i am wiring a detached garage, and putting a 100 amp sub panel in the garage, feeding it from a 200 amp main panel in the house. i am going thru the house from the main panel, to a junction box then thru the outside block wall underground with pvc to the garage, is it akay to use #4/3 romex in the basement or do i have to use #2 ser.

I just read an interesting take on this from Gerald Newton. We all know you can't use 310.15(B)(6) for the feeder but he points out the #4 is good for 85a at 75c and the next standard breaker size is 100. You would still have to have a calculated load of 85 or less but it seems to be legal to protect this with 100a.
edit
You do still bump up against that 60c thing with Romex tho. Do it with THHN/THWN in Smurf.



[This message has been edited by gfretwell (edited 12-11-2006).]

Greg, are you sure Gerald says the next standard size breaker from 85 amps is 100?

It is actually 90.


Roger

People frequently misinterpret the "exception" that permits #4 CU or #2 AL for RESIDENTIAL 100 amp services; for other uses, the sizing must be calculated just like any other feeder or branch circuit.

They also frequently abuse the 2/0 Cu & 4/0 Al 200 ampere service ampacity by using the next size breaker exception. I asked one GC if he intended to tell the customer that he had ordered a 370 ampere service installed in place of the 400 ampere one he had sold the customer and he went ballistic and ordered me out of the trailer. The inspector had already told me that he would amend the permit down to 370 amps if the two runs of 4/0 were used to feed the two two hundred ampere panels. I think we should be required to post a warning label on the service equipment cover when the ampacity of the service entry conductors is smaller then the over current protective device/s that they supply.
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Tom Horne

[This message has been edited by tdhorne (edited 12-12-2006).]

Roger, I went back and looked, You are right the discussion was 4/0al and 200a feeds not #4 and 100a.
It's not my fault, I was whacky on the Nyquil

Sorry for the confusion

Tom shouldn't a qualified person opening the panel to add load realize the conductors are sized for 370 amps?

iwire Wrote: "Tom shouldn't a qualified person opening the panel to add load realize the conductors are sized for 370 amps?"

Sure he/she should check the ampacity of the service conductors and do a service calculation for any increase in the service load but how many do? I'm just frustrated by this type of penny pinching fraud. If the service size is the ampacity of the conductors then the customer should be told that the service that is being installed is 370 rather than being told that they have a four hundred ampere service which is a bold faced lie. A good definition of that particular type of General Contractor is a fella who would cut his own mothers throat from ear to ear for five cents off per hundred board feet of lumber. It just gives me the creeps working for them. In forty years of wiring it has only gotten ugly with two of them. One was the guy I already mentioned and another was a guy who wanted me to sign a statement that the home had been wired to the highest standards of the industry when many multi entry rooms did not have switches at every entry, the floored storage attic was unlighted, the basement lighting consisted of pull chain fixtures, and the list went on. He said I would never work in the industry again. I laughed at him to his face and dragged up. I had work on another site the following day.
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Tom Horne

I wrote a proposal to the 99 code about a similar issue. I was seeing 400a switchgear installed with 225a fuses in it and 4/0 copper feeders. My concern was it would be trivial for anyone to see 400a switchgear assume larger fuses would be OK. CMP 10 said only qualified people should be replacing fuses and they should know better. Changing a breaker would probably involve a better qualified person.

We will have to remain in disagreement that what you describe is 'fraud'

In most cases if the prints had already determined the service size the conductor sizes would also be on the prints.

If the service size was left for the EC to calculate and design than the customer has no expectation of anything other than a NEC compliant installation.

Another important point is this.

IF a service qualifies for and the conductors are sized using Table 310.15(B)(6) than it is 'that size' service.

A homes 200 amp service using 2/0 CU based on Table 310.15(B)(6) is in fact a '200 amp service' and can be loaded to that level.

This is different than when we apply 240.4(B) where we can round up but we can not load past the 310.16 ampacity.

A very common commercial installation will use 6 service disconnects that total far higher than the actual service conductor ampacity.

Here is another common example. This is a service a buddy of mine did.



Notice the size of the riser, it has a capacity of about 200 amps. If you add up the six main breakers inside you would get 600 amps.

Is this 'fraud'?

Should he have run a 600 amp riser for no particular reason and charge the customer a lot more money?

Bob



[This message has been edited by iwire (edited 12-26-2006).]

Greg how could CMP 10 agree with that proposal when disconnecting means only come in certain sizes?

I may have an RTU that the nameplate requires 125 amp fuses and of course I will need to use a 200 amp disconect.

Also don't forget about 230.90(A)Ex. 3


That is the exception that allows the service in the picture above.

If each of the five tenants adds say an electric range the riser will be undersized.

My point is there are many instances where the NEC relies on qualified people to ensure conductors are not overloaded.


[This message has been edited by iwire (edited 12-26-2006).]

I was just curious what they would say about the possibility of overfusing the installation. It sure looked likely to me since this was already cruising at a significant portion of the service capacity and it was brand new. They had no way to go but bigger. It is at the rest stop at MM 60 on I75. The DOT maintenance staff will be the ones who look after it. I was pretty unhappy with the size vs the length of the feeder too but I couldn't do anything about the expected voltage drop either. Usually the state overbuilt everything but they sure cheaped out on this one.