Reduntant HVAC remote com shelter

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JesterMT

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Hello all,
I am new to the site.
I am trying to understand the Load calculation rules for calculating the load for Reduntan HVAC system.
I am tasked with setting an electrical service for a remote pre-cast building that has two complete HVAC systems. By design they will never operate at the same time. They are conroled by a programable thermostat that changes the lead unit weekly each being sized sepratly to handel the total Heating or cooling load required. Do I have to calculate the load of the second unit at all or a precentage of that load? If anyone can point me to the relevant article in the NEC or if someone has dealt with this issue specificly I would be very gratefull.
Thanks
 
If the two systems are intentionally designed to not operate at the same time as indicated by you, they are considered noncoincidental loads and one can be omitted from the service/feeder calculation as permitted by 220.60 of the NEC 2005.
 
Sounds like one of my companys buildings.
Normally we do not design our buildings to lock out the Lag A/C unit. it is allowed to run if the heat load becomes to much for the lead unit to handle or if the lead should fail. Communication companies get highly perturbed if a sight goes down from a failed A/C. Lost revenues and all that stuff!
Redundant systems are designed for 1 A/C unit to carry the heat load and by design the lag should never run unless there is a failure of the lead unit.
So if your building is designed to lock out the lag unit I would say that someone has told the manufactuer that the available current will not support the load of both A/C units if they are both on at the same time.
Just my 2 cents worth. You should probably get clarification on the design.
Something could have gotten lost in the interpetation.
 
One thing to consider that may NOT affect your official calculations for service size per Article 230, but will effect your calculation for feeder size.
A 2N system (2 systems where only one is needed to carry load), often is designed for simultaneous maintainability. So I might run one system to carry load, but I'm running the other for testing or prev. maint reasons. So both run at the same time, and the feeder (or service) better not open due to overload ..... or else.
 
Things to consider

Things to consider

Thanks Guys for the input,
My big challange seems to be overkill on the size of the A/C units and a max 150 KVA transformer size. Our bid documents originaly asked for 2 15 ton units for a com shelter that is only 20'X40' and has a heat load of 50kbtu/hr.
Even in the hot dry conditions that these units will oporate in they can easily overcome 10k Btu/hr pre ton. My costomer is very sensitive to down time as these locations are also Data nodes as well as com huts. I am trying to convince them that it would be far better to have two smaller units that the 400 amp service, and 100KVA Gen-set can support than overkill the units so bad they might short cycle, remove to much humidity or even shorten their own life span or that of the Gen-set. This type of overkill engineering is prevelant in all our statement of requirments that we bid to, so you can probably guess who my costomer is.
 
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