Service Conductor

[inspector 102]

If installing a 400 amp meter base on a residential application that will serve 2-200 service panels, where can it be found that the grounded conductor may be decreased one size under the current carrying conductors. Is there a typical installation diagram available that I could review for this type of installation.

 

[bennie]

Re: Service Conductor

250.24(B), will give the information requested. I am assuming you are not an electrical inspector. May I ask why the interest?

 

[rick5280]

Re: Service Conductor

Though not answering your question directly, I would suggest you read Article 220.22 of the 2002 NEC, and look at the examples in the back of the codebook. This is for figureing the load, by which you will then size the wire.

Rick

 

[roger deas]

Re: Service Conductor

You should also look at article 220, specifically 220.22

Roger

 

[roger deas]

Re: Service Conductor

Sorry Rick, I was typing as you were posting.

Roger

 

[bphgravity]

Re: Service Conductor

Its funny (funny bad, not funny good) how many times I have heard an electrician give the old "reduce the grounded conductor one or two sizes" without actually performing a load claculation misconception. Was this an old code allowance, or was this common practice at some time in the past?

 

[inspector 102]

Re: Service Conductor

Like many of you, I have always heard of the reduction allowed however have never seen anyone actually do the load calculations. As an inspector I typically do not see the reduction on residential service but have notice that some SE cable has a smaller grounded conductor. I am looking for a utility detail sheet for typical installation that I can reference on the job site when discussing installations with installers. Our jurisdiction does not require electrical licensing which make the job more difficult. After discussing an installation with an installer (note: I did not say electrian) I wanted to use proper code references the next time we speak. I to am guilty of reading only the section of the NEC that I want to and was looking for other reference to this installation. Thanks for all the help and if anyone knows of a site that has an installation diagram, I am still interested.

 

[bphgravity]

Re: Service Conductor

Inspector 102: There really cant be a diagram because the installation is reliant on the calculation of the neutral load which is variable with each and every job. The calculation is very simple and only requires knowledge of the connected loads. All you need is 220.22 of the NEC. The code even gives example of this calculation in Appendix D.

PS - Most power quality engineers dont recommend neutral reduction on services and feeders. In most cases, they even suggest increased neutral sizing. With the growing problem of harmonic currents and non-linear loads, neutral sizing needs to be reconsidered above code allowances. (Not really a residential issue. Commerical 3-phase primarily.)

[ May 09, 2003, 03:16 PM: Message edited by: bphgravity ]

 

[charlie]

Re: Service Conductor

inspector 102, don't forget that the bare concentric in Type SE cable is rated the same ampacity as the phase conductors even though is smaller in size.

I would like someone else to verify this statement, I can't find a solid reference but I am reasonably sure it is correct (of course, I'm never wong).

 

[tom baker]

Re: Service Conductor

The NEC used to say the neutral would be two trade sizes smaller. I recall this went away in 1996, the neutral is sized based on the maximum unbalanced load or table 250.66 which ever is larger.
However, when we buy cable or conductors for a residential application, it comes in 4/0-4/0-2/0, hence we may still tend to believe the neutral requirement is for two trade sizes smaller.
I don't know about SE cable. In Washington State I have never seen a residential application with SE cable. Our standard is 200 A panels, 42 circuit, with 2 inch conduit. I typicall see a 42 circuit main breaker panel out the door at the wholesale house for $60.00

 

[iwire]

Re: Service Conductor

Originally posted by inspector 102:
I to am guilty of reading only the section of the NEC that I want to and was looking for other reference to this installation.

That is big of you to admit, I know that I can be guilty of this too, you sound like an inspector I would want to work with.

Back to the topic, it gets so depressing to always hear of people trying to do the absolute minimum.

How much cost savings is realized on a typical residential service by reducing the neutral size.

I have never done this and would not know how to do the calculations that Bryan pointed out to make me fell comfortable with down sizing the neutral.

[ May 10, 2003, 08:43 AM: Message edited by: iwire ]

 

[charlie]

Re: Service Conductor

Bob, think about this, you are wiring a small home that has gas appliances (heat, WH, range, dryer). A 100-ampere service is requested and your calculations indicate a 100-ampere service so you put in a 100-ampere service. I would assume you would feel comfortable about the installation.

Now suppose another home was built by the same builder and it is exactly the same except he now wants all electric appliances (heat, WH, range, dryer). A 200-ampere service is requested and your calculations indicate a 200-ampere service so you put in a 200-ampere service.

In the second example, where is the big increase in neutral current? Why wouldn't you feel comfortable with about the same size neutral that you installed in the first house after you took into account the 70% for the range and dryer?

 

[iwire]

Re: Service Conductor

Charlie, you are right.

It is strictly a personal thing for me, I just do not think the limited cost savings on a typical residential service would be worth it to me not to provide a service that can deal with any changes down the road.

No NEC reason not to do it, and I would not want there to be.

 

[russellroberts]

Re: Service Conductor

In my area,I can't remember the last time I saw a 100a service on a new home no matter how small.

150a loadcenters are almost nonexistent here,so most EC's just build a 200a service reguardless of the size of the home. Mostly I think that it's a case(around here anyway) of it's only a few dollars more to build a 200a as opposed to 100a.And if copper thhn is used,very,very few downsize the neutral.

I guess this could be considered lazy in some respects as far as doing a load calc.

But then again, most customers ask the question,what if I want to add an outbuilding,shop,heat pumps,etc.

russell

 

[bennie]

Re: Service Conductor

Making provisions for future expansion is an ideal situation, however, it is not practical when you count the money.

California State once required spare conduit from the panels, to the attic, or underfloor space.

This sounds like a good idea until you look at the entire picture. There is millions of homes, with empty conduit, to this day.

Sure, some of these conduits were used, but the cost savings is small, compared to the initial total cost.

[ May 11, 2003, 09:56 AM: Message edited by: bennie ]