ELECTRICAN GURUS:

   / ELECTRICAN GURUS: #1  

machmeter62

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May 24, 2004
Messages
638
I have a question for you gentlemen? Our home has a 400 AMP service panel. This spring, I want to run an electrical line for a future small care-takers home... (two bdrm), that will be about 250-300 feet from our existing home. The line will come from the main house service panel.

Can I go with a direct burial wire, or is that a local code decision, and what type of wire and size in aluminum or copper? The prices have seemed to have fallen with copper....would that be the best, and somewhat economical route; or go with a larger aluminum line to carry a 100 AMP service panel there?

If I go with copper wire and conduit, what size conduit would be suitable? Will an aluminum wire size conduit have to be larger too, because of its ampacity? The property is located in Northern Calif.

Thanx in advance for your help and suggestions.
LEE
 
   / ELECTRICAN GURUS: #3  
I'm not an electrician but I play one on my farm.

First off, codes are all local, your best bet is to talk to the local inspector.

That said, there are two things you have to worry about. One is the ability of the wire to carry current without overheating. The other factor is losses due to resistance in the wire. Thinner wires can carry less current, and they have more resistance for the same length, than thicker wires.

At that length, resistance is going to determine your wire gauge rather than current carrying capacity. A 1/0 wire will carry 100 Amps, but it will have a drop of 8%, so your voltage will be 111 volt instead of 120.

Is that OK? This is where it gets local. The national codes are non-commital as to what is an acceptable voltage drop. You also have to think about what you're going to be doing with that electricity -- lights work fine at lower voltages, motors less well. It might be a good idea to measure what your current voltage is at the panel -- if you currently have a big drop, you probably want to be more conservative.
 
   / ELECTRICAN GURUS: #4  
When I built my house and shop, I had the utility company run primary underground to a transformer smack dab between my shop and the house. Now, the runs from the transformer to either the shop or the house are under 150 feet. This minimised voltage drop as well as conenction costs.
 
   / ELECTRICAN GURUS: #5  
Exactly as mentioned above. Your 400 amp panels distance from the last transformer + the additional run to the new panel will dictate the need for another transformer (or not). Call the local power provider, the co. that owns and maintains the actual lines and have them come out and do an audit per your needs. In our area when we are doing projects requiring upping power needs, the utility will regularly provide transformers, bigger gauge drops, etc. The only catch is the labor is a hit and miss deal, sometime free, sometimes not.
 
   / ELECTRICAN GURUS: #6  
check your local code.
that said, we put ours in 2" gray (Carlon) PVC, which is considered overkill in our area, but also makes future repair/work easier because i won't have to dig a new trench.
our electric company installed a meter with 3 sets of lugs for running primary service as well - so I have the option of slaving my shop (when I get it built) off the house panel or running it's own service from the pole - which in my case will be the shorter distance.
 
   / ELECTRICAN GURUS: #8  
:)
even in a "no-code" county (state?) you still have to meet federal minimum standards.
on the other hand, not having to have an inspection every time you turn around makes the construction process quicker.
 
   / ELECTRICAN GURUS: #9  
Here's my two cents, and feel free to ignore it:D

I do not like direct bury wire. If there is ever a problem with one wire (damaged insulation), it can not be fixed with direct burial, you will have to replace. If it is in conduit, and one wire is damaged, you can pull it out and replace it.
That being said, 11/4" conduit would work for copper wire at minimum size. 11/2" would work for aluminum. Neither would be much fun at this distance, so I would suggest 2", which would leave room for future upgrade, and make pulling a little easier.
Wire size: there is a table in the NEC for residential feeders which says. #4 copper or #2 aluminum is enough for a 100 amp service. If you take voltage drop into account (no more than 5% recommended to the furthest point of the system) then wire needs to be upsized to the area of #1 or #2 copper or 1/0 or 2/0 aluminum.
For feeder conductors, I have no preference of copper or aluminum, other than copper is more fun to scrap:p
One final note, if possible, make sure you get THHN (THWN, THHN-2, THWN-2) type insulation. It is a thinner coating of insulation, and this type of insulation slides easier in conduit than XHHW (XHHW-2).
Good luck, post photos.
 
   / ELECTRICAN GURUS: #10  
quicksandfarmer said:
I'm not an electrician but I play one on my farm.

First off, codes are all local, your best bet is to talk to the local inspector.

That said, there are two things you have to worry about. One is the ability of the wire to carry current without overheating. The other factor is losses due to resistance in the wire. Thinner wires can carry less current, and they have more resistance for the same length, than thicker wires.

[[[At that length, resistance is going to determine your wire gauge rather than current carrying capacity. A 1/0 wire will carry 100 Amps, but it will have a drop of 8%, so your voltage will be 111 volt instead of 120.]]]
Using the equation p=ie (Power = current x voltage) You will see that at 100A you are conducting 12kW on a 120V line. You are losing 900Watts of this (100x [120-111] in the wire. I assume you will be running both legs and taking 240V to the addition. If you divide the load equally on each leg at the addition, with 12KW you will be conducting just 50 A on each leg and you will lose only 4.5V instead of 9v on the round trip. Now P=IE = 50 X 4.5=225W lost. Still that Wattage is wasted heating the earth. If you are anticipating high loads at the addition use the biggest wire you can justify. Otherwise the electricity you pay for to heat the earth on the way there will eat back at your initial savings.
larry
 
 
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