# 240.4B2 breakered above wire ampacity???



## 1capybara (Feb 27, 2009)

what does 240.4B2 mean?
here's an example:
1/0 XHHW is rated for 150amps. we have air conditioner [or whatever] with a
actual load 125 amps. 125% of 125 amps = 156 amps but these is
no such breaker. if the next higher breaker is 200A, would I
be allowed to use that?
this is an example only, please dont tell about the nameplate, thats
not what this example is about.
I dont understand this because it doesnt seem safe to use a 200A
breaker on wire rated for 150 Amps. or since we are now using a 200A breaker, do we now have to increase our wire size to 2/0 ???
:001_huh:
....................................................................................................
240.4B Devices Rated 800 Amperes or Less. The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all of the following conditions are met: 
(1) The conductors being protected are not part of a multioutlet branch ckt
(2) The ampacity of the conductors does not correspond
with the standard ampere rating of a fuse or a circuit
breaker without overload trip adjustments above its rating
(but that shall be permitted to have other trip or
rating adjustments).


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## jwjrw (Jan 14, 2010)

1capybara said:


> what does 240.4B2 mean?
> here's an example:
> 1/0 XHHW is rated for 150amps. we have air conditioner [or whatever] with a
> actual load 125 amps. 125% of 125 amps = 156 amps but these is
> ...




They make a 175A.

240.6 Standard Ampere Ratings.
(A) Fuses and Fixed-Trip Circuit Breakers. The standard
ampere ratings for fuses and inverse time circuit
breakers shall be considered 15, 20, 25, 30, 35, 40, 45, 50,
60, 70, 80, 90, 100, 110, 125, 150, *175*, 200, 225, 250, 300,
350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000,
2500, 3000, 4000, 5000, and 6000 amperes. Additional
standard ampere ratings for fuses shall be 1, 3, 6, 10, and
601. The use of fuses and inverse time circuit breakers with
nonstandard ampere ratings shall be permitted.


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## BBQ (Nov 16, 2010)

You have it close to right but you need to look at 240.6 for the standard sizes.

In your case you would move up to a 175 amp over current device not a 200.


It is safe just like using a 8 AWG EGC for a 100 amp breaker is safe. In either case if these is a fault the conductor can handle the fault current for the time it takes the over current device to open.


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## Dennis Alwon (May 9, 2009)

Many people have a problem with this but it is quite simple. A/C units have built in motor overload protection. This overload will protect the wire from having more current then it's ampacity. The circuit breaker is to protect from short circuit and ground fault protection only.

In your example you need to look at 440.22. You can rate the breaker at 175% of the motor rated load. So you could use, IMO 200 amps.


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## BBQ (Nov 16, 2010)

Dennis Alwon said:


> Many people have a problem with this but it is quite simple. A/C units have built in motor overload protection. This overload will protect the wire from having more current then it's ampacity. The circuit breaker is to protect from short circuit and ground fault protection only.
> 
> In your example you need to look at 440.22. You can rate the breaker at 175% of the motor rated load. So you could use, IMO 200 amps.


Dennis we can forget Article 440 here, 240.4(B) applies to all most all loads not just HVAC loads.


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## Dennis Alwon (May 9, 2009)

BBQ said:


> Dennis we can forget Article 440 here, 240.4(B) applies to all most all loads not just HVAC loads.


Are you saying that the op's a/c cannot be on a 200 amp breaker. I know what 240.4(B) is about but his question goes beyond that with a/c


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## BBQ (Nov 16, 2010)

Dennis Alwon said:


> Are you saying that the op's a/c cannot be on a 200 amp breaker. I know what 240.4(B) is about but his question goes beyond that with a/c



I am saying we have no idea, the OP asked about 240.4(B) and specifically stated ...



> we have air conditioner* [or whatever]*





> this is an example only,* please dont tell about the nameplate, thats
> not what this example is about.*



I was just thinking it may be better to stay on 240.6 till the OP is satisfied with that part of things.


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## 1capybara (Feb 27, 2009)

ok, so I was looking for an example of 240.4B2 like this:
2AWG is rated 115A. we have an actual load of
80% of 115A = 92A.
the next highest standard breaker is 125A.
is everyone ok with using 2AWG on a 125A breaker
because 240.4B2 says its ok ???


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## NolaTigaBait (Oct 19, 2008)

1capybara said:


> ok, so I was looking for an example of 240.4B2 like this:
> 2AWG is rated 115A. we have an actual load of
> 80% of 115A = 92A.
> the next highest standard breaker is 125A.
> ...


Is there a nameplate?...If so, just follow that...

also, thhn? Copper? aluminum?...You are missing some info


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## Dennis Alwon (May 9, 2009)

BBQ said:


> I am saying we have no idea, the OP asked about 240.4(B) and specifically stated ...
> 
> I was just thinking it may be better to stay on 240.6 till the OP is satisfied with that part of things.


You can do that but I felt it okay to add a bit extra. Sorry I didn't do what you want. If he just want that then he didn't have to use an a/c as an example.

Your statement about the fault current of a #8 and a 100 amp breaker is based on more than 240.4--- If you can so can I, so there. :laughing:

Anyway, if he wants 240.4(B) only then yes we must use the next size breaker but the calculated load must not be higher then the ampacity of the conductor.

So lets go back to the example if the a/c unit is 125 amps without the 125% then the 1/0 xhhw is not enough since it is 150 amps and the load is 156.


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## BBQ (Nov 16, 2010)

1capybara said:


> ok, so I was looking for an example of 240.4B2 like this:
> 2AWG is rated 115A. we have an actual load of
> 80% of 115A = 92A.
> the next highest standard breaker is 125A.
> ...


As long as the load with any required adjustments is below the ampacity of the conductors with any derating you are can roll up to he next standard size found in 240.6.


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## BBQ (Nov 16, 2010)

Dennis Alwon said:


> You can do that but I felt it okay to add a bit extra. *Sorry I didn't do what you want.*


OK I did not know it was your PMS week.


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## Dennis Alwon (May 9, 2009)

1capybara said:


> ok, so I was looking for an example of 240.4B2 like this:
> 2AWG is rated 115A. we have an actual load of
> 80% of 115A = 92A.
> the next highest standard breaker is 125A.
> ...


The #2 is good for 115 amps so as long as the load is not greater than 115 then you can use a 125 amp breaker


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## Dennis Alwon (May 9, 2009)

BBQ said:


> OK I did not know it was your PMS week.


 Actually I was thinking that about you. Nit picking.


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## 1capybara (Feb 27, 2009)

BBQ said:


> As long as the load with any required adjustments is below the ampacity of the conductors with any derating you are can roll up to he next standard size found in 240.6.


but thats NOT what 240.4B says:
"(B) Devices Rated 800 Amperes or Less. The next higher
standard overcurrent device rating (above the ampacity of
the conductors being protected) shall be permitted to be
used, provided all of the following conditions are met......"
...
check it out - it says OCPD above the ampacity of the conductors.
i couldnt believe it either when someone at work showed me that!!!


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## Dennis Alwon (May 9, 2009)

1capybara said:


> but thats NOT what 240.4B says:
> "(B) Devices Rated 800 Amperes or Less. The next higher
> standard overcurrent device rating (above the ampacity of
> the conductors being protected) shall be permitted to be
> ...


You are misreading Bob's(BBQ) post. He is saying, in short, if the calculated load on the conductor is less than or equal to that of the ampacity of the wire, after derating, and the ampacity is not a standard breaker size then you can go to the next size breaker. 

Example: I have a 6/3 NM cable that is rated at 55 amps that feeds a subpanel. If the calculated load of the subpanel is 55 amps or less then I can install a 60 amp breaker on it those conductors.


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## 1capybara (Feb 27, 2009)

Thanks Dennis, now i get it


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## brian john (Mar 11, 2007)

NolaTigaBait said:


> Is there a nameplate?...If so, just follow that...
> 
> also, thhn? Copper? aluminum?...You are missing some info


In his example he specifically said it has nothing to do with name plate data.



> this is an example only, please dont tell about the nameplate, thats
> not what this example is about.


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## NolaTigaBait (Oct 19, 2008)

brian john said:


> In his example he specifically said it has nothing to do with name plate data.


Uhhh..thanks?...Could it be that I skimmed through the first post?...And, maybe , possibly....I realized that now?....But...thanks...


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## brian john (Mar 11, 2007)

NolaTigaBait said:


> Uhhh..thanks?...Could it be that I skimmed through the first post?...And, maybe , possibly....I realized that now?....But...thanks...


I skim over many post and miss important facts all the time.


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