First, let me explain the project. It's a 4-story office condo building circa mid 80's with a 75KVA and a 30KVA transformers providing all the low voltage for the 25,000s.f. 4th floor. I need to add load for a copy center (about 19KVA equipment). The 75KVA has 95 20A loaded breakers in 3 panels, and the 30KVA has 42 20A loaded breakers in a 100A panel. There's no room in the electrical room for another transformer (the two are stacked.) or another panel for that matter. If I use 720VA per breaker and more for the occasional water heater, these transformers appear fully loaded. However, if I were to use NEC2005 220.44 demand for receptacle loads, they are not fully loaded...but what about computers that are on every desk in most offices? Is the NEC demand legitimate for today's offices? On the 75KVA main panel, there's space for a 3-pole breaker that I could sub yet another panel (in the TI space.) Or should I put a transformer in the TI ceiling space (the tenant is extremely cost conscious.)
Should I use the 10KVA/50% receptacle demand for offices (computers galore now days)?
- Thread starter donw
- Start date
- Status
hmspe
Senior Member
- Location
- Temple, TX
- Occupation
- PE
I don't generally use the diversity option, but I tend to be very conservative in my designs. I'd recommend a full day recording of the actual loads or a physical count of receptacles. If the maximum load during a typical work day is substantially less than the maximum available it should be OK to use the diversity calc.
If that's not an option, I'd look at the receptacles more closely. Do you have a receptacle count? Do you have a computer count? Are all the receptacles in use or only a fraction? I don't see a basis for using a flat 720VA per breaker. With a real receptacle count and a list of other loads you could do a reasonable base-line calc.
If neither of these worked I'd be looking at adding a transformer. Transformer above a ceiling? Unlikely on one of my designs. Legal? Yes, if the Code requirements are followed. A good idea? I don't think so. If you go that route be sure to have a structural engineer involved.
Martin
If that's not an option, I'd look at the receptacles more closely. Do you have a receptacle count? Do you have a computer count? Are all the receptacles in use or only a fraction? I don't see a basis for using a flat 720VA per breaker. With a real receptacle count and a list of other loads you could do a reasonable base-line calc.
If neither of these worked I'd be looking at adding a transformer. Transformer above a ceiling? Unlikely on one of my designs. Legal? Yes, if the Code requirements are followed. A good idea? I don't think so. If you go that route be sure to have a structural engineer involved.
Martin
Hi, Martin. Thanks for the reply. It's hard to count all the recepts and computers, as the floor is 25K feet and offices are owned by different owners. I, too, was thinking about measuring the current over time. Also, I thought about moving three of the circuits on the 30KVA transformer to give me enough space for a sub panel on it. Then divide up my loads among the two transformers. How are you doing?
- Status