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Joined: Jul 2001
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JBD Offline
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Originally Posted by pdh

This would mean much larger transformers. For example if the data center loads needed 375 kVA in an isolation transformer, a 50 kVA 240:32 transformer would do where its secondary were supplying 1/7.5 of the voltage.

Have you ever tried to find a 50kVA unit with a 32V output? Talk about special equipment.



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PC were sold into the mass market where the 120v receptacle was the norm.
Noise tolerance got a whole lot better by the end of the 70s too. TTL logic had a lot to do with it and CMOS was even better than that. IBM removed all of the requirements for IG and such in the early 80s. We stopped "zap testing" machines on installation and they stopped worrying about star grounding. The modern switch mode power supplies have such a wide mouth that we don't even care that much about sags and spikes on the input voltage.
A lot of the things people "know" about noise and power problems in computer rooms aren't really urban legends but they are mostly ancient history. In my last 15 years at IBM I have to say, every time we "fixed" something by correcting a power or noise problem, we were back in there finding the real problem soon thereafter.


Greg Fretwell
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pdh Offline OP
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Originally Posted by LarryC
My guess as to why not many people make 277 compatible equipment is that for UL safety ratings. The spacing and clearance requirements for line voltage carrying components become larger above 265 VAC.

Larry C

Right. Although it really isn't much more to go from the spacing needed for 240V (NEMA 6-15) to the spacing needed for 277V (NEMA 7-15). Even SOJ cable is rated for 300V. Switch mode power supplies, including those in lots of small wall warts, are safe as high as 240V and are listed as safe for use in places like Europe.

The big reason power supplies are not made in any quantity for 277V is the lack of market. The lack of market exists because 277V is not a domestic or even customary receptacle voltage. You MIGHT have 277V fluorescent lights in the office, but not 277V convenience outlets. If someone has a machine that needs 277V they call an electrician to run a new circuit and install a special outlet for it (assuming 277V is even available in the building).

But, I'm not trying to make computer run directly on 277V. Even if we could convince manufacturers to design them to work and be safe on 277V, flushing out the old ones already integrated in equipment would be years, if not a couple decades.

They do work on 240V. Since +/- 10% is considered an acceptable range, I'm guessing they should be fine on the 244.5V I'd get from the 240+32:240 buck-boost arrangement derived from 277V. That does limit my upper voltage swing to 7.9%. And it might be easier to get the utility to tap their transformers down to deliver around 272V, than to get them to deliver a 416Y/240 service (easy to do with common pole pigs, but for pad mount that would require a special order for three phase, or 3 separate single phase units).

If it turns out the 244.5V is an issue, one fall-back is to go with a 240+48:240 buck-boost arrangement, which would step 277V down to 231V. That would change the buck-boost capacity ratio from 7.5 down to just 6.

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pdh Offline OP
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Originally Posted by JBD
Have you ever tried to find a 50kVA unit with a 32V output? Talk about special equipment.

But that's a one time special order in the worst case. Try making special order power supplies that run on 277V for every computer, every network switch, every router, etc.

It's not practical to change out every piece of equipment to run on 277V. If it were, that's the direction I'd go. However, it is practical when building up a data center designed for around 240V L-N circuits to special order transformers needed to step the 277V down to around 240V.

What may be a useful question is weighing the cost of an off the shelf 277V to 240V isolation transformer at full capacity against the cost of a special order buck-boost transformer at the partial capacity buck-boost can work with. That's part of what I am pondering: could the buck boost approach be made practical. Several transformer manufacturers apparently will do a special order like that. The exact size would depend on the scale of the data center involved.

And there is the option to distribute 277V down to a group of rack cabinets and step it down to 240V or 244.5V on the scale that can utilize an off the shelf buck-boost transformer. One 7.5kVA buck boost transformer in 240:32 configuration could support 56.25kVA of load on that one phase (1/7.5 of the load is served by the 32V secondary in series on the 277V side, where the 1/7.5 is from the 240/32 ratio).

Joined: Mar 2005
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Maybe we here in the US should just give up and adopt 400/230V 50Hz like they have in europe. Wires would be smaller, coffee pots would heat up faster, everyone would be happy.

Is it too late to split the difference for the best of both worlds and make the whole world 400/230V 60Hz?

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pdh Offline OP
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Changing to 50 Hz would be very hard unless there was a 16.7% voltage reduction (although going from 480 to 400 is that percentage). That kind of voltage reduction would mean more current and more wiring losses. Or all those transformers have to be changed out. Keeping 60 Hz would be fine. Keeping the voltages we already have would be fine. Adding to it would be desired.

We already have 240 V in homes. We just need to learn to use it, which includes tweaking NEC 210.6 a bit. We can run fluorescent lighting on the existing 240 V if we use double pole switches. Computers, and an increasing number of other electronics, will run fine on 240 V. There are NEMA-6-15P to IEC power cords available.

Making 400/230 or 416/240 (at 60 Hz) available on request, where 480/277 would be available, and making 240/480 available on request, where 120/240 would be available, would help. Allowing 2 different systems at the same time in electrical service (avoids the extra transformer losses where both 120/208 and a higher voltage at the same time, are desired) would help. They just need to bring it all in on a combined neutral to avoid the dual path issue across ground bonding. This would be a utility tariff issue.

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The US will never abandon the good old NEMA 5-15 and 120/60hz, we have too much equipment out there.
Just look at how long it took to get rid of analog TV ... if it ever really goes away and that is a fairly minor amount of equipment, compared to everything else we plug in.


Greg Fretwell
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pdh Offline OP
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It still might be fun to put up a humorous web site about the coming electrical power transition grin

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There is already a thread on alt.electrical.engineering about the upcoming switch to digital electricity.


Greg Fretwell
Joined: Mar 2005
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Originally Posted by pdh
Changing to 50 Hz would be very hard unless there was a 16.7% voltage reduction (although going from 480 to 400 is that percentage). That kind of voltage reduction would mean more current and more wiring losses. Or all those transformers have to be changed out. Keeping 60 Hz would be fine. Keeping the voltages we already have would be fine. Adding to it would be desired.
That much is easy- all they have to do is slow the generators 20% and you get both a 20% decrease in voltage and frequency, and that 480V 60Hz generator is now putting out 400V 50Hz! Pretty much every 3-phase emergency generator in production can do this, too - drop from 1800rpm to 1500 and that's about it, it's reconfigured from US to Europe. Most feature 12-tap coils that can be configured for 208V 60Hz, 480V 60Hz and 400V 50Hz. Often they can be derated for 208V 50Hz if required.

The transition, especially in residential areas, would be difficult. People love their NEMA plugs. Fortunately, the trend nowdays is that even though the cords have NEMA plugs, the appliances don't really care WHAT they get. I can't tell you how many different countries I've plugged a NEMA plug into with just a plug adapter- 100V 50Hz, 230V 50Hz, it all works. Sure, it all has to be designed this way, since a 60Hz transformer would saturate and burn up and an unregulated device built for 120V getting 240V isn't happy either, but just about everything is built for easy manufacture and export anymore.

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