being partially or completely dissolved.

2 parallel runs Means that you are using 2 smaller wires instead of one unavailable and very difficult to handle single wire for each conductive pathway of the circuit. For this to work the conductors must be as equal in length as is possible. A difference in the length of the two wires would cause one to carry more current than the other because of the resultant difference in the impedance of each wire. The wire carrying the larger amount of current would always be operating at a higher temperature than the other one would. The greater the difference in length the worse that problem is. In order to maintain the normal magnetic field cancellation of balanced electrical circuits a full set of each conductor of the circuit must be run together in the same trench, raceway, or cable. You need to have one neutral, one Equipment Grounding Conductor (EGC) and 2 energized conductors from opposing voltage points of the transformer's secondary winding in each set of conductors. The characteristics of each conductor must be identical. Not equivalent but identical! Same insulation same conductor material, same stranding, same twist...

3% is not even a requirement of the NEC. It is nothing more than a suggestion. If you achieve limiting the voltage drop to 3% you should call it good.

About the 2 conduits. Sized for the regular stranded 600 kcmil wire each conduit would have to be 4 inches per set of conductors. It would appear that from the conduit ends to the terminals you would need to run #3/0 AWG copper the rest of the way to the terminals at the other end of the conduit.
special connector to transition from aluminum to copper.
The transition will have to be made using High Pressure Crimp Connectors which are suitable for direct burial and are indefinitely submersible to the depth of burial once insulated. Some of that can be mitigated by installing some kinds of splices in ground boxes. Please don't expect me to remember the details off the top of my head.
 

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Anyway to answer. Your questions...

Paralleling ain't gonna happen.

600 kcmil is fine for a 200A feeder.

The connector you're looking for is called a Polaris.

You must make all splices inside junction boxes, and the junction box covers must remain accessible forever without the use of tools. The junction boxes for large wires are positively enormous and you'll want some engineering help sizing those boxes.

You can put all the wires in one huge conduit. The oversizing you are doing for voltage drop purposes more than takes care of the 310.15(B)(3)(a) derate. You can put telecomm fiber in there too if it has no conductive parts.

You must assemble the conduit complete, bury it, tamp it, and then pull the wires. Pulling 600 kcmil wires x3 will require serious pulling gear. Block and tackle, Jeep winch, etc. Pulling forces will tear apart poorly constructed conduit that hasn't been tamped well at the elbows.

You cannot assemble the conduit around the wires, it's a code violation. Novices always think that is clever, but I install a lot of conduit and I think it sounds like the most difficult way possible to do that job. Makes a huge work area mess, gets the conduit filled up with dirt and rocks, forces you to do the job in one continuous operation, etc. And damages the wire, not least with PVC cement. Realistically you won't get it all done in one day. So you'll have to leave $7000 worth of wire splayed out all over your property. That's just asking to get it stolen.

Pulling it afterwards is a monster pull, but it's less than an hour of actually pulling. And you can hire that part of the job done if you want to.

Oh, one other thing. You will have two main panels at the house. One of them will have a 100A breaker feeding the shed. PLEASE... don't just spam circuits into both panels higgledy piggledy or based on some "system". Please put all the circuits you would want on generator in one panel. Putting one panel on a generator is easy. Putting both panels on a generator is exceedingly awkward and difficult.

I know you may want to have one fat 400A feeder because you want to do a 400A gen interlock, heh heh. Might want toidentify and price those specific components before you commit to that plan - don't just count on the market having that stuff at sane cost.
 

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Oh, and grounds. Your required conductor size given 200A per feeder ( given 310.15(B)(7) ) is 4/0 which is 212 kcmil.

You are choosing to use 600 kcmil.

So you are using hot wires 3x larger than required.

Your ground wire must be the same proportion larger. Minimum ground is #4 (42 kcmil). So your ground must be 3x larger. 2/0 wire is 133 kcmil, so that'll do it for a ground!
 

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Have you talked to the power company about setting a transformer closer the the house? Have you considered direct burial cable?
 

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Very good points @Jim Port. Direct burial will reduce theft risk, though I'm still not a fan because 400' of length is an awful lot of "risk surface area" for a rock penetration and the like.

And yeah, the proposal is to spend Seven Thousand Dollars on wire... simply so the PoCo can put the transformer at the edge of your property (i.e. on their easement) instead of more usefully near your house. Even if you had to buy your own transformer, a 100 KVA transformer brand-new is only $4000.

Then get the PoCo to give you 480V single-phase service (better: 600V), which they shouldn't mind since your 120/240V transformer would be single-phase anyway. At that higher voltage, vastly smaller wires are needed - 3% drop happens on 4/0 wire (2/0 wire) at $1000 ($550). Pays for the transformer and then some.



Though if it were my $7000++, that's a rather large "piggy bank" to select more efficient appliances or other mitigation tech. For instance dump the tankless electric for a large tanked water heater using heat-pump tech.

So tell us what your loads are, and why you want Class 320 aka 400A service, and we can advise if there are options that don't cost $7000 for wires alone.
 

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Discussion Starter · #8 ·
Thank you all for your expertise and for being so detailed in your responses. So according to

However here's the other thing about "400A" service. There's no such thing as a 400A breaker or panel in the consumer-affordable space. Class 320 aka 400A service is actually wired as two 200A main breakers feeding two 200A panels. Most 400A meters are sold with double lugs intended for two 250 kcmil wires. Most 400A meter-mains are sold as dual 200A breakers. I have never seen one with a 400A breaker.
We're fine with 300A, but it sounds like because of the meter panel's breakers, one run isn't likely to bring up 300A to a main panel in the house then add a breaker off that panel that directs 100A to the garage panel (another 300 ft away). We need to verify the meter panel has that capability, as well as the connection at the panel in the house. The panel in the house would have to be a 400A panel to get that extra 100A breaker on there, right? Cost wise, that 400A panel looks like $1,500—which seems like a better alternative than running an extra 400ft of 350kcmil in conduit.

Have you talked to the power company about setting a transformer closer the the house? Have you considered direct burial cable?
Yes. So they don't provide quotes upfront, which is a reason we want to limit their scope of work. But their representative gave estimates, if we put the meter near our house, they'll add in a transformer about midway (ballpark estimate for them to take care of the long run is $30,000) vs the other ballpark quote (less than $5,000) of putting the meter within 50ft of their current hookup point which, at that location, leaves us with a 400 ft run to the house panel (700 ft meter to garage panel).

So tell us what your loads are, and why you want Class 320 aka 400A service,
Our current loads don't necessitate a large capacity, and we understand we're not going to be using all of it at the same time. We want room to expand in the future and not find out we want to bring up more and have to redo something.
  • We're all electric, no gas.
  • Our major load in the house is the tankless water heater that uses three 40A breakers,
  • then we have the other usual appliances:
    • 30A dryer,
    • 40A stovetop,
    • 30A oven/microwave,
    • then the A/C heater which I don't have the amps on hand but it's a 3.5 ton system in total.
  • The garage panel powers
  • an electric car charger 40A,
  • well pump (not sure on the specs)
  • and other small things: pressure tank's pump, lights, outlets
We might down size that water heater if it is too problematic, but our space is limited to accommodate a traditional tank water heater.
 

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1) Have POCO run THEIR line and transformer up to your house or you'll be paying for line losses forever.
Or
2) As said above, install a transformer and boost voltage to use smaller wire.
Or
3) Put in a solar system and generator and go off-grid. Solar systems are cheap and gov gives 26% tax credit... batteries are expensive, but they've come way down in price. A good lithium ion battery will last you over a decade. And for those bad solar days, you have a generator to keep the batteries charged.
 
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