much confusion over sub panel

[LarryOZ]

I have read a number of threads here about sizing conductors for sub panels and have not found a definitive answer.

Here is a scenario that is common in the part of the world:
400 amp ces with 2 200 amp main breakers.
Panel is mounted on the exterior of an attached garage as per the PoCo.
Service is underground.
Ambient air temp in summer normally can reach 49c
The confusion starts when the (#2)main breaker feeds an existing 200 amp panel which currently is serving the living area.
The (#1) main breaker supplies the panel board which will feed various equipment in the garage, a well pump, swimming pool and a hot tub.

Table 310.15(B)(6) can be used to size 3-wire, single-phase, 120/240V service or feeder conductors that supply all loads that are part of or associated with the dwelling unit.

Using this table 3/0 cu thhn 90c would be the correct conductor as I interpret it.

Let the different views be heard.

 

[Dennis Alwon]

I would agree that you need 3/0 copper since the house doesn't carry the entire load of the service. Unfortunately, it is an attached garage so that makes it part of the dwelling unit loads.

 

[jaggedben]

Just realize that the table you're referencing makes no logical physical sense and the conclusion you reach is totally arbitrary, and then your mind will be at peace over this. :lol: In my opinion you ought to get to use that table, but I can't give you a bullet proof argument based on code chapter and verse. Dennis might as well be right.

You are referencing an older code, by the way, which code cycle are you on?

 

[Smart $]

Short answer... you cannot use that table because neither subpanel supplies the entire load of the one-family dwelling unit. That table does apply to the service conductors on the line side of the 200A breakers... provided they are under NEC purview.

I should add that if the hot tub and pool equipment were the only items on one subpanel (not the well pump; i.e. equipment external, and not serving the dwelling unit), you could apply the table sizes to the feeder supplying the dwelling unit itself.

 

[electrofelon]

Table 310.15(B)(6) can be used to size 3-wire, single-phase, 120/240V service or feeder conductors that supply all loads that are part of or associated with the dwelling unit.

Using this table 3/0 cu thhn 90c would be the correct conductor as I interpret it.

I agree with Smart. But I am confused by your statement i quoted above. Would it not be 2/0 (if you could use that table which you cant)?

 

[LarryOZ]

I agree with Smart. But I am confused by your statement i quoted above. Would it not be 2/0 (if you could use that table which you cant)?

This topic is quite confusing for sure.

The csed used is normally a Square D Model # SU3040D400CN

Smart $ - The NEC states "all loads that are part of or associated with the dwelling unit."

Logic would dictate that the pump supplying service water to the residence, the occupants hot tub, the garage attached to the dwelling unit ,etc are covered under the working part of or associated with the dwelling unit.

electorfelon - I arrived at 3/0 using the temperature correction table, which as most things I do may not be correct!

me :dunce:

 

[jaggedben]

...
electorfelon - I arrived at 3/0 using the temperature correction table, which as most things I do may not be correct!

me :dunce:

I'll ask again what code cycle you're on, exactly. The 2011 NEC just said you can use the conductor sizes in the table, which to me implies you can forget about temperature correction. Whereas the 2014 NEC says you can use a conductor with only 83% the normally required ampacity, so that to me says you do all the normal things including temperature correction and then get to downsize from there.

 

[GoldDigger]

But others, including some CMP panel members I believe, felt that the corrections and adjustments always applied and the change simply clarified that.

Sent from my XT1585 using Tapatalk

 

[LarryOZ]

I'll ask again what code cycle you're on, exactly. The 2011 NEC just said you can use the conductor sizes in the table, which to me implies you can forget about temperature correction. Whereas the 2014 NEC says you can use a conductor with only 83% the normally required ampacity, so that to me says you do all the normal things including temperature correction and then get to downsize from there.

2011 National Electrical Code

 

[Smart $]

Smart $ - The NEC states "all loads that are part of or associated with the dwelling unit."

Logic would dictate that the pump supplying service water to the residence, the occupants hot tub, the garage attached to the dwelling unit ,etc are covered under the working part of or associated with the dwelling unit.

The water pump for sure, assuming it's for household potable water and not just for the hot tub and pool. But if the hot tub and pool are both exterior to the dwelling, these could be considered accessory structures. Debatable for the hot tub if it does not have completely detached decking.

Otherwise, you are correct. In weighing this out the AHJ must consider whether accessory structures on the premises are necessary to provide the utility required of a dwelling unit.