New 400 Amp Service to 2-200 Amp Subs to 2-50 Amp RVs.

Reply

  #1  
Old 11-08-04, 01:54 PM
Norm L
Visiting Guest
Posts: n/a
New 400 Amp Service to 2-200 Amp Subs to 2-50 Amp RVs.

Hi folks. I just finished installing a new 400 amp meter base and two 200 amp sub-pannels at the enterance to my property (holy cow I even passed the electrical inspection!!!). I want to run two 200 amp runs, one each to my house and shop (yet to be built). Figure I'll use 4/0 4/0 2/0 aluminum xlp for the runs in pvc conduit. Do I need to run a ground wire too? And/or do I need grounding rods instead? The runs are 250 feet each in different directions.

Also, from the shop pannel, I want to run a couple of 50 amp RV hook-ups. They will be 260 and 310 feet from the shop. If I read the charts correctly, I should use 4 awg with a ground??? Since the wire will run in a pvc conduit, do I need to buy direct bury cable for the RVs?

I appreciate the advice, as I am a confirmed screw-up and cheap-skate, and best known for my quality re-doing of jobs screwed up.
 
Sponsored Links
  #2  
Old 11-08-04, 06:49 PM
Member
Join Date: Jan 2004
Location: Oregon
Posts: 1,219
Direct buried Aluminium conductors have a bad reputation for reliability. One small pinhole in the insulation, water gets in, the aluminium corrodes, and then the conductor fails. At the very least, I would recommend running the cable in conduit, so that you can pull it out and replace it later. If you can do copper, so much the better.

4/0 Aluminium is allowed to serve a 200A feeder for a dwelling, by NEC 310.15(B)(6). For other applications, 4/0 Al has an ampacity of 180A. Strict interpretation of the electric code says that for the 200A shop panel, you will need something larger than 4/0 Al, but most inspectors will pass this use.

You should give careful consideration to making these wires larger than required by code because of 'voltage drop'. Because the run is so long, the wires have considerable resistance, and thus will 'lose' a bit of voltage when current flows. For example, with a 250 foot 4/0 feeder operating at 150A, you have a voltage drop of 250 ft * 2 * (0.1 ohms/1000 feet) * 150A = 7.5V, or 3.1%. Depending upon the actual loads drawn, this is right on the borderline of acceptable.

What you should really do is a 'demand' calculation, to figure out how much capacity you need on each of these feeders, but for most homes 200A is well oversized, same for most shops. The demand load will help you determine if you need to increase the size of your feeders.

For your application, you should run _both_ ground wires and install ground rods at each structure. You must always install ground rods at each structure, and while you don't always have to install ground wires, doing so makes other aspects of the job easier.

You will have the same voltage drop problems with the RV hookups. 4ga is the minimum permitted by code for overheating the wire (I presume aluminium again), but for reasons of voltage drop, you should be looking at 2ga or even 1ga.

-Jon
 

Last edited by winnie; 11-09-04 at 07:47 AM.
  #3  
Old 11-09-04, 06:53 AM
Norm L
Visiting Guest
Posts: n/a
Thanks!

Thanks very much. I appreciate the info and will follow your advice.
 
  #4  
Old 11-09-04, 12:30 PM
Norm L
Visiting Guest
Posts: n/a
Woops, one more question

On the two 200 amp feeders, what size ground wire do I use? 4 awg copper? I have not seen any formulas or web calculators for figuring out the correct ground wire guage for a given length of wire or load.
 
  #5  
Old 11-09-04, 05:25 PM
Member
Join Date: Jan 2004
Location: Oregon
Posts: 1,219
It is all in article 250 of the NEC. Given that you are taking on a very large project (service plus wiring for two buildings plus), buying a copy of the NEC and a couple of other books will pay many times over, and probably greatly increase the safety of your installation.

You should doubly check out 250.32(B)(1) for installing the equipment grounding conductors between the buildings, 250.52 for grounding electrodes themselves, 250.66 for the size of the grounding electrode conductors, and 250.122 for the sie of the equipment grounding conductors.

Article 220 steps you through the calculations needed to figure out how large the circuits need to be.

There are so many details to installing a complete electrical system, that we simply can't tell you all that you need to know. There is a reason that books on this topic exist. Once you start learning the subject from the books, then we can help by answering the questions that you come up with. But there is no way we can tell you all you need to know from scratch.

-Jon
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
Display Modes
'