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CorinthWest 12-05-2010 03:58 PM

Wiring Size Calculations for New Service
 
Greetings, experts! I'm hoping someone out here can help me with a few problems I am facing while installing new service into a utility building.

Here is the scenario:

I have a small metal utility building ("pump house") situated on a lake. There is commercial service on a pole 20' away from the building. The pump house will control 2 submersible 2HP 220V single phase pumps in the lake (which may be upgraded to 5HP in the future). These pumps are controlled by an "Autocon" which alternates the pumps when pressure drops and calls for a pump to kick on (IE. only 1 pump will ever be running at any point in time). The pump house will also have some ammenities such as a 5KW heater, a small 1.5A refrigerator, a 900W microwave, a 400W exterior flood light, a 150W interior light, a small TV and some duplex outlets.

We live in an unincorporated area and therefore have no requirement for permits or inspections. However, I want everything up to code, and "overkill" is my specialty. I plan on having an inspector come out and critique my wiring when finished.

I've spoken to the electric company and they are ready to drop my 220V single phase power to the building when I tell them I am ready. However, they require a "main service disconnect" next to the meter loop (understandable) before they will hook me up. A knife switch type disconnect would work, but being the masochist that I am, I elected to go with something that would offer me "some" measure of additional protection, and wouldn't require me to be standing in the pouring rain (during a lightening storm) trying to change out fuses (as in a fused knife switch type disconnect). Therefore I decided to go with a circuit breaker type main service disconnect, with an external lever. The box I purchased is a new Square D J250DS, which can handle up to a 250A circuit breaker (which I have not purchased as yet, as you will see).

I elected to go with a Square D 100A breaker panel inside the building with a 100A main disconnect circuit breaker factory installed in the panel. All breakers except for the pump breakers and the breaker for the refrigerator are on GFI circuit breakers because of the potentially wet environement inside the building. The breaker panel is a Square D QO112M100 and the box and all breakers are new.

I already have a 200A Milbank single phase 220V meter loop that will be installed on the outside of the pump house building, and I will mount the main service disconnect next to it (within 12"). From the main service disconnect, I will go through the wall to a raceway inside the pump house building, and from there to the circuit breaker panel (mounted on the opposite side of the wall from the main service disconnect, a 24" loop at most).

Here is/are my dilemma(s):

Because I want to eventually build a small house on the end of the 100' pier directly behind the pump house, I want to size the meter loop and main service disconnect such that it will be able to service the circuit breaker panels for both the pump house and the "lake house". The lake house will also use a 100A circuit breaker panel identical to the one I am using in the pump house (eventually). Therefore, I initially thought I would go with a 200A meter loop, a 200A main service disconnect circuit breaker feeding 2 seperate 100A circuit breaker panels (1 to be installed later, after the lake house is built at the end of the 100' pier). I elected to go with a 100A panel in the pump house because, as I previously stated, I "may" want to upgrade the pumps to 5HP at some point in time, and didn't want to have to upgrade the panel or wiring. I "may" also have some additional power requirements in the pump house such as a small window air conditioner.

Since I am using a J250DS main service disconnect circuit breaker encloseure, I thought "hey, the 250A breaker isn't much more expensive, why don't I just go with a 250A circuit breaker in case I ever need some additional capacity?". Then an electrician told me that I had to run 4/0 wires from the mast head all the way through to the 100A breaker panel.....huh? He explained that "if" there was ever a short BETWEEN the main service disconnect and the circuit breaker panel inside the building, the wiring had to be sized such that it would not fry before the main service disconnect circuit breaker tripped. Okay, that sorta makes sense, except that if I had elected to use an unfused knife switch as the main service disconnect, there would be NO protection and the wire size wouldn't matter!

So, if I use a 200A main service disconnect circuit breaker, and 4/0 wire, I need to run 2" conduit all the way to the 100A breaker panel inside the building...??!!??!! And now the problem.....the QO112M100 lugs ONLY accept wire sizes up to 1 AWG!!! Using this scenario, I will also eventually have to run 2" conduit and 4/0 wire 100' down the pier to the lake house at the end to supply power to the 100A circuit breaker panel in the lake house??!!??!! I may be a man who is known for his "overkill", but this is getting rediculous, even for me.

There is clearly something I am missing, and I think it may have to do with "continuouse load" vs. "non-continuous load". "IF" a short happens between the main service disconnect and the breaker box, the load on the wires would only be there for an instant before the main service disconnect circuit breaker trips, whether it is 200A or 250A. Is there a calculation that can be used to detemine the wire size that would withstand this "instantaneous" load and not damage the wires? What does the NEC code say about situations like this? I've tried to read it, but my eyes start to glaze over after about 30 minutes.....smiles.

My current plan is this:

Run 2" rigid from the mast head to the 200A meter loop (10'). Then run 2" threaded EMT from the meter loop to the main service disconnect circuit breaker box (12"). Then run 1.5" threaded EMT from the main service disconnect circuit breaker box through the wall to the raceway inside the building (6"). Then run 1.5" threaded EMT from the raceway to the 100A circuit breaker panel inside the pump house (4" nipple). I am going to use 4/0 down the mast head to the 200A meter loop, then continue the 4/0 from the 200A meter loop to the 200A (not going to 250A since I think I will need to change out the meter loop to accomodate the higher ampacity) main service disconnect circuit breaker. THEN I am going to use 1 AWG from the main service disconnect circuit breaker to the circuit breaker panel inside the pump house (via the raceway).

So my questions are:

1.) Am I missing something as far as the wire size from the main service disconnect circuit breaker to the circuit breaker panel inside the building? Is there a calculation? Will 1 AWG be okay?

2.) What wire size would I use to feed the 100A circuit breaker panel in the house at the end of the 100' pier when it is built? I would think I could also use 1 AWG in a 1.5" conduit.

3.) Would I need to upgrade the 200A meter loop if I use a 250A main disconnect circuit breaker (I think the answer here is yes)?

4.) If I upgraded the main service disconnect circuit breaker to 250A, how would that affect the answer to question #1 and #2 above?

5.) I was told by Square D that the circuit breakers for the J250DS are rated at "80%". Does this mean that I have to use a 250A main service disconnect breaker to achieve a true 200A service feed to both of my 100A circuit breaker boxes?

6.) I know I need to bond the neutral to ground in the main service disconnect circuit breaker box. Do I need to (or should I) bond it again in the circuit breaker panel inside the building? None of the wire runs inside the building will be longer than 10' from the circuit breaker box. I think the answer here is that I don't need to, but can do it if I want ("overkill" - I like overkill - smiles).

7.) I understand I "can" use a neutral wire that is 70% of the size of the load wires. Where is this applicable? Due to the short runs, I probably won't do this, but it would be nice to know where I can do this (IE. between the meter loop and the main service disconnect circuit breaker, between the main service disconnect circuit breaker and the 100A circuit breaker panel inside the building).

I should note that because of the relatively short runs, I will only be using THHN Cu (even down the 100' run to the house at the end of the pier, when the time comes - I know this may be somewhat expensive, but I had a bad experience with Al many years ago and will never use it again).

If you've read this far, I sincerely appreciate any advice you may have. What is most important to me is that the finished wiring conform to NEC specs. I just can't believe this has become so difficult! I didn't have this much trouble when I ran the service to our HOUSE!!!

Thank you in advance for your assistance, and for taking the time to read my novella!

Sincerely,
Rikki K.
Corinth West Missile Base

brric 12-05-2010 05:30 PM

The very first issue to consider is that your proposed initial installation is not a dwelling unit which means your service conductors must be larger. A 200 amp service will require 3/0 copper or 250 MCM aluminum.

CorinthWest 12-05-2010 05:35 PM

"I am going to use 4/0 down the mast head to the 200A meter loop, then continue the 4/0 from the 200A meter loop to the 200A (not going to 250A since I think I will need to change out the meter loop to accomodate the higher ampacity) main service disconnect circuit breaker. THEN I am going to use 1 AWG from the main service disconnect circuit breaker to the circuit breaker panel inside the pump house (via the raceway)."

So you're saying I could use 3/0 instead of 4/0 to feed a 200A main service disconnect circuit breaker.

Thanks.

brric 12-05-2010 06:00 PM

3/0 COPPER down the mast to the meter base and 3/0 Copper to the disconnect. In my opinion you should use a meter main or a 200 amp, 3R feed-thru panel for this installation.

CorinthWest 12-05-2010 07:35 PM

"I should note that because of the relatively short runs, I will only be using THHN Cu (even down the 100' run to the house at the end of the pier, when the time comes - I know this may be somewhat expensive, but I had a bad experience with Al many years ago and will never use it again)."

Al isn't in my vocabulary.

I understand your idea of using a CSED or feed-thru. But I don't understand why this would be any better than using a meter socket to a seperate main service disconnect circuit breaker, subsequently feeding 2 seperate circuit breaker panels? And this wouldn't solve my wiring gauge question/problem, I don't think.....:huh:

CorinthWest 12-07-2010 10:51 AM

Okay, your idea of a feed-thru or meter main WOULD solve my problem of having to run 3/0 all the way to the 100A non-service panel(s). now that I think about it. But, if I take this one step further, could't I use a second main service disconnect on the other side of the meter (physical location), both disconnects being fed by the same meter? In one disconnect, I could install a 150A circuit breaker to feed the 100A non-service panel inside the pump house. That would allow me to use #3 (?) wire from the disconnect to the panel, and since the panel can accomodate up to a #1, this would work. As for the second main service disconnect that would feed the non-service panel in the lake house, I would use another 150A circuit breaker, and would be able to use "residential" rules (#1?) for the 100' run to the house. The only problem I see here is running 2 main disconnects from the same meter loop. I think I would need to upgrade the meter socket to a higher ampacity and dual lugs, and upgrade the feeder wires since I will be using 2 x 150A main service disconnects off the meter loop.

I need to figure this out now before I move on with the wiring for the pump house or else I am going to end up changing stuff out later on when I add the service for the lake house. I'm also starting to wonder if 100A service to the lake house will be adequate in the end. It will be a VERY small bungalow, maybe 400 Sq. Ft., but it will be all electric.

I know I am reluctant to change out the J250DS disconnect. I really like this box and the fact that I can kill the service to the pump house with the pull of a lever. It looks clean, and will be functional. It would be nice to use an identical box to control the lake house power. But it MUST be on the same meter.

Your thoughts?

brric 12-07-2010 11:28 AM

The feed-thru panel should accept the breakers you are talking about and will provide the flexibility for disconnecting you are seeking. Yes, you probably could do it the way you have proposed. Simpler, easier and cheaper appears not to concern you.

HouseHelper 12-07-2010 11:36 AM

I would second the meter-main, a 200A with 8 breaker spaces would allow you to install two 100A breakers to feed your subpanels, and still leave space for additional circuits.


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