We are talking power plant levels of current to power the the thing. Did it just have 2 inch lugs?

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    – Chenmunka
    May 10, 2023 at 13:05
  • For comparison the fast charging ports for EVs is about 100kw. Last month I was at a shopping mall in China (Wanda Plaza, Shuangliu, Chengdu), there are a row of 50 EV chargers, of different vendors and operators. And there are 6 transformer rooms that supports them. Yeah, that's at least 30MW, in a parking lot. May 17, 2023 at 13:55

5 Answers 5


Captain Obvious here: By Cable.

We are talking power plant levels of current to power the the thing.

No. 115 kW is what the lowest end Mercedes C180 of 2008 offered - that's a quite average car. Not anything special. Not even for domestic. Power plant level starts somewhere in the multi megawatt range(*1). My 1990 build single family home is connected at 3x64A at 400V: that's roughly 45 kW. Even 3x160A (which is about 110 kW) are still considered household level - at least in Europe.

Did it just have 2 inch lugs?

Well, wire size depends on current rating (amperes). Also (early) Cray and CDC used to work from motor generators (*2) running at mains voltage and frequency, outputting ~210V at 400 Hz (*3).

To carry 115 kW the following 'wire' sizes might have been used:

Section Current Area
400 V Main 150 A 95 mm²
210 V MG 300 A 185 mm²
5 V Logic >12000 A 6000 mm²

While the first two are rather common wire sizes, the third row is more of a theoretical value, as, for one, only part of that power was feeding the logic. Most would have been used for cooling and so on - but more importantly, it wouldn't have been a single power supply but a series of independent ones. Also their output would have been on copper bars, not round(ish) wires.

Those parts using wires could be handled manually - though might need some force. I had to do it several times back in the good old days of mainframes.

*1 - Average size of German power plants was ~300 MW(el) during the 1950s, ~600 MW(el) in the 1970s and over 1-2 GW(el) since the 1980s. Today a single wind turbine delivers about 10 to 30 times the need of a Cray 1 installation.

*2 - By being physical moving mass devices they also doubled as surge protectors and short term UPS. Beside increasing frequency to reduce transformer size, that is. Motor-generators were an all in one solution.

*3 - IIRC that's some kind of WW2 military standard.

  • 1
    Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on Retrocomputing Meta, or in Retrocomputing Chat. Comments continuing discussion may be removed.
    – Chenmunka
    May 8, 2023 at 21:05
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    Minor side note: the Cray-1 used dual power supply levels for the logic: -5.2V and -2V. May 10, 2023 at 2:56

Most big iron of the era used 3-phase power, and tended to use motor generator ('MG') sets to shield the load from impulses and from voltage sags and surges. Some computer MG sets had a flywheel in between the motor and generator for added momentum, I think for some kind of brief 'UPS' capability.

The Cray was no exception. It had two 150 kVA MG sets, each one powered by a 200 HP 460 V, 60 Hz, 3-phase motor, driving a generator providing 208 V, 400 Hz, 3-phase to the power distribution cabinet. That's a big motor! Each MG set weighed 3900 pounds, around 2 US short tons (1800 kg), and the motor needed to be started by a 10 HP (7.5 kW in round numbers) 'pony motor'. Each set had a control cabinet weighing 3/4 of a US short ton or 680 kg.

Cray 1 Preliminary Site Planning (pdf)

I am pretty sure these dimensions are in inches...

enter image description here

  • Thank you for the link. Extremely interesting! This thing had a CARD reader! No teletype or video terminal. Just enter a big computing job in a deck of cards, and press run!
    – Roland
    May 9, 2023 at 8:34
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    @Roland I used a card reader in university. There was a big room full of keypunches instead of terminals, and we queued up to submit our jobs into the hopper (stacks of cards with some JCL magic cards to get the right compiler invoked). And a massive chain printer. Good times. May 10, 2023 at 5:41

Power Plant Level is not recognized jargon. Yes, there are power plants of that size. Wind turbines and hydro powerplants exist as small as a few kW. But the fun starts with higher levels. Large power plants (hydro, coal, gas) may consist of one or several units of hundreds of MW each. Large consumers like aluminum smelters are preferably located close to such power plants, but a Cray could be located just at your company or university.

100 kW is not that big of a load.

Your electric vehicle may, for fast charging, need a power level not much lower than that Cray.

Today's supercomputers may use far more than that miserly 100 kW but are still located near the user instead of near a power plant. But I heard that bitcoin mining farms do get located near power plants, because of low energy prices.

Then trains. Electric trains have engines in the MW range, and receive power not through cables, but through overhead contacts, with the rails as return conductor. Those overhead contacts consist of blocks of carbon against a wire of copper or bronze alloys, and operate at high speed, some trains run up to 300 or 400 km/h. In order to keep the current low, the voltage must be high, up to 30 kV, but then sparking may be a problem.

Update: on my way home I saw this mobile power plant rated 100 kVA. If you realize that this is (almost) big enough to power your Cray, you would not ask about crazy cabling or lugs, would you?

100 kVA mobile generator

That means, cabling at the line distribution voltage, e.g. 230 V, is trivial, but not at the electronics component level. The Cray probably is built with 5 volt logic. Divide your 110 kW by 5 V to arrive at 22 kA, and you understand that no single 5 V power supply would be capable. Such computers consist of lots of circuit boards spread out over several racks, where each rack has one or several 230 V AC to 5 V DC power supply units, so that the cable requirements of each unit are modest, and of the same scale of the power supply in your desktop PC.

On another level, a single modern CPU or GPU chip can draw up to a few hundred Watt, at a voltage in the order of 1 Volt. This means that that centimeter scale chip must be fed with around 100 Ampere. Here the same strategy is followed. Close around the chip you will see a dozen tiny DC/DC buck converters, that convert 12 V or 24 V DC power from "the pc power supply" down to that 1 V. The total IC supply current is spread out over a lot of smaller currents that flow through copper PCB traces with a small cross section, but also very small path length of just a few millimeters, then through a fairly high number of IC pins, then through a lot of internal IC interconnects. Yes, it needs a lot of trouble to get all that power to the actual transistors.

Someone posted a link to the description of the Cray's power supply system. Note the similarity to the situation around today's CPU chip. Including the heat pipe cooling in some desktop / laptop PC's.

After all, that Cray question was not about a simple problem.

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    Megawatt range at 30 kV is literally nothing, just 30A. However there are still many electric trains that use overhead systems with 3 kV or even 1.5 kV, this is where THICK wires are required, lots of traction substations, and sometimes even several pantographs per locomotive in order not to literally burn the wire at low speeds.
    – lvd
    May 8, 2023 at 14:49
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    @lvd In southern England there is a large area of railway electrified at 750V DC, via a third rail (and I don't mean just 'subway' lines, main lines up to 140 miles long). The trains can draw 2000A or more at starting, and some substation circuit breakers are set at 6800A. I once saw a French 1500V DC loco at night, start a heavy freight train. The track was a bit uneven and the loco rocked a bit. There was arcing at the 2 pantos and you could see the (dual) contact wires glowing bright red and fading at the points where this happened. May 8, 2023 at 15:54
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    @lvd correct. But for a high speed train of 45 MW peak level, 45 times "nothing" yields over a KA, not exactly "nothing" :-) My point was not about trains, but just that a Cray's 110 kW is not that big of a thing
    – Roland
    May 9, 2023 at 8:10
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    @MichaelHarvey interesting, thanks. I once saw, during a test run of a Thalys high speed train that the carbon at the pantograph broke because of sparking. That was end of test that day.
    – Roland
    May 9, 2023 at 8:12
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    @Roland - All Thalys sets are dual voltage 25 kV AC French and 1500v DC Dutch, some also have 3000 V DC Belgian, and some have 15 kV AC German as well. DC traction current is a brute. You should see the 1955 SNCF speed record run on Youtube. They got a 1500 V DC loco and three coaches to do 206 MPH/331 KPH between Bordeaux and Dax on the old Midi 1500 V system. The panto seemed to be sparking continuously in the film shot from a plane. They added extra mobile substations and boosted the voltage to 1900 V. The track was wrecked behind the train in many places. May 9, 2023 at 8:40

As the voltage is reduced, the current is increased. So the really serious wiring is in the last (thankfully short) run from the power distribution cabinets to the computer proper. Here are a couple pictures.

Here's a shot of one of the power supply cabinets (and part of another).

enter image description here

See those yellow wires? Here's a little closer shot:

enter image description here

So yeah, each of those is about the size of a battery cable for a car. And as you can see, one power supply cabinet has what looks like 4 separate power supply modules, and each module has around a dozen of those cables.

Altogether, those power supplies deliver 770 amps at 5.2 volts.

The Cray also used a 2 volt power supply, but this was used only for line termination, so I believe the current draw on it was relatively low.



Easy on commercial power

115kW will need a 125% continuous-load derate to 138kW, unless you want to pay through the nose for "100% rated" gear. That's not a problem, though.

In North America, if you talk to your power company about 150kW tier service, the assumption will be 480V 3-phase power unless you're residential. That is bog-standard industrial power found in any industrial park or strip mall... just look around for a forest green box guarded by some bollards. (Except some places in Canada where they're real into pushing insulation limits by running at 575V). I swung by a shoddy little mall near me to get a shot of the nameplate on their transformer, but some neatnik had painted it over. I'm sure it was big enough; the site has a laundromat.

So 115,000 watts divided by 480V divided by sqrt(3) gives 138 amps actual. That's perfectly reasonable for run of the mill equipment. As a continuous load, that must be derated 125% giving 173 amps, and so a 175A circuit breaker would be called for. That would use 4/0 AWG, type THHN or XHHW aluminum feeder wire which is rated 180A per NEC 310.15(B)(16). The stuff is a commodity - Home Depot stocks it.

The 3-phase panelboards are a little tougher to come by, but not much - Home Depot tends to stock the ones rated for 120/280V (120V to ground) and we need slightly improved ones rated 277V to ground. But still, this is not costly kit.

Doable on residential too.

For that matter, if you do need to do it with residential tier 120/240V split phase service, the next size up from 200A (48kW) is 400A (96kW) - and the next size from there is 600A (144kW). Remember where the Cray needed 138kW derated? There you go. Order service to an outbuilding. Though at that service size you can probably have the conversation about getting 480V 3-phase.

Due to the sheer difficulty of making a single 15,000 amp power supply, I'm sure the Cray uses a large array of DC power supplies in parallel - easy to do with DC. And then each one takes a modest size AC power feed. So I'm sure the Cray supports simply having the distribution equipment having a bunch of small circuit breakers (e.g. 20A @ 480V or 40A @ 240V). That's easy to do with common kit.

Tesla Model S car chargers do the same thing, 2 blades in a USA unit and 3 blades in a Euro unit, 5.7 kW per blade. We're also seeing that approach taken in DC fast chargers. You have a 350kW DC fast charger, but inside it's like fourteen little 25kW blades. Then you buy a 50kW fast charger from that same company and it's 2 of the same blade.

And Europe is not a problem

On AC power distribution, Europe does the same thing as America does with 480V "wye" industrial power (277V phase to neutral) - except they are 18% weaker at 400V phase-phase (230V phase-neutral) and then, they use that for absolutely everything, including residential service. So the stuff is literally on every pole. Industrial power everywhere. As such, 115kW would be pretty easy to acquire in Europe - if the local residential transformer doesn't have the headroom, then they can just grab another one from the yard and slap it on a pole.

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