Specifying Electrical Safety Switch for UPS

We are installing a large tower UPS into a computer room, and I want to make
sure I specify the safety switch correctly.
- The UPS is a 230V 50A UPS, single phase input
- Output is 208V single phase
- The main panel is a commercial panel fed by three phase to the building
but all of the circuits coming from that panel are single phase circuits.
My questions are:
1) Should the main panel should be equipped with a 50A breaker or smaller
(based on projected load)?
2) Given the UPS is a single phase input, is it required by code that the
breaker also be single phase? I have seen contractors use existing three
phase breakers but wire them for a single phase application. Since the
documentation for this is rarely done well, I'm skeptical about doing that.
3) In the UPS room, in front of the UPS itself, we would like to have a
safety switch with a fuse. Again, by code and convention, are we required
to get a single phase safety switch? Or can we wire into a three phase
safety switch and just not use one of the three pins?
4) I assume a safety switch rated for 100A fuses can be equipped with
smaller fuses?
5) On the output from a UPS, is it generally recommended that you install
another fused safety switch? I guess that gives you the ability to shut
off the load in an emergency to the end equipment, but aside from that would
run the risk that if you were at the border of the UPS capability you might
accidentally turn off the entire machine room. Presumably any decent
large UPS would start to warn you of an impending overload condition?
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Assuming you have spaces for more breakers, you might as well put the maximum sized breaker the panel is rated for unless the supply conductors are undersized.
You don't gain much "protection" by replacing a main breaker with a smaller model and you may get an unncessary trip.
Is the UPS supposed to be on it's own breaker? If so, it should have only the number of poles the UPS wants.
Why are you using fuses in this day and age? A breaker can combine the functions of safety switch and over current protection.
But, yes, you can usually find lower rated fuses in the same physical size.
BUT if the appliance is on a "single circuit" the CB at the distribution panel should be rated to protect the wiring and the breaker/safety switch should be rated to protect the UPS. If the UPS gets into trouble you want a local breaker to trip. If the local breaker is the same size as the panel breaker, you are better off replacing THAT breaker with a simple switch or disconnect. You don't want to be re-setting two breakers because of a single fault.
The nature of a UPS is that disconnecting the power to its input doesn't stop the output.
If the UPS doesn't have a switch then it's useful to have a means of quickly cutting power to the loads.
If you have "significant" loads, it may make sense to monitor both current and power to these loads. The wiring only cares about current but the battery and inverter care a lot about power also. Be advised that once you get beyond "consumer grade" power monitoring stuff, it can get expensive. You might just occasionally check currents with a clamp on meter as part of your routine maintenance and record your measurements. If the currents start to creep up, you might want to check out the cause (e.g.: incipient bearing failure, dirty filter, fluid valve either too open or too restricted.) If you do install a "safety switch" use an oversized box and arrange the wires so that you can check currents without disturbing the wiring but by only removing a cover.
But don't kid yourself, most problems don't give you much warning.
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John Gilmer
Hi W
You do not say where you are, unfortunately the requirements vary depending on where you are. If you are in the USA most areas follow the National Electric Code NEC 70 (NFPA 70). If you are in the UK you need the follow the UK wiring regs BS 7671 (17th Edition). Other countries other requirements.
Leeds Lad in exile
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For the US... Branch circuit breaker for the UPS - I would assume 50A. I wouldn't reduce it below the input rating without a manufacturer OK.
A 230V 3-phase source is probably high leg delta. If you are using the high leg does the manufacturer allow that?
I don't know why 2 phases of a 3 phase breaker wouldn't work if there is an existing breaker. Somewhat tacky...
Same answer. Three phase will be more expensive (new).
Do you need any fuses? Protection is at the circuit breaker panel? Does the manufacturer require fuses ("maximum fuse size..."?
There are fuse sizes - up to 30A, 35-60A, 70-100A. There are reducers go between the sizes for some types of fuses. Reducers are not a great idea.
Computer rooms are covered by the NEC in 645. A disconnect is required by 645.10, 645.11. There are requirements for where can be tripped, and HVAC and/or smoke dampers may have to be shutdown. You want to shut down the entire machine room.
These are newsgroup opinions without seeing anything. You should really get someone competent on-site.
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