100% Rated Circuit Breaker for Service Disconnect

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Sparky2791

Sparky2791

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I did service calculations for a school based on 220.86 and the service load after calculation comes to 580A. This is a complete renovation of the school including upgrading the existing main distribution panel with a new service disconnect (Main Circuit breaker) According to the calculations the 600A service is adequate, obviously sized very close but the school is served by a campus distribution and upgrading the transfomer to bring in a larger service is not only cost prohibitive but the lead time for a transfomer in this current market is unreal. I recommended that a service upgrade be considered moving forward but the 600A is adequate to serve their immediate needs. My question is if the main service disconnect must be a 100% rated circuit breaker? Connected load is 711A so realistically the building will only see 60% of that as a peak demand an that is being generous.
The building does have a full 600A of conductors feeding it .

Thanks for your replies.
 
The total KVA of all loads in the main distribution panel. The KVA used to perform the NEC 220.86 calculations for service.
 
Using a 100% rated OCPD comes into play if you are attempting to use Exception 2 to 230.42(A). I would give 230.42(A) a read.

Per 230.42(A)(1), the split between non-continuous and continuous loads isn't super clear after applying the demand factor of 220.86 because 220.86 requires you to essentially lump all your loads together before applying the VA/sf demand factors. You could potentially apply the 125% continuous multiplier to whatever continuous loads are in the school prior to applying the demand factor of 220.86. If that calculation yields a service size <600A, I believe you would be justified in using a standard circuit breaker.

I'm interested to see what other members think about this.
 
I'm not well versed in the details of Article 220 for different types of buildings. But I do know that the output you need from Article 220, in order to apply Articles 215 and 230, isn't just "the total load is 580A" but is a statement of the form "the total load is 580A, of which X amps is continuous". And Article 220 isn't clear on exactly how to do that, and how it relates to demand factors. Some of the examples in Annex D might help clarify that, I haven't checked.

Once you have determined X, then as mentioned, see 230.42.

Cheers, Wayne
 
I have considered the Article 220 calculations resulting in non-continuous loads, so the 600A service disconnect (standard 80% rated molded case breaker) should be fine.
 
Do you have past demand data to tell you what the true demand has been?

You did mention complete renovation so depending on what actual load changes might be could be useful or totally useless. Guessing that if anything more efficient lighting and HVAC were possibly installed and that most the remaining load is similar to what there was before unless maybe there was significant change of use of certain spaces.
 
ron said:
I have considered the Article 220 calculations resulting in non-continuous loads
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I would think that is only defensible if all of the loads used as inputs to the Article 220 calculations are non-continuous. If some of them are continuous, then obviously the continuous load on the service is non-zero.

If the Article 220 calculations just use + and * (i.e. add up some loads, multiply by a demand factor, add up some other loads, multiply by a different demand factor, add the results, etc.), then it's pretty clear how to calculate the continuous loads per Article 220. You just label each input load term as continuous or non-continuous, and to calculate the continuous loads, set all the non-continuous loads to 0. The coefficients (demand factors) remain the same. E.g. if the calc ends up as 30% * (200A n.c. + 100A c.) + 70% * (50A n.c. + 250A c.), then the total load is 300A, of which 205A is continuous.

The difficult arises when Article 220 calls for a calculation like "first 3 kW at 100% plus the rest at 40%." If the inputs to that are a mix of continuous and non-continuous loads, should the continuous loads go into the first 3 kW to the maximum extent possible, the minimum extent possible, or proportionally?

Cheers, Wayne
 
kwired said:
Do you have past demand data to tell you what the true demand has been?

You did mention complete renovation so depending on what actual load changes might be could be useful or totally useless. Guessing that if anything more efficient lighting and HVAC were possibly installed and that most the remaining load is similar to what there was before unless maybe there was significant change of use of certain spaces.
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Unfortunately I do not, it is a campus wide distribution so building is not metered by utility except by way of the primary meter which meters the whole campus. Otherwise I would have loved to see that data. Ultimately since a complete reno yes kind of useless but still gives a good snapshot of past peak demand on the building. Last renovated 25 years ago so more energy efficient equipment will help,
 
wwhitney said:
I would think that is only defensible if all of the loads used as inputs to the Article 220 calculations are non-continuous. If some of them are continuous, then obviously the continuous load on the service is non-zero.

If the Article 220 calculations just use + and * (i.e. add up some loads, multiply by a demand factor, add up some other loads, multiply by a different demand factor, add the results, etc.), then it's pretty clear how to calculate the continuous loads per Article 220. You just label each input load term as continuous or non-continuous, and to calculate the continuous loads, set all the non-continuous loads to 0. The coefficients (demand factors) remain the same. E.g. if the calc ends up as 30% * (200A n.c. + 100A c.) + 70% * (50A n.c. + 250A c.), then the total load is 300A, of which 205A is continuous.

The difficult arises when Article 220 calls for a calculation like "first 3 kW at 100% plus the rest at 40%." If the inputs to that are a mix of continuous and non-continuous loads, should the continuous loads go into the first 3 kW to the maximum extent possible, the minimum extent possible, or proportionally?

Cheers, Wayne
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I would expect that NEC assumes you calculated your loads accordingly applying the 25% to continuous loads where required as the Code directs you so the demand factors in 220.86 were based on the fact that the service load was calculated correctly.
 
LibertyEngineering said:
I would expect that NEC assumes you calculated your loads accordingly applying the 25% to continuous loads where required as the Code directs you so the demand factors in 220.86 were based on the fact that the service load was calculated correctly.
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Nothing in Article 220 directs you to add 25% to continuous loads, so I don't really follow your comment.

Cheers, Wayne
 
I don't think I would worry about the 100% breaker. I understand your reasoning that it gives you extra service capacity on paper. In practice, there is no difference between a continuous breaker and a standard breaker, it's really just about the enclosure it's mounted in. If you are talking about a service disconnect, I'm guessing it's in its own enclosure or compartment anyway, not smashed together with a bunch of other breakers, which indeed could add extra heat and does result in the 80% breaker derating. If having a proper 100% breaker setup makes you sleep well at night, go for it, but in this application I don't really see it giving you much over a standard circuit breaker and enclosure or safety switch, plus this has been fine on a standard 600 amp for how long? Is there any reason that the load will be greater after this renovation? Finally, Remember, If a NEC calc says it's fine you probably have A LOT of fudge room.
 
electrofelon said:
I don't think I would worry about the 100% breaker. I understand your reasoning that it gives you extra service capacity on paper. In practice, there is no difference between a continuous breaker and a standard breaker, it's really just about the enclosure it's mounted in. If you are talking about a service disconnect, I'm guessing it's in its own enclosure or compartment anyway, not smashed together with a bunch of other breakers, which indeed could add extra heat and does result in the 80% breaker derating. If having a proper 100% breaker setup makes you sleep well at night, go for it, but in this application I don't really see it giving you much over a standard circuit breaker and enclosure or safety switch, plus this has been fine on a standard 600 amp for how long? Is there any reason that the load will be greater after this renovation? Finally, Remember, If a NEC calc says it's fine you probably have A LOT of fudge room.
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I was thinking since the NEC says the service load is 580A I should make sure the breaker is capable of handling the calculated service load but you are right there is a lot of fudge room in the service calculations. What is a total connected load of 800A in the building is likely to only have a peak demand of 65% of that load or less.
 
David Castor said:
The 100% rated breaker requires 90 deg C insulated wire, applied at max rating for 75 deg C - but you probably know that.
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Actually we comply per our specs which require 90 deg C insulation and of course use 75 deg C for sizing wire 100A and above. 60 deg C below 100A but I was unaware of that requirement. . Curious, where in the NEC does your statement come from?
 
LibertyEngineering said:
Actually we comply per our specs which require 90 deg C insulation and of course use 75 deg C for sizing wire 100A and above. 60 deg C below 100A but I was unaware of that requirement. . Curious, where in the NEC does your statement come from?
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It comes from the UL Listing, and thus 110.3(B).
 
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