Load calculation and 80%

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Epalmateer

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Michigan
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Electrician
Am I correct with this, when doing a load calculation for a service, if you calculate all continuous loads at 125%, you can load your service disconnect 100%. But if you calculate continuous and non continuous loads at 100%, you can only load your service disconnect 80%, unless you have a listed 100% panel and ocpd?
 
Epalmateer said:
Am I correct with this, when doing a load calculation for a service, if you calculate all continuous loads at 125%, you can load your service disconnect 100%. But if you calculate continuous and non continuous loads at 100%, you can only load your service disconnect 80%, unless you have a listed 100% panel and ocpd?
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Not exactly. The service OCPD must be sized for at least 125% of the continuous load plus 100% of the non-continuous load, unless it is listed for operation at 100% of it's rating.

For example, if you has a 200A load that was half continuous loads and half non-continuous loads, the min service OCPD size would be 225A...it would be loaded to 88.9% of it's rating.
 
david luchini said:
Not exactly. The service OCPD must be sized for at least 125% of the continuous load plus 100% of the non-continuous load, unless it is listed for operation at 100% of it's rating.

For example, if you has a 200A load that was half continuous loads and half non-continuous loads, the min service OCPD size would be 225A...it would be loaded to 88.9% of it's rating.
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Ok. But if you were to calculate your continuous loads at 125% from the get go, wouldn't that ensure the same result as calculating everything at 100% and then sizing the ocpd accordingly?
 
Epalmateer said:
Ok. But if you were to calculate your continuous loads at 125% from the get go, wouldn't that ensure the same result as calculating everything at 100% and then sizing the ocpd accordingly?
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I'm not sure I follow.

If I calculate my load at 100% to be 200A and size the OCPD to be 200A, then it is undersized if 100A of the 200A load is continuous.
 
david luchini said:
Not exactly. The service OCPD must be sized for at least 125% of the continuous load plus 100% of the non-continuous load, unless it is listed for operation at 100% of it's rating.
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I was just looking over Article 230, do you have a reference for the above?

230.42(A)(1) requires the above sizing for service entrance conductors. But 230.90(A) just requires an OCPD "that has a rating or setting not higher than the allowable ampacity." Which is somewhat different that the comparable sections of Articles 210 and 215.

Cheers, Wayne
 
Reviewing (2017) Article 230, this is what I came up for a hypothetical overhead service with a single (non-100% rated) service disconnecting means/OCPD and a calculated load of 100A continuous plus 100A non-continuous:

Utility Lines and Service Drop--sized by utility
Service Point
Overhead Service Conductors (if any)--minimum ampacity 200A, 230.23(A)
Service Entrance Conductors--minimum ampacity 225A, 230.42(A)(1)
Service Disconnecting Means--minimum rating of 200A, 230.79
Service OCPD--rating not to exceed the ampacity of the service conductors, with exceptions (including 240.4(B)), 230.90(A).

Now if the Service Disconnecting Means rating is the rating of the Service OCPD therein, the Service OCPD would have to be at least 200A. Otherwise, it appears there is no minimum size for the Service OCPD.

Cheers, Wayne
 
I guess I need to learn how to properly do a load calculation. Once you have completed the standard method, how do you account for continuous and no continuous loads?
 
Epalmateer said:
I guess I need to learn how to properly do a load calculation. Once you have completed the standard method, how do you account for continuous and no continuous loads?
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I'm not super familiar with that, and Article 220 doesn't discuss continuous loads. But my assumption has always been that every number input into the Article 220 calculations should really be a two component vector (continuous load, non continuous load). Then you just track those two components through all the calculations so that your result is also a two component vector.

Cheers, Wayne
 
wwhitney said:
I'm not super familiar with that, and Article 220 doesn't discuss continuous loads. But my assumption has always been that every number input into the Article 220 calculations should really be a two component vector (continuous load, non continuous load). Then you just track those two components through all the calculations so that your result is also a two component vector.

Cheers, Wayne
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I'm curious about how others do it.
 
david luchini said:
What's downstream of the service OCPD?
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Ah, the service OCPD is also the feeder OCPD, so 215 applies to that also. Thanks, basic oversight on my part.

Thus in my subsequent post, the Service OCPD would have to be 225A per 215.3. Which means the Overhead Service Conductors would have to be 225A as well, and everything on the service has to be 225A.

So the allowance on sizing the Overhead/Underground Service Conductors at 100% of the calculated load is only useful when there are two or more service disconnects. E.g. say you have a service with two disconnects supplying two different feeders, each with a load of 40A continuous, 60A non-continuous. Each service OCPD and set of service entrance conductors would have to be rated at 110A. But the Overhead/Underground Service Conductors (if any) would only need to be 100A.

Thanks, Wayne
 
wwhitney said:
Ah, the service OCPD is also the feeder OCPD, so 215 applies to that also. Thanks, basic oversight on my part.

Thus in my subsequent post, the Service OCPD would have to be 225A per 215.3. Which means the Overhead Service Conductors would have to be 225A as well, and everything on the service has to be 225A.

So the allowance on sizing the Overhead/Underground Service Conductors at 100% of the calculated load is only useful when there are two or more service disconnects. E.g. say you have a service with two disconnects supplying two different feeders, each with a load of 40A continuous, 60A non-continuous. Each service OCPD and set of service entrance conductors would have to be rated at 110A. But the Overhead/Underground Service Conductors (if any) would only need to be 100A.

Thanks, Wayne
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david luchini said:
I'm not sure I follow.

If I calculate my load at 100% to be 200A and size the OCPD to be 200A, then it is undersized if 100A of the 200A load is continuous.
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At what point do you separate continuous and non continuous loads in residential load calc? And say you have a 320/400 amp meter, if you sperate continuous from non continuous, and calculate continuous at 125%, could you load that meter to 400? Since you have already accounted for continuous loads? Sorry man I am all over the place right now
 
Epalmateer said:
At what point do you separate continuous and non continuous loads in residential load calc? And say you have a 320/400 amp meter, if you sperate continuous from non continuous, and calculate continuous at 125%, could you load that meter to 400? Since you have already accounted for continuous loads? Sorry man I am all over the place right now
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There really aren't continuous loads to deal with in residential other than the largest motor.
 
Epalmateer said:
Ev charging and grow lights. Would you multiply the VA by 1.25 before inserting that value into the calculation?
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let me ask you this. where in the load calculation articles does it say anything about calculating continuous and non-continuous loads differently?

pretty sure there is only one place it is mentioned for something pretty unusual in the optional calculations section.
 
petersonra said:
let me ask you this. where in the load calculation articles does it say anything about calculating continuous and non-continuous loads differently?

pretty sure there is only one place it is mentioned for something pretty unusual in the optional calculations section
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So are you saying that you calculate everything at 100%? I'm just trying to gain some knowledge. If you have a 320/400 amp meter base, couldn't you put a full 400 amps on if you calculate 125% of continuous and 100% of non continuous?
 
Epalmateer said:
So are you saying that you calculate everything at 100%? I'm just trying to gain some knowledge. If you have a 320/400 amp meter base, couldn't you put a full 400 amps on if you calculate 125% of continuous and 100% of non continuous?
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I assume you have the NEC--- Look at annex D and example D(1) (b) for single family dwellings. It is an example of a dwelling calculation.
 
petersonra said:
let me ask you this. where in the load calculation articles does it say anything about calculating continuous and non-continuous loads differently?
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It doesn't say anything about calculating them differently. But it is implicit in the other articles that you calculate them separately, as you need to be able to come up with a non-continuous load and a continuous load to apply the various sizing sections.

The lack of any guidance on that in Article 220 is a bit mysterious to me. For example, take the Optional Method and the General Loads, 220.82(B). [For simplicity assume the HVAC Load 220.82(C) is 0. If it's bigger it wouldn't change the nature of the question, just the numbers I'd need to use.] You're supposed to take the first 10 kVA and then 40% of the remainder. At 240V 10 kVA is 42A (rounding).

So suppose before doing the 40% you have a load of 105A non-continuous and 60A continuous (say there's a couple EVSEs). Your total load is definitely 42A + 40% (165 - 42) = 91A. But how much of that is continuous? Is it all 60A? Is it 40A + 40% (60A - 40A) = 48A? Is it 60/165 * 93 = 34A? Is it 40% * 60A = 24A? The answer to the question determines whether a 100A service is enough.

Cheers, Wayne
 
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