Calculations for existing buildings - 30 Day Metering

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minesh21

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Hi,

I wanted to get a consensus on this topic because I have read conflicting arguments on how to calculate total load on a feeder using existing metering data and new load. Let's assume we have done a 30 day load study and have and multiplied the load times 1.25 as stated in the code, NEC 220.87. I believe this is agreed upon and confirmed by everyone. The confusion occurs when adding in new load. Do we (assume load is continuous):

A. Take 1.25 of the continuous load + (1.25 X existing load) < breaker rating
B. Take continuous load + (1.25 X existing load) < breaker rating
C. Take continuous load + (1.25 X existing load) < 80% of breaker rating

Honestly I see (C) done more often than not on electrical drawings. Essentially (C) is the same as:

[Take continuous load + (1.25 X existing load)] x 1.25 < breaker rating

This doesn't really make sense,but the city usually will freak out if they see you have done (A) and show your final amperage greater than 80% of the main breaker. So I usually do C. Problem is I feel like we are being overly conservative and the code doesn't really require this.

Thoughts?
 
Perhaps this will help answer your question. Technically, Article 220 does not factor a continuous load by 125%. The 125% required by 220.87 is shall we say padding to account for any infrequent load combination that may not have been on concurrently with other loads during the 30 days (and that you also forgot to compensate for).
 
Smart $ said:
Perhaps this will help answer your question. Technically, Article 220 does not factor a continuous load by 125%. The 125% required by 220.87 is shall we say padding to account for any infrequent load combination that may not have been on concurrently with other loads during the 30 days (and that you also forgot to compensate for).
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YES. This is what I was trying to find out. The 125% is for unknown loads that may have not been on. Therefore, if you treat the existing load as continuous and the existing load as continuous then you can essentially take 80% of the breaker rating. Correct? So then (C)?
 
minesh21 said:
YES. This is what I was trying to find out. The 125% is for unknown loads that may have not been on. Therefore, if you treat the existing load as continuous and the existing load as continuous then you can essentially take 80% of the breaker rating. Correct? So then (C)?
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I don't like to use 80% of the breaker rating as the limit. It may work for some, but it can be misleading. That limit applies only if the load is all continuous. It seldom is for general service. I prefer to use 125% continuous load plus the noncontinuous load must be not greater than the OCPD rating.

That said, the load determined per 220.87 is not factored for any continuous load. Keeping this value at not more than 80% of the OCPD rating assures compliance with the 125% continuous + noncontinuous requirement... but it is technically an inaccurate method for determining compliance. Consider a load that is 100% noncontinuous. The accurate assessment is the OCPD cannot be less than the 220.87 value. An OCPD sized to 80% of its rating is 25% greater than the minimum required.
 
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Smart $ said:
I don't like to use 80% of the breaker rating as the limit. It may work for some, but it can be misleading. That limit applies only if the load is all continuous. It seldom is for general service. I prefer to use 125% continuous load plus the noncontinuous load must be not greater than the OCPD rating.

That said, the load determined per 220.87 is not factored for any continuous load. Keeping this value at not more than 80% of the OCPD rating assures compliance with the 125% continuous + noncontinuous requirement... but it is technically an accurate method for determining compliance. Consider a load that is 100% noncontinuous. The accurate assessment is the OCPD cannot be less than the 220.87 value. An OCPD sized to 80% of its rating is 25% greater than the minimum required.
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Ok so don't worry about continuous and non-continuous for metered load just take 1.25 for metering existing load and then worry about continuous and non-continuous for the NEW load only?
 
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jumper said:
Since you are using the 30 day metering allowance of 220.87 we do not know what is continuous or non continuous in the existing so 220.87 simply factors in 125% as I did above for the existing load.
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Ok I think I now understand what you are saying. We don't worry about "continuous" and "non-continuous" in regards to metered loads. Essentially we should do

Existing load x 125% + New continuous load x 125% + New non-continuous load x 100% < breaker rating
 
minesh21 said:
Ok I think I now understand what you are saying. We don't worry about "continuous" and "non-continuous" in regards to metered loads. Essentially we should do

Existing load x 125% + New continuous load x 125% + New non-continuous load x 100% < breaker rating
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In a nut shell yes. This takes you to the breaker and conductor sizing by factoring in the 125% for continuous and existing loads now.
 
minesh21 said:
Ok so don't worry about continuous and non-continuous for metered load just take 1.25 for metering existing load and then worry about continuous and non-continuous for the NEW load only?
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minesh21 said:
Ok I think I now understand what you are saying. We don't worry about "continuous" and "non-continuous" in regards to metered loads. Essentially we should do

Existing load x 125% + New continuous load x 125% + New non-continuous load x 100% < breaker rating
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Works for me until forced to do otherwise. :happyyes:


This section was first added to the NEC in 1981 as section 220.35. I looked through the ROP and ROC documents. It appears to have been added without a proposal. As such, I cannot say with certainty the reason for factoring the recorded data by 125%.
 
I am going to split a hair here, and probably not change the final answer anyone would arrive upon.
  • The peak measured load is not the "existing load." Rather when you take 125% of the peak measured load, the result is the "existing load."
  • You calculate the "added load" as 100% of non-continuous plus 125% of continuous.
  • The project can proceed so long as the "existing load" plus the "added load" is within the rating of the panel.
  • If this calculation takes the total load on the panel to 100% of its nameplate rating or to 100% of its main breaker's rating, that is still code-compliant. You are not limited to 80% of the breaker's rating, specifically because you already took the 125% factor into account for the continuous portion of the load.
 
charlie b said:
You are not limited to 80% of the breaker's rating, specifically because you already took the 125% factor into account for the continuous portion of the load.[/FONT][/SIZE]
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Yeah I see that fairly often. Someone will say, "your calc is XXXX which is too high because you can only use 80%...." They forget that the extra 25% for continuous loads was already figured in :rant:
 
Smart $ said:
This section was first added to the NEC in 1981 as section 220.35. I looked through the ROP and ROC documents. It appears to have been added without a proposal. As such, I cannot say with certainty the reason for factoring the recorded data by 125%.
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Just a theory, but perhaps it is to account for lack of perfect balance and power factor. In the case of a load study the recorder is probably recording amps and we would know the highest loaded phase, however if using utility data we get KW and the code doesnt really advise on taking into account power factor or lack of balance.
 
electrofelon said:
Just a theory, but perhaps it is to account for lack of perfect balance and power factor. In the case of a load study the recorder is probably recording amps and we would know the highest loaded phase, however if using utility data we get KW and the code doesnt really advise on taking into account power factor or lack of balance.
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Short of someone providing first-hand information, all we can do is speculate. ;)
 
i thought that the code specified to use actual vs nec calculated load and use whichever is larger?


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It should be noted the metering must be on the highest phase, at occupied time and include the highest heat/cool equipment plus periodic loads.
 
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