Subpanel

[curt swartz]

I think you are adding too much if factor to this. A small home would not require a very big air condtioner any any case so that load would be diversive also. You certainly arent going to have more than one ac unit on the panel anyway. I know what the code says but I dont agree that is necessarily logical when it comes to this specific area. Anyhow just use 2 al and a 90 amp breaker and you are good to go.

I think you are missing the point. What if the feeder is not in a small house? It's not uncommon to have 100 amps of heating or cooling load in a larger dwelling. The feeder might be supplying pool equipment, hot tub or combination of loads that may be continuous.

 

[kwired]

I think you are adding too much if factor to this. A small home would not require a very big air condtioner any any case so that load would be diversive also. You certainly arent going to have more than one ac unit on the panel anyway. I know what the code says but I dont agree that is necessarily logical when it comes to this specific area. Anyhow just use 2 al and a 90 amp breaker and you are good to go.

If you use the 2 al and a 90 amp breaker you are in compliance. If you put same 2 al on a 100 amp breaker you must meet rules of 310.15(B)(6) or you are not in compliance.

As far as a feeder to a sub in a small house if you did a load calculation on what you connect to it, if it is nothing more than general purpose lighting and receptacle loads you likely do not need a 100 amp feeder to be able to handle the load.

A 60 amp feeder will supply a lot of lights and outlets. If you start to add HVAC, water heaters, or other heavy loads you need to be careful but are often better to feed them directly from the service panel if possible unless there is just one unit in a remote area plus other circuits in that area you wish to feed - that is a good example of when the load diversity may not be enough to justify the smaller feeder conductors allowed in 310.15(B)(6). I don't think the NEC wants you to try to determine what the load diversity will be though -that is something for an engineer to determine. It is not worth paying the engineer his fee to save maybe $20-50 in cost difference of conductors on this feeder.

 

[romex jockey101]

when your sizing feeders for your main disconect you can use art 310.15b6 but when you sizing feeders for a subpanel suddenly its withen the dwelling unit you would use table310.16 90* thhn 4awg is rated at 95 amps so use 4awg andput it on a 70Iocp...why would you have to take it at the 60*colum if ure using sec

 

[infinity]

when your sizing feeders for your main disconect you can use art 310.15b6 but when you sizing feeders for a subpanel suddenly its withen the dwelling unit you would use table310.16 90* thhn 4awg is rated at 95 amps so use 4awg andput it on a 70Iocp...why would you have to take it at the 60*colum if ure using sec

Whay code cycle are you referring to? the 2005, 2008 and 2011 NEC all have different requirements for SE cable installed indoors.

 

[Twoskinsoneman]

The feeder in the OP is not feeding a "dwelling unit" it is feeding loads in a dwelling unit but not the entire unit. Size of service has nothing to do with it what the feeder is supplying does.

T310.15(B)(6) has taken into account that on the main feed to a dwelling that not all load in the dwelling will be used at the same time. They have come up with the values in the table as being acceptable for this situation. If the feeder is not supplying the entire load of a dwelling it is harder to determine the load diversity in generic terms and create such a table. If the feeder happens to feed only HVAC or something like that it may very well be loaded 100 percent quite frequently.

There are cases where you can run conductors smaller than required by 310.16 but they require engineering supervision to do so. 310.15(B)(6) has done the engineering supervision part to a certain extent for you but only for a feeder or service conductor that supplies the entire load to a dwelling unit.

This logic can not apply to having feeder LARGER than the service conductors...
Whatever the diversity loads are the feeder current isn't going to be higher than the service conductors.

I was convinced about this before but 215.2(A)(3) seems to say the feeders don't need to be larger than the service conductors. I'm not certain "for individual dwelling units" makes this rule apply ONLY to feeder carrying the entire dwelling load.... I think that make be making the NEC say what you believe is in line with what you believe. If it IS what they meant they made a poor effort getting that point across.

 

[Dennis Alwon]

when your sizing feeders for your main disconect you can use art 310.15b6 but when you sizing feeders for a subpanel suddenly its withen the dwelling unit you would use table310.16 90* thhn 4awg is rated at 95 amps so use 4awg andput it on a 70Iocp...why would you have to take it at the 60*colum if ure using sec

2008 NEC says se when used indoors as a feeder must be used at 60C- same as nm cable. #4 can be on a 70 amp breaker max in that case. 90C is only good for derating.

 

[infinity]

2008 NEC says se when used indoors as a feeder must be used at 60C- same as nm cable. #4 can be on a 70 amp breaker max in that case. 90C is only good for derating.

What about the 2011?

 

[Dennis Alwon]

What about the 2011?

Most areas haven't adopted it yet but there are some changes there. If the se cable is not run in any insulation than you can use 75C.

 

[infinity]

Most areas haven't adopted it yet but there are some changes there. If the se cable is not run in any insulation than you can use 75C.

I thought that I remembered reading that somewhere.

 

[iwire]

Most areas haven't adopted it yet

I think I am in the only state under the 2011 NEC ...... lucky me! :roll::grin:

 

[infinity]

I think I am in the only state under the 2011 NEC ...... lucky me! :roll::grin:

Yup we have about a year to go, and then a 6 month grace period.

 

[romex jockey101]

Whay code cycle are you referring to? the 2005, 2008 and 2011 NEC all have different requirements for SE cable installed indoors.

2005

 

[romex jockey101]

2008 NEC says se when used indoors as a feeder must be used at 60C- same as nm cable. #4 can be on a 70 amp breaker max in that case. 90C is only good for derating.

ct is under 2005.is that something new were 90*is only for derating im confused ive always used 90*colum when finding ampacity of conductors...in a dwelling unit

 

[Dennis Alwon]

ct is under 2005.is that something new were 90*is only for derating im confused ive always used 90*colum when finding ampacity of conductors...in a dwelling unit

90C was always for derating. The terminals you connect to are rated 75C so your OCPD cannot be larger then the weakest link. In the case of NM cable it is 60C because the wire is rated that.

 

[infinity]

ct is under 2005.is that something new were 90*is only for derating im confused ive always used 90*colum when finding ampacity of conductors...in a dwelling unit

Here's the breakdown:
2005-SE Cable can be used at the 75? C ampacity
2008-SE Cable must be used at the 60? C ampacity
2011-SE Cable can be used at the 75? C ampacity when not in thermal insulation

Confusing? :grin:

 

[Little Bill]

90C was always for derating. The terminals you connect to are rated 75C so your OCPD cannot be larger then the weakest link. In the case of NM cable it is 60C because the wire is rated that.

Actually NM is rated 90C but only for derating. Question is why does it have an insulation rating of 90C but 60C is all the ampacity that is allowed. I know nothing is 90C but you would think that since most terminals are 75C that NM would have a rating of that.

 

[Dennis Alwon]

Actually NM is rated 90C but only for derating. Question is why does it have an insulation rating of 90C but 60C is all the ampacity that is allowed. I know nothing is 90C but you would think that since most terminals are 75C that NM would have a rating of that.

I realize NM has a rating of 90C but the final ampacity of nm cannot be more than 60C-- that is the weakest link as I was trying to point out.

I think the reason NM is 60C is because of all the insulation it is run thru but that is just a guess.

 

[Little Bill]

I realize NM has a rating of 90C but the final ampacity of nm cannot be more than 60C-- that is the weakest link as I was trying to point out.

I think the reason NM is 60C is because of all the insulation it is run thru but that is just a guess.

Because the sheath, when hot or melting, puts off fumes that the insulation could trap in walls, ceilings, etc.?

 

[Dennis Alwon]

Because the sheath, when hot or melting, puts off fumes that the insulation could trap in walls, ceilings, etc.?

Well the reason, I am told, that seu cable was lowered to 60C was because the wire was breaking down when loaded to the 75C. The thought is apparent in 2011 where they allow 75C if not run in insulation. I guess the extra heat gets trapped and affects the conductors.

 

[kwired]

This logic can not apply to having feeder LARGER than the service conductors...
Whatever the diversity loads are the feeder current isn't going to be higher than the service conductors.

I was convinced about this before but 215.2(A)(3) seems to say the feeders don't need to be larger than the service conductors. I'm not certain "for individual dwelling units" makes this rule apply ONLY to feeder carrying the entire dwelling load.... I think that make be making the NEC say what you believe is in line with what you believe. If it IS what they meant they made a poor effort getting that point across.

This point has been debated here before and if I recall the general consensus was the feeder must feed the entire dwelling. That works if you hit your service disconnect say on outside of building then run a feeder to an interior distribution load center. If you hit the service disconnect outside catch the air conditioner from the service then run a feeder to same interior distribution load center you are no longer carrying entire dwelling on the feeder (you do have less load because the A/C is not on the feeder) but that is not how the NEC is worded.

Actually NM is rated 90C but only for derating. Question is why does it have an insulation rating of 90C but 60C is all the ampacity that is allowed. I know nothing is 90C but you would think that since most terminals are 75C that NM would have a rating of that.

NM cable is 60? because 334.80 says it is. I do not know the reason for 334.80.

Not all terminations are rated 75?. Especially on older equipment. Maybe this could have a little to do with it. And the fact that NEC has become a little more geared to handimen and DIY's although most of them would not pay attention to temperature rating or understand it anyway.