Lots of variables, depends on the load
Lots of variables, depends on the load
There are lots of variables, depends on the site, the loads, and the quality of the UPS.
Computer loads cannot be accurately sized by nameplate. Most likely, the nameplate rating is the power supply rating only, say 1 kW, but the max actual load due to the way the equipment is built is much less, say .2 kW on a 1 kW rated power supply. I will not offer a code opinion, I will say on a large UPS I would look at the actual load and add.
Efficiency on lightly loaded UPS's is pretty bad. For illustration purposes efficiency could go from 75% to 85% as load increases from 25% to 70%. Adding load could increase efficiency up to a sweet spot.
Look at the manufacturer's rating and ask them for recommendations when you reach that point. Look at the KW rating, not the KVA rating, and look if the manufacturer rates their KW output at 100% for continuous loading or at 80% for continuous loading. Get a solid target number.
Site variables:
Are there lots of dual source, dual power cord loads that will double the kW drawn if the alternate redundant source goes down ? If worst case and all the loads are loaded 50% - 50% on two redundant cords, you are already at max at 50% on the UPS with 50% on another UPS. In some arrangements, that 50% transfers to the B source when A goes down. Dual power cord hot swap power supplies are pretty common in a certain equipment class.
How much does the load vary with demand. What is peak load, what loads are added or cycle with demand, what is in standby and must be met when the load calls.
Is there constant onsite KW load monitoring, how often, do the guys know what the numbers measure and the source / load limits. With no onsite talent, the guys will add loads until something trips. Are there already lots of outlets prewired that something will be plugged into later. Is there a supervising EE / PE power person for the site.
I would be tempted to survey the site and add load confidently but business wise, if I am liable for adding load in excess of capacity, you may want to test the customer's appetite and budget in addition to testing their loads. If the onsite guys are very clueless and the cost of downtime is huge, maybe they want another UPS and an array of dual power cord rack loads.
Poor quality, cheap, power supplies as loads draw noise and harmonics from the source. Harmonic distortion will increase with the increased load and probably interfere with the max rating.
High quality data center rated power supplies as loads are pretty clean, good loads I would be confident in adding.
