SE Cable Temp Ampacity?

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inspector141

inspector141

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Westminster, MD
The 2008 code has deleted the reference for excluding the temp ampacity in 334.80 for SE cable. According to the 2008 code article 338.10(B)(4), SE cable installed as branch circuit or feeders must use the 60 degree ampacity column for interior wiring according to part II of article 334 which includes 334.80. Most code update books do not even recognize this change. Did anyone notice this change? This will require many SE cable installations to be upsized.

Marty
 
Yes, this has been discussed before and will catch quite a few electricians off guard. I agree that some publications have failed to adequately address this change since it can have a large impact on residential installations.
 
Does this change still allow the use of Table 310.15(B)(6) to size the Main Power Feeder between a outside Main and a inside MLO panel?

I'm still on the 05 code, so I'm just curious.
I'll eventually get a 08 code....but...not just yet.

thanks
steve
 
hillbilly said:
Does this change still allow the use of Table 310.15(B)(6) to size the Main Power Feeder between a outside Main and a inside MLO panel?

I'm still on the 05 code, so I'm just curious.
I'll eventually get a 08 code....but...not just yet.

thanks
steve
Click to expand...

Yes, but the definition of main power feeder was revised in the 2008. The change is more restrictive then the old wording. Here is the new wording of what constitutes a "main power feeder"310.15(B)(6)

" For application of this section, the main power feeder shall be the feeder between the main disconnect and the panelboard that supplies, either by branch circuits or by feeders, or both, all loads that are part or associated with the dwelling unit."

Chris
 
raider1 said:
Yes, but the definition of main power feeder was revised in the 2008. The change is more restrictive then the old wording. Here is the new wording of what constitutes a "main power feeder"310.15(B)(6)

" For application of this section, the main power feeder shall be the feeder between the main disconnect and the panelboard that supplies, either by branch circuits or by feeders, or both, all loads that are part or associated with the dwelling unit."

Chris
Click to expand...


So let me get this straight.:smile:

If I set a 200A Meter Main outside and this Meter Main has additional space for (2) double pole breakers which I use to power the (2) A/C compressors that are sitting right beside it,

and

From the Meter Main, I run a feeder to a inside 200A MLO panel,

and

Just because I used the breaker spaces in the Meter Main to power the A/C pumps, I'm now not allowed to use Table 310.15(B)(6) to size the feeder between the Main and the MLO panel...
It's no longer the Main Power feeder.:mad:

so


If I power the A/C pumps directly from the MLO panel.... which will add substantial, additional load the the Main Power feeder, I can then use table 310.15(B)(6)? :roll:

Doesn't make much sense to me....but hey...it doesn't have to.:smile:

Just my opinion
steve
 
hillbilly said:
So let me get this straight.:smile:

If I set a 200A Meter Main outside and this Meter Main has additional space for (2) double pole breakers which I use to power the (2) A/C compressors that are sitting right beside it,

and

From the Meter Main, I run a feeder to a inside 200A MLO panel,

and

Just because I used the breaker spaces in the Meter Main to power the A/C pumps, I'm now not allowed to use Table 310.15(B)(6) to size the feeder between the Main and the MLO panel...
It's no longer the Main Power feeder.:mad:

so


If I power the A/C pumps directly from the MLO panel.... which will add substantial, additional load the the Main Power feeder, I can then use table 310.15(B)(6)? :roll:

Doesn't make much sense to me....but hey...it doesn't have to.:smile:

Just my opinion
steve
Click to expand...

Yep, I believe that you have it correct.

This change, in conjuntion with the new requirement that SE cable ampacity be sized using the 60 degree column when installed as feeders or branch circuits, gets the most negative comments when I teach 2008 code update classes. Even more negative comments then the AFCI and the tamper resistant receptacle requirements.

Chris
 
SE Cable Ampacity

SE Cable Ampacity

Steve,

310.15(B)(6) does not require the main feeder to have a greater ampacity than the service conductors. Interesting example though, but not very likely.

Marty
 
inspector141 said:
Steve,

310.15(B)(6) does not require the main feeder to have a greater ampacity than the service conductors. Interesting example though, but not very likely.

Marty
Click to expand...

Actually, the example that Steve used is realtively common in my area.

Here is another example that I use in my classes.

Take a 320 amp meter/main combo with 2 200 amp breakers that constitute the service disconnecting means. You run two feeders from the two breakers to two 200 amp panels within the home. With the combine changes from the above section you could no use Table 310.15(B)(6) for the feeders. So assuming that we use SE cable for the feeders, we would have to use Table 310.16 and the 60 degree column to get the allowable ampacity for the cable.

So to make the above work you would need to use 300 kcmil aluminum SE cable or a 4/0 copper SE cable.

Chris
 
raider1 said:
Take a 320 amp meter/main combo with 2 200 amp breakers that constitute the service disconnecting means. You run two feeders from the two breakers to two 200 amp panels within the home. With the combine changes from the above section you could no use Table 310.15(B)(6) for the feeders. So assuming that we use SE cable for the feeders, we would have to use Table 310.16 and the 60 degree column to get the allowable ampacity for the cable.

So to make the above work you would need to use 300 kcmil aluminum SE cable or a 4/0 copper SE cable.
Click to expand...

Chris,

I guess I'm "missing it". Wouldn't the feeders you described in your example be the main power feeders? I know the NEC is singular when it speaks of the panelboard and the feeder, but would it be possible that the intent of the section has been met in regard to your example?

Pete
 
pete m. said:
Chris,

I guess I'm "missing it". Wouldn't the feeders you described in your example be the main power feeders? I know the NEC is singular when it speaks of the panelboard and the feeder, but would it be possible that the intent of the section has been met in regard to your example?

Pete
Click to expand...

Pete, the reworded definition of "main power feeder" would not permit two feeders from the main service to two panels in the house to use Table 310.15(B)(6). The new wording is clear that only a single feeder that carrys all loads for the dwelling unit can utilize Table 310.15(B)(6).

Chris
 
SE Cable ampacity

SE Cable ampacity

The change in the 2008 only clarifies or strengthens what was already there.
We have for years known that two 200 amp service/feeder panels for a single family dwelling that are fed with feeders or from a meter technically were not allowed to be fed with 4/0 alum if the load for each panel exceeded 180 amps. The reason is that one feeder/service serving the entire dwelling provides a tremendous diversity of loads as opposed to splitting up the feeder or service panel loads. For example, one 200 amp feeder in a dwelling of a 400 amp service,may have all the A/C loads and other common loads in one panel but does not give much of a diversity than the other 200 amp feeder. The first panel conductors might be overloaded based on 310.15(B)(6), but would not be overloaded when applying 310.16, because larger conductors might be required. The second panel would not be overloaded at any rate. Usually the calculated load for a 200 amp service would not exceed 180 amps (310.16)which would allow the installer to use 4/0 alum conductors regardless of if the allowable ampacity in 310.15(B)(6) applies. 310.16 still allows 4/0 on a 200 amp service if the load does not exceed 180 amps (240.4(B).
 
inspector141 said:
310.16 still allows 4/0 on a 200 amp service if the load does not exceed 180 amps (240.4(B).
Click to expand...
I would hope that the continuous load on a 200a panel does not exceed 160a.
 
310.16 still allows 4/0 on a 200 amp service if the load does not exceed 180 amps (240.4(B).
Click to expand...

Not in the 60 degree column of Table 310.16. Using the 60 degree column 4/0 SER cable would have an ampacity of 150 amps and you could not use the next size up rule because 150 is a standard sized breaker. So with the two changes in the 2008 a 4/0 SER feeder, that is not a main power feeder, would have an ampacity of 150 amps.

Chris
 
SE Cable Ampacity

SE Cable Ampacity

Larry,

The calculated load would have already taken into account whether the load is continous or not. A double calculated continous load would not be correct.

Marty
 
raider1 said:
Not in the 60 degree column of Table 310.16. Using the 60 degree column 4/0 SER cable would have an ampacity of 150 amps and you could not use the next size up rule because 150 is a standard sized breaker. So with the two changes in the 2008 a 4/0 SER feeder, that is not a main power feeder, would have an ampacity of 150 amps.

Chris
Click to expand...
Chris,
My point is there is no real change when applying table 310.15(B)(6). If there are two power feeders for one dwelling, applying table 310.15(B)(6) would not be allowed under the 2005 code as well as the 2008. Under the 2008 code, if table 310.15(B)(6) is applicable, the 60 degree rating for SE or SER would not apply. Table 310.15(B)(6) would rule over 338.10. You are correct under the 2008, if two or more SER feeders are used to supply the panels and are installed as interior wiring, then according to the 60 degree ampacity table, 4/0 alum is only rated for 150 amps. But if the service conductors are 4/0 alum, they are not interior wiring and would be rated at 180 amps from the 75 degree column.

Artcle 310.15 (B)(6) says:

"The feeder conductors to a dwelling unit shall not be required to have an allowable ampacity rating greater than their service-entrance conductors."

Therfore 4/0 alum SER would have a rating of 180 because they are not required to be larger than the service conductors. This makes sense. It would be silly to have 4/0 alum serv conductors and 300kcmil alum feeder conductors.

Marty
 
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