New construction questions

Old 12-17-02, 09:34 AM
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New construction questions

Hi all,

We're in the process of building a new 3800 sq ft house (plus basement to be finished at some point) and I want to do the wiring myself. (Don't let me have it yet!!!)

I'm not an electrician, but consider myself an advanced amateur with a degree in electrical engineering who admits that a degree does not an electrician make. My point is, I understand and respect electricty. I also know my limits, and plan on having my builder's electrician give it a critical once-over at a couple of stages.

Anyway, I really enjoy this type of work and read everything I can get my hands on. I've got it all planned out on paper. I'm still researching the power requirements of some appliances before I do my service calculations.

OK. There are my disclaimers.

Here are a number of issues I'd like input on.

1) I plan on having two geothermal heat pumps installed. Each of these units will require an emergency strip heating backup, one at 15 KW and one at 8 KW. I hope to never need them in the area I'm in, but need to include them in the load calculation of course. As I understand it, I need to include their full value (no derate) at 23 KW (which will be MUCH higher than the AC load). Correct?

2) My instinct (having not done the actual calculation yet) is that I'd like to go with a 400 Amp service--2 200 Amp panels in parallel. My builder gave me one of those looks--the "nodody does that--300 is enough" look. My reasons are that the house is pretty big, the strip heaters are high demand items, we plan on having a pool with associated requirements (I'll hire the electrician for THAT job--I know my limits), I'll have a workshop with several high demand items, etc.

In other words, why not go big up front? Is 400 Amp service really that uncommon? Will this be an issue with the power company that I need to resolve upfront?

3) How many cables am I allowed to run through EACH load center knockout after installing a romex clamp? Some load centers I've seen seem pretty sparse when it comes to the number of knockouts they supply.

4) I know that I shouldn't "bundle" cables beyond 24" to avoid derate issues. But I'm a little confused. When running several cables along the same path (i.e. through several joists) together, am I OK? I believe that is OK, but is there a practical limit to the number of cables one should do this with?

That's a long enough post for now. I enjoy this kind of workd and by doing it myself, feel I can afford to do a lot more than I could otherwise do including lots of recessed lighting, plenty of receptacles, some home automation (X-10), structured wiring, whole house audio, etc.

Thanks in advance.

Old 12-17-02, 10:17 AM
Join Date: Jun 2002
Location: Brethren, Mi
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I like 400 A service. The poco has to feed it better than a 300 also. (they like to be frugal with wire) I have a conversation with the engineer before they come and have him spec the incoming wire right on the work order. Some are even using contractors to do installs now and they really feed with the absolute minimum and if you dont get it straight you will take what you get. I dont remember what the size of mine is exactly but its as big as your thumb anyway. I think they started out wanting to go 3/0 or 4/0 and I finally get them to go 500 mcm. Some connectors are rated for 2 wires and I believe you can go 3 wires to a hole in a joist. We will be corrected here if I am wrong. I assume you read up on proper location of drilled holes in joists.
Old 12-17-02, 11:40 AM
Join Date: Feb 2002
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The most practical installation of a 400 amp residential Service would utilize an out-door enclosure rated at 400 amps and equipped with a meter-socket and two 200 amp breakers which are the Service Dis-connects. Any service equiptment with metering connections must be approved by the local utility co.You'll need to go to an electrical supply-house for specific, detailed information on this product.-----NEC, Art. 230, Services, Part VII, Over-current protection, Art 230.90 (A) exception No. 3 reads--------"The sum of the ratings of circuit-breakers shall be permitted to exceed the ampacity of the Service conductors."----- The Service conductors are the conductors between the utility connection and the Line side of the Service Dis-Connects.This execption allows you to use Service conductors with an ampacity of less then 400 amps. The ampacity of the Service conductors is determined by Art. 230 which is a "optional" calculation for a dwelling. This requires an exact summation of the volt-ampere ratings of all the connected loads.-------For an installation of this scope I suggest 100 amp. "distribution"sub-panels at different locations to minimize the lenth and grouping of the Branch-Circuit cables.------I'm curious about your decision to use heat-pumps-I hope you have consulted with HVAC contractors.You may want to consider automatic emergency generators in your design which will have to power your heating systems during a power outage.Only 8 amps could power an oil-fired forced-air furnace.-----Good Luck!!!!

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