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Old 08-11-2010, 08:56 AM   #16
UAW SKILLED TRADES
 
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Stubbie, are you sure about those ampacities?
Yes .. but I think you feel there is an error. So I am open to what that error is if you would point it out.

I base the ampacities I listed on the provisions provided by 110.14 (C). This is assuming that I have 75C terminations at both panels. Technically not knowing that information would require 60C ampacities. So in that respect I'm on shakey ground....
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Old 08-13-2010, 05:40 PM   #17
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Sorry Stubbie, I just noticed your reply today.

EDITED, incorrect information.
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Old 08-13-2010, 07:08 PM   #18
UAW SKILLED TRADES
 
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I don't agree at all ...

How do you intrepret 240.4(B) and the 3 requirements. If I have a 55 amp conductor in table 310.16 I am allowed for a feeder using #6 UF-B to use a 60 amp breaker because its ampacity (55 amps) does not fall on a standard fuse size in accordance with 240.6(A)...no?
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Old 08-14-2010, 08:07 AM   #19
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I don't agree at all ...

How do you intrepret 240.4(B) and the 3 requirements. If I have a 55 amp conductor in table 310.16 I am allowed for a feeder using #6 UF-B to use a 60 amp breaker because its ampacity (55 amps) does not fall on a standard fuse size in accordance with 240.6(A)...no?
I agree, and never failed an inspection using this method!
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Old 08-14-2010, 09:28 AM   #20
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I've been looking into why I thought that way. From what I can tell, you can't upsize the breaker if the calculated load is higher than the cable's ampacity. In all other cases you can. You were right Stubbie. I really don't know why such an elementary code issue has me confused.
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Old 08-14-2010, 02:01 PM   #21
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I've been looking into why I thought that way. From what I can tell, you can't upsize the breaker if the calculated load is higher than the cable's ampacity. In all other cases you can. You were right Stubbie. I really don't know why such an elementary code issue has me confused.
Hey it happens.... No sweat. Speaking of feeders they only have to be capable of carrying the calculated load. If the calculated load is more than the ampacity of the feeder then your going to have to upsize the feeder. I never try to 'squeeze' a feeder to meet a calculated load... give yourself some room.. In many cases a 60 or 70 amp feeder is fine to a 4 space 70 amp panel that uses tandems for 120 volt circuits.. #6 thhn/thwn on 60 or 70 amp breaker. Depends on how many expected 240 volt circuits the customer needs. But many garages doubling as small hobby shops fall in this catagory.

Anyway getting back on the subject a little.... I have two file links I saved that kinda goes over all this.... I go back to it quite often when I get a little fuzzy on what we have been discussing.. I'll post them and you can save for yourself.

One goes into a lot of other considerations and one is less detailed. You will notice in the detailed article that it isn't always as simple as "next size' up' ... and remember to that feeders supplying only motors are 'next size down' .. So no surprise at all when I hear it is confusing .. I've been there myself many times.

I'm posting these because they are excellent references not because I think you need them. Given time on this forum your going to catch me (if you haven't already) in one of those fuzzy moments ...and return the favor....

Sizing Conductors for All Load Conditions | IAEI Magazine Online

Overcurrent Protection for Conductors | IAEI Magazine Online

And here are a couple good ones for motors and the calculations required


Motor Calculations Part 1: Motors and Branch-Circuit Conductors

Motor Calculations Part 2: Feeders
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