80% Rule for switchgear

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tlquick

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WE have an existing 800-amp 3-phase 4-wire 120/208V service feed to a floor in a high rise building in downtown Chicago, The building wants to add another bank of meters to the floor to divide the space for more tenants. The existing gear has (2) 350amp breakers feeding the current meters and a 250 amp breaker feeeding a public meter. My question is can we modify the buss to install another 350 amp breaker or do we need to upgrade the service to 1200 amps. I know there is an 80% rule but not sure how to calculate this. If I add all my breakers in my sub-panels they go way over 80%. Does this rule apply only to service feeds?
Thanks for your input,
TQ
 
Re: 80% Rule for switchgear

The ratings of the breakers means nothing. All that matters is the calculated load. That is to say, you need to perform a load calculation. If the total calculated load is within the rating of the existing switchgear (within 100% - I know of no "80% Rule"), then you can proceed.
 
Re: 80% Rule for switchgear

The only 80% rule of which I am aware is that a circuit should not be loaded "continuously" to more than 80% of its capacity. This only applies to continuous loads. You will need to do a load calculation on the building or refer to past billings to determine existing load. Then you will need to calculate the load that you will be adding to determine if the existing service is adequate. There is no shortcut. You will need to do the calculations. Adding existing breaker sizes tells you nothing other than what size breakers are installed. A residential 200-amp panel may have 600 amps worth of individual breaker for example, and the calculated load may be only 100 amps. The service must be sized for the load, not the breakers.
 
Re: 80% Rule for switchgear

i have been contracted by engineering groups to monitor service loads over a thirty day period in regards to load expansions, but i don't know if they used the data as "calculated" or to check their "calculations". i have been involved in figuring load calculations and in a large office building --- this gets out of control --- they give you prints that are not usually up to date and many things are changed without reflection to the prints. just consider how many times a tenant space is remodeled and new panels are set---hay, it's the easiest way for engineers!!! it was one of my pet peeves!! all the original circuits get buried in the walls and ceilings!!! how is that load "calculated"? this drives the total calculated load through the roof!!! i've seen where the calculated load figures couldn't be supported by the service size!!! and i know monitoring the loads for a month doesn't cover many things.....................
 
Re: 80% Rule for switchgear

Originally posted by charlie tuna: i have been contracted by engineering groups to monitor service loads over a thirty day period in regards to load expansions, but i don't know if they used the data as "calculated" or to check their "calculations."
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I have contracted electrical companies to do the same work. I use the results as follows:
</font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">Take the highest reading during the 30 days.</font>
<font size="2" face="Verdana, Helvetica, sans-serif"></font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">Convert to KVA (if the readings were in amps)</font>
<font size="2" face="Verdana, Helvetica, sans-serif"></font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">Add 25% of the highest reading.</font>
<font size="2" face="Verdana, Helvetica, sans-serif"></font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">Apply three "correction factors" that account for possible differences in the readings that might arise due to season, due to occupancy level, or due to some "other" factor (that I might apply just to that project). Most of the time, all three correction factors are equal to 1.0. But if the building has a major Air Conditioning load in the summer, and if you take the 30 day readings in the fall, you might have to apply a "seasonal correction factor."</font>
<font size="2" face="Verdana, Helvetica, sans-serif"></font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">Add the proposed amount of new load.</font>
<font size="2" face="Verdana, Helvetica, sans-serif"></font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">Compare the "total load," as obtained above, to the rating of the equipment. If this does not exceed the rating, I issue the design.</font>
<font size="2" face="Verdana, Helvetica, sans-serif">The State of Washington has a formal process for performing this calculation, and submitting it (along with the plans) for their "permit review."
Originally posted by charlie tuna: . . . and i know monitoring the loads for a month doesn't cover many things.....................
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That is why we are required to add 25% to the highest measured load, and why Washington requires us to apply the three correction factors I described above.
 
Re: 80% Rule for switchgear

Originally posted by charlie b:
Take the highest reading during the 30 days.

Convert to KVA (if the readings were in amps)
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Charlie,

When you get a load profile from utility company, it shows in terms of kw (peak demand). Do I have to worry about the power factor here? Do I have to convert kw to kva, or kw is considered equivalent to kva for this purpose?
 
Re: 80% Rule for switchgear

The math must be performed in terms of KVA. So if you are given info in terms of KW, then you do need to assign a value of power factor. The standard form used by Washington State Dept. of Labor and Industries includes a line at which you state your assumed PF and divide that into the measured KW.

By the way, since you are using utility data, may I presume that you have one year's worth? A 30 day sample is not adequate for utility data.
 
Re: 80% Rule for switchgear

Charlie tuna, I would interested to know in your experience what have you been seeing as a typical demand load for office buildings, in the watss/sf range?

Thanks

[ January 29, 2006, 08:11 PM: Message edited by: bwyllie ]
 
Re: 80% Rule for switchgear

Originally posted by charlie b:
..if you are given info in terms of KW, then you do need to assign a value of power factor
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How do you assign a PF value?

Using Pf=Er/Et works with lowest-running volts (Er), since measuring (Er) at device terminals is in series (voltage varies), as does device (I)=P/Er, or device VA=Et*(I) where Et=Nominal or applied votage.

But, Pf=P/VA prevails where voltage is constant in parallel loads, as does VA=Et*It, where It=Et/Z or Z=Et/It

To get KVA without a Power Factor in hand, it seems we must use a cl/amp meter at feeder/panels to log peak (It) Amps. Even if we got (Z) at feeder/panels using Z=VD/I, where VD=(Et-Er), the Amp probe is still needed.
 
Re: 80% Rule for switchgear

Thanks for all your help in this matter, we have submitted a bid for the gear modification with the alternate to do load calculations based on Edisons KW readings and sq ft calcs on the unoccupied spaces on that floor. This will determine the total connected load.
The entire floor is approx. 20,000 sq ft. The xfrm feeds the 24th 25th & 26th floors from the ComEd vault.
We are working with the ComEd engineer to determine if they need to upgrade their service or not. I will follow up with you once we have load calcs completed.
Thanks again,
TQ
 
Re: 80% Rule for switchgear

Originally posted by ramsy:How do you assign a PF value?
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SWAG.

I think that stands for "State of Washington Allowance for Generalities," or perhaps "State of Washington Analysis of Geometry," or something along those lines. Others might suggest it stands for "Swinging a Wild Arbitrary Guess," but I'm sure that would not meet with professional standards and practices. :D
 
Re: 80% Rule for switchgear

Compare the "total load," as obtained above, to the rating of the equipment. If this does not exceed the rating, I issue the design.

Charlie in your quote above, shouldn't this be 80% of the nameplate rating of the equipment? figuring the calculated load to be a continuous load, I don't believe you can go to the full current rating.
TQ
 
Re: 80% Rule for switchgear

Let's be clear, by using an example. I'll do it in terms of amps, but it should really be done in VA, and then converted to amps at the end. But this is just an example.

Suppose your load calculation showed that you had 75 amps worth on non-continuous load, and 20 amps worth of continuous load. You add the two, and then you add 25% (or 5 amps) of the continuous load, and you get a total of 100 amps. I believe you can use a 100 amp panel to serve this load. I base that on first sentence of 408.30 (2005 NEC).

The basic concept is that you figure in an extra amount, when you calculate the load. You don't need to figure it in a second time, when you select the panel.

By the way, for my example, I would have really chosen a panel with a higher rating than 100 amps. But that is a design choice, not an NEC requirement, and would be intended to provide the owner with room for future expansion.
 
Re: 80% Rule for switchgear

Of course, the SWAG is the easy part. The State form also requires the engineer (i.e., the form must be sealed and signed by a PE) to state the basis for the selected value of power factor. I believe I have some skill in technical writing, but I never was very good at creative writing. :D
 
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