80% Design on Calculations

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Andrew Jackson

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Hello, Trying to find the answer in the NEC and having issues. Been an electrical contractor for 20+ years and from time to time I run into "the new rule" (albeit old) being enforced by our local municipalities. We do small design build commercial projects as well as residential custom homes and when I design the loads we try to stay at 80% or under the max amperage rating for circuits electrical panels or other.
On a sfr remodel we are doing an inspector is calling us on exceeding the 80% capacity of an existing 100 amp sub-panel with three existing ac units and one new water feature pump. Total load calculation on the sub-panel comes in at 86 amps. So I am trying to find the answer in the NEC to show this inspector he is misunderstanding or that I am wrong. And if it is the case that I am mistaken our industry needs to just change all the rated breakers, panels, etc to what is real. WHY would we have a 20 amp rated breaker that can only have 16 amps on it or for that matter a 3,000 amp 480 volt panel is only good for 2,400 amps??? And that would open a can of worms when selling a 3,000 amp service (which we are in the bidding process of) and is only good for 2,400 amps.....law suit ensues..........

Thanks for any assistance, Andy
 
As a general rule you would only need to worry about the 125% when dealing with "continuous loads" (full current 3 hrs or more).
In your case with air-conditioners and motors the required calculations could be more difficult.
Can you give us the specifics (MCA, MOCP, HP, etc)
 
infinity said:
If your total load calculation is 86 amps a 100 amp panel is code complaint.
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Many refer to calculated load as including the extra 25% for continuous loads. After summing 100% of the noncontinuous load plus 125% continuous load, you have a value that is not the calculated load... but rather the minimum circuit ocpd rating and the minimum conductor ampacity.
 
And in this case the minimum circuit ampacity applies to the panelboard bus as well as the wires.
Note that a 100% rated breaker in a non-100% rated panel does not give you any benefit.
 
Smart $ said:
Many refer to calculated load as including the extra 25% for continuous loads. After summing 100% of the noncontinuous load plus 125% continuous load, you have a value that is not the calculated load... but rather the minimum circuit ocpd rating and the minimum conductor ampacity.
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That's merely semantics, the wording in 220.40 and the examples in the Informative Annex D would seem to disagree with you. :)
 
infinity said:
That's merely semantics, the wording in 220.40 and the examples in the Informative Annex D would seem to disagree with you. :)
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Call it semantics if you want... 220.40 and the Annex D Examples do not disagree with me.

Where in Article 220 does it factor any continuous load by 125%?

What is the section title in any Example where continuous load is multiplied by 125% (or 1.25)?
 
infinity said:
That's merely semantics, the wording in 220.40 and the examples in the Informative Annex D would seem to disagree with you. :)
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Smart $ said:
Call it semantics if you want... 220.40 and the Annex D Examples do not disagree with me.
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I agree with Smart$...

Annex Example D3(a) shows a calculated load of 99,000VA (@480V-3ph = 119A.)

It also shows a the calculate load plus 125% of the continuous load being 113,200VA (@480V-3ph = 136A.)

It goes on to show a 2/0 feeder conductor with an ampacity of 131 to supply this load. If the load was calculated to be 100%noncont + 125%cont (or 113,200VA in the Annex Example,) then you'd have to say the conductor with an ampacity of 131 is too small for the load.
 
I got an idea.....have the manufacturers reliable their breakers, i.e. 20A becomes a 15A, 30A becomes 20A, etc. Now they are 100% rated. Problem solved.

Look at all the calcs that would be simplified.

It amazes me that IEC breakers are all capable of 100% loading yet in the US we have to deal with the BS of only loading to 80%.

Imagine a 100A main breaker actually capable of carrying 100A continuously.
 
And I think many can. Just look at the trip curves -- a 100% continual load can be handled on most trip curves for "normal" breakers. I think where you run in to trouble is if you have a continual load, a high ambient temperature, small wires (heat sink), and a breaker at the wrong end of the trip curve tolerance. So to handle that few percent of installs where this happens, you get hit with the 80% rule. Perhaps breakers degrade over time if loaded this high.

I'm not really sure why we have this limitation. Its probably from the same place as the 25 ohm ground rod.
 
Smart $ said:
Call it semantics if you want... 220.40 and the Annex D Examples do not disagree with me.

Where in Article 220 does it factor any continuous load by 125%?

What is the section title in any Example where continuous load is multiplied by 125% (or 1.25)?
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I was merely commenting about the words load calculation as mentioned in the OP.
 
The OP does not state that the 86A is continuous, non-continuous or combination, just 86A. 80% of 100A is 80A. Pretty basic.

Unless the breaker/panel is 100% rated, than it exceeds the 80% loading.

Interesting, in the OP it says he tries to stay at or under the 80%, but yet 86A is above 80%. Would seem to answer his own question; and why suddenly is it a question at all.

Simply change panel to 125A panel, and make more money.
 
kingpb said:
The OP does not state that the 86A is continuous, non-continuous or combination, just 86A. 80% of 100A is 80A. Pretty basic.

Unless the breaker/panel is 100% rated, than it exceeds the 80% loading.

Interesting, in the OP it says he tries to stay at or under the 80%, but yet 86A is above 80%. Would seem to answer his own question; and why suddenly is it a question at all.

Simply change panel to 125A panel, and make more money.
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Slight flaws with that reasoning...

86A calculated load is perfectly acceptable on a 100A panel if it is all or partially non-continuous such that noncontinuous plus 125% continuous does not exceed 100A.

Replacing panel would be at contractor's expense if it is covered under fixed-rate contract.
 
Smart $ said:
Slight flaws with that reasoning...

86A calculated load is perfectly acceptable on a 100A panel if it is all or partially non-continuous such that noncontinuous plus 125% continuous does not exceed 100A.

Replacing panel would be at contractor's expense if it is covered under fixed-rate contract.
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I reread the OP, and since Andy hasn't chimed in, I suspect, based on the description of loads, they are all continuous.

Therefore, until they can be clarified, its all speculation. Including the contract terms :p
 
Andrew Jackson said:
...
On a sfr remodel we are doing an inspector is calling us on exceeding the 80% capacity of an existing 100 amp sub-panel with three existing ac units and one new water feature pump. Total load calculation on the sub-panel comes in at 86 amps. ...
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kingpb said:
I reread the OP, and since Andy hasn't chimed in, I suspect, based on the description of loads, they are all continuous.

Therefore, until they can be clarified, its all speculation. Including the contract terms :p
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The highlighted portions of Andy's post indicate to me that few if any loads are continuous.

But we can wait for him to chime in if you want. :D
 
Smart $ said:
... few if any loads are continuous....
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Every water feature that I have seen uses a continuously running pump. But probably not very large.
My pond uses a very large appearing 1/8 horse motor, since the flow and head are rather small. (The same pump flange is used with 1/8, 1/2 and 3/4 HP 1800RPM motors, so the motor cases are almost identical.)
 
GoldDigger said:
Every water feature that I have seen uses a continuously running pump. But probably not very large.
My pond uses a very large appearing 1/8 horse motor, since the flow and head are rather small. (The same pump flange is used with 1/8, 1/2 and 3/4 HP 1800RPM motors, so the motor cases are almost identical.)
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Motors may run continuously but all the load factoring is done in Article 430/440. When doing a load calculation, motor loads are kept separate from non-continuous and continuous load listings. If they had to be included in one or the other, they'd be in the non-continuous grouping, so 125% factoring isn't applied twice.

Another issue is that many motors do not run at full load for periods greater than 3 hours.
 
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