Table 310.15(B)(6)

[Dennis Alwon]

I am thinking of a proposal for 2011 on this table based on the changes in 338.10. I have never done this before so I really don't have a good feel for it but I will do my best.

Here is the current wording of the table

Table 310.15(B)(6) Conductor Types and Sizes for 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders. Conductor Types RHH, RHW, RHW-2, THHN, THHW, THW, THW-2, THWN, THWN-2, XHHW, XHHW-2, SE, USE, USE-2.

I would like to add an asterisk after "SE" and add a note that states when SE cable is installed outdoors.

Proposed Text

Table 310.15(B)(6) Conductor Types and Sizes for 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders. Conductor Types RHH, RHW, RHW-2, THHN, THHW, THW, THW-2, THWN, THWN-2, XHHW, XHHW-2, SE*, USE, USE-2. *when installed per article 338.10(B)(4)(b)

My basis is that since NM cable is not listed and rated 60C then neither should SE if installed indoors.
It seems that currently under the 2008 SE cable is now limited to 60C when installed indoors as per article 338.10(B)(4)(a). This would mean that a 2/0 SE copper cable would be rated 145 amps and still be allowed on a 200 amp breaker. Dwellings only, of course.. It seems the intent was to lower the rating of SE but somehow I think it got overlooked in this section.



Feedback please.

 

[iwire]

Dennis, IMO Table 310.15(B)(6) is not a conductor ampacity table like 310.16 etc.

It is just a 'you can use this for this' table, there are no temperature derting requirements etc.

The reason they can say 2/0 CU can be used for 200 amp dwelling unit service is because they are counting on the load to never reach that amount, and it will not as long as the calculations in Article 220 are followed.

If I had my way Article 220 would be changed to be provide more realistic service size and Table 310.15(B)(6) would go away.

 

[Dennis Alwon]

So you don't think the fact that a 75C wire is rated 175 amps in the last code and now rated 145amps has no bearing on how they est. table 310.15..

 

[iwire]

So you don't think the fact that a 75C wire is rated 175 amps in the last code and now rated 145amps has no bearing on how they est. table 310.15..

No I do not, I bet the real load is under 145 and if does exceed 145 it will be for very short duration.

What is the ampacity of the power companies conductor supplying the house?

A power company engineer has stated on this forum they figure the real load as about 50% of the NECs calculated load (for a dwelling).

All that said, make the proposal, with all the changes about wire temp it could be accepted. :smile:

 

[Dennis Alwon]

Let me add that my thinking is that Table 310.15(B)(6) appears to be based on one conductor size lower than it would be if the table did not exist. Given that it seems that this is an issue.

I agree totally with Bob on the actual load on a service being much lower and the fact that there are millions of services out there under the 2005 and earlier code that allows this install. It just appears to not be consistent with the changes in 338.10.

Maybe one day 310.15(B)(6) won't exist.

 

[ryan_618]

I would leave it alone. I would hate to see this table become less usable than it already is. With the changes in the 2008, it has increased the size of conductors for about 90% of the houses that I inspect.

 

[tom baker]

Our local utility uses 5KW as the average connected load to a dwelling unit.

 

[Dennis Alwon]

I never thought the NEC was about what the utilities use but here we have many heat pumps with electric strip heat. Not unusual to see a 2o kw or 2 10 kw units in a house with a 200 amp service-- electric water heater, stove etc.

I concur that this will probably never cause a problem but does anyone think there is inconsistency here in these sections. For years everyone was using 75C termination for SE cable. Now we can't--- hum , maybe the error was in dropping the excluding 334.80. There is no gray area to designate change....???

 

[iwire]

Dennis first let me say I do understand what you noticed with the changes and I think your proposal is logical and makes things more consistent.

I am still not convinced it is a necessary change for safety.

but here we have many heat pumps with electric strip heat. Not unusual to see a 2o kw or 2 10 kw units in a house with a 200 amp service-- electric water heater, stove etc.

It does not matter one bit what equipment the home has in it as long as the service for that home was calculated per Article 220.

Now what happens when people add equipment with out doing service calculations?

I don't know, but I do know the NEC already counts on the calculated load to protect the service conductors.

600 amps worth of breakers 'protecting' the service conductors in that riser.