100A subpanel

[blackhat]

Further, will your feeder breaker accept #2 Al conductors?  You may need to include a box for transition connections. $40 here, $20 there will negate money saved on wire pretty fast.

[JosephP]

At some point in the future, I may, maybe will put an office on that side of the building.  If so, it may have a small AC unit there.  Maybe a fridge.  Maybe even an oven???  While I'm running it, I figured 100A made sense.  Maybe it doesn't.  I'm not opposed to dropping to 60A.

The subpanel I'm using did have a 125A breaker on it.  Not saying it was "right" before...just that was there.  Pics of equipment should help.

Thanks again!

[srv52761]

......
The Nausau spec sheet includes "Use-2" which on the chart you provided is listed for 100A.  ......

If that quad URD is composed of only USE cable, it can not be used indoors at all, conduit or no conduit.  Look at the individual conductors and see if it has the letters RHW or RHW-2 on it.  If so it can be used indoors completely within conduit.  Most USE-2 manufactured today is dual rated.

[JosephP]

Here is the box I have that I was planning to use for the subpanel

   

[JosephP]

OK...paid more attention to where I would run the wire...

First the north wall...the main panel will be on the north wall.  So if I run it without conduit (conduit up wall, then none across the top), should I run it behind the posts on top of the wall instead on the ceiling joist.

   

Then it looks like I would need to come inside the corner post

   

And behind the posts of the west wall and down through conduit again to the sub

   

[srv52761]

I was still flummoxed by that 155 amp rating, so I asked my fellow members on Mike Holt's forum of electricians, electrical engineers, inspectors and such.  Several explained.  This was typical.

Q.  Why is #2 alum rated at 155 amps in this URD cable.

R.  A cable needs to have an NEC designation to be used in applications governed by the NEC. When you use it for applications governed by the NEC, the ampacities in the NEC for that designation, will override larger ampacities indicated on the product datasheet. URD is not one of these designations. It would have to carry additional designations to be used where governed by the NEC.

URD stands for underground residential distribution, and is used by utilities for distributing power underground on the utility's side of the service point. URD secondary cable is what the utility would use, for feeding a neighborhood of homes from a common pad-mount transformer that the utility owns. The utility isn't governed by the NEC, and has different standards for what ampacities they can use on theIr wire.  The NEC ampacities are more conservative, because of the greater risk of burning down buildings, if there is a failure of the wiring.

So in essence, URD is designed to be used by utilities.  Utilities are not governed by the NEC. The NEC or NFPA 70 is published by the National Fire Protection Association to protect buildings and such.   URD is not an NEC designation, but rather a  National Electric Safety Code designation, the code that governs utilities.
When URD is used by utilities it is serviced only by trained professionals in an environment with few flammables;  hot wires just get hot.  
Not so much if used in residential environments.  When used for residential, then the NEC is usually the governing body.  Then one would need to look for an NEC label. In this case you found the URD was assembled with conductors with the USE-2 designation.  So then you would look that up on the NEC charts to determine ampacity and other characteristics for use in residential or commercial.   In this case, 90 amps, no indoor use.

[JosephP]

Great - Thanks for digging that up!

So....no code here.  Is it unsafe to use that and run 100A or should I jump down to a lower amperage (90?) and use 2-2-2-4 mobile home feed?

[srv52761]

Great - Thanks for digging that up!

So....no code here.  Is it unsafe to use that and run 100A or should I jump down to a lower amperage (90?) and use 2-2-2-4 mobile home feed?

#2 aluminum is #2 aluminum, whether in MHF or in the USE-2 in this URD, the ampacity is the same. The difference is in the insulation.  USE-2 insulation has no protection from light and gives off extremely noxious fumes when it melts.  MHF has uv protection, and also gives off noxious gas, but not as bad. It is allowed indoors if entirely enclosed in conduit.

Your entire run is indoors.  SER is the wire you should be considering.  That can be run exactly the same as romex. 
#2 aluminum is rated for 90 amps, but is allowed for 100 amps if supplying all the power to a residence, the thinking being you will never use all your power at the same time. And even then, as indicated by the NESC designation, can handle the current.  (As an aside, the members on Mike Holt’s forum have stated using #2 aluminum for a 100A sub is common practice.   Our local inspector acknowledged this disparity in usage and did approve #2 aluminum for my 100A sub to my workshop.  I tell you this as a matter of fact and in no way am I recommending this.  Putting this in print gives me the willies.... )

[JosephP]

Really appreciate the time you've taken to run through all this!  Not only does it get me the right info...but I have an understanding as to WHY it is, so can feel confident I am doing the best thing.

[TDKPE]

Correct me if I’m wrong, but I thought the rules were ‘clarified’ to require SE and SER used as other than service entrance cable (feeders and branch circuits, in other words) be used at the 60C ampacity, same as NM.  That would make it 1/0 aluminium for 100A sub panel or non-dwelling unit feeder, where 2 AWG is the Table 310.15 value for dwelling unit service entrance and service feeder applications.