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Old 09-24-2010, 02:48 PM   #1
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Load Center Replacement Question


I have a very old 100a fused load center that I'd like to update. I know occasionally I'm close to being at max load for a short period of time. Between the load center and meter is a disconnect with 100a fuses. I'm afraid if I replace the load center with a 100a unit I will be getting trips since breaker characteristics are quite a bit closer to the actual rating than fuses are at least for shorter time periods. I'd like to drop in a 125a unit since I know I still have 100a protection with the disconnect fuses and quick term protection at 125a. My wire is tw-2 so 125a is not terribly over it's rating.
Does anyone see any issues with this?

Of course the correct thing to do would upgrade from the post in but it's not feasible at this time.

Thanks

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Old 09-24-2010, 04:14 PM   #2
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Load Center Replacement Question


You can put in the 125 amp rated panel, but does it have a 125 amp breaker?
As long as the wires from the outside disconnect are protected for the rated ampicity, then you can install any larger size box you want.

100 amp fues protecting a 125 amp panel is fine.

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Old 09-24-2010, 07:31 PM   #3
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You can put in the 125 amp rated panel, but does it have a 125 amp breaker?

As long as the wires from the outside disconnect are protected for the rated ampacity, then you can install any larger size box you want.

100 amp fues protecting a 125 amp panel is fine.
It does have a 125a main but the disconnect has 100a blade fuses.
I just wanted to make sure there wasn't some rule I didn't know about!

Thanks!
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Old 09-24-2010, 09:36 PM   #4
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Load Center Replacement Question


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My wire is tw-2 so 125a is not terribly over it's rating.
if you are in an area that accepts the NEC, you are not over the rating at all. #2 cu is what is listed as a minimum for a 125 amp residential service.
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Old 09-27-2010, 07:29 PM   #5
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if you are in an area that accepts the NEC, you are not over the rating at all. #2 cu is what is listed as a minimum for a 125 amp residential service.
I'm confused by this, from what I read in the NEC 2008 sticky under subpanels tw-2 is good for 95a. I also see it says "See 310.16 for wire sizing" and what I'm looking at is 300.13, how are they different?

Thanks Again
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Old 09-27-2010, 07:30 PM   #6
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Originally Posted by EmilG View Post
I'm confused by this, from what I read in the NEC 2008 sticky under subpanels tw-2 is good for 95a. I also see it says "See 310.16 for wire sizing" and what I'm looking at is 300.13, how are they different?

Thanks Again
that is because for a residential service 310.16 is not applicable. 310.15(B)(6) applies

310.13 doesn't deal with ampacity.It is conductor construction.
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Old 09-27-2010, 09:00 PM   #7
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Load Center Replacement Question


Quote:
Originally Posted by EmilG View Post
I'm confused by this, from what I read in the NEC 2008 sticky under subpanels tw-2 is good for 95a. I also see it says "See 310.16 for wire sizing" and what I'm looking at is 300.13, how are they different?

Thanks Again
They are diffrent if you used SE or NM cables they have diffrent rating than THHN/THWN type conductors and they will show two listing on the tables one for 60C while the other set will be on 75C that how we read the ampacity rating I know it can get confused.

But if you have med to large alum conductors they will automatic bump up one size due the conductor rating on alum cable are diffrent than copper.

Merci.
Marc
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Old 09-28-2010, 07:02 AM   #8
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Load Center Replacement Question


Guys,

Sorry to drag this out but Table 310.15(B)(6) says it's for:

Conductor Types RHH, RHW, RHW-2, THHN,
THHW, THW, THW-2, THWN, THWN-2, XHHW,
XHHW-2, SE, USE, USE-2 [ROP 6–66]

If the wire type is not listed (TW) does it default to the 310.15(B)(6)?

Thanks!


BTW, I have 3 TW-2 copper conductors in conduit (built 1955).

Last edited by EmilG; 09-28-2010 at 07:37 AM. Reason: Added wire info
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Old 09-28-2010, 11:18 AM   #9
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Load Center Replacement Question


Guys,

I realize I've errantly been calling my wire TW-2, it's actually TW 2ga. I learned the -2 means a 90deg. rating which gives it a much higher acceptable ampacity.

Thanks
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Old 09-28-2010, 07:22 PM   #10
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Originally Posted by EmilG View Post
Guys,

I realize I've errantly been calling my wire TW-2, it's actually TW 2ga. I learned the -2 means a 90deg. rating which gives it a much higher acceptable ampacity.

Thanks
The straight TW conductor is only rated at 75C{ much older verison will be only rated at 60C} while the TW-2 is rated at 90C so it will make the diffrence.

Merci.
Marc
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Old 09-29-2010, 12:11 PM   #11
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Load Center Replacement Question


My plan is to install the 125a load center with 100a FRN-R-100 dual element time delay fuses in the disconnect box. This way I'll make sure I don't trip prematurely near 100a but yet I'll still be thermally protecting the lines. Looking at the fuse chart it will take about 4 minutes to blow the fuse at 125a.

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