5 ms·

The article says that you're are not allowed to use the full rated capacity of a circuit. Can anyone tell me why? This seems weird to me. Also, is his concern

The article says that you're are not allowed to use the full rated capacity of a circuit. Can anyone tell me why? This seems weird to me. Also, is his concern about earthquakes really warranted? I'd have thought that a DC would have been able to withstand more or less anything thrown its way.

tptacek 14y ago
The concern is probably mostly about the power grid. Power problems have caused outages at large colo centers in the last 5 years.
- kintamanimatt 14y ago
  Why wouldn't they just rate a 20A circuit as 16A if that's all that's usable? What's the benefit to rating it as a 20A circuit if 4A are unusable? I feel like I'm missing something.
  - kika 14y ago
    Because it looks better in the pricelist? 20A for $500 is better than 15A for $500.
  - tedunangst 14y ago
    Peak draw is less than typical draw. If the fuse blew at 16A, you'd only be able to use 12A. So they give you some headroom. But if everybody draws at 20A all the time, the building melts down. Soft quotas vs hard quotas.
  - Daniel_Newby 14y ago
    1. As mmt mentions in another comment, that's the way the U.S. safety codes rate circuit capacity. 2. A 20 A circuit breaker is allowed to trip at 20.000001 A. Derating the continuous load provides the safety margin for reliable operation, even using inexpensive circuit breakers.
  - manuscreationis 14y ago
    Believe it or not, this is standard electrical engineering. The breakers in your home breaker box operate the exact same way. A 20A breaker will trip once you exceed a certain threshold, which I believe is 16 or 17. They're rated to go to 20 by the factory, but to prevent problems with overloading and causing fires/explosions/etc, they purposefully break at a lower threshold. So this isn't just a colo price scam thing, this is the way the world works.
duskwuff 14y ago
Most electrical equipment, including computers, draw significantly more power as they're starting up than they do at steady-state. If everyone's server were pulling the maximum rated capacity at steady-state, you'd probably trip a breaker when everything started up (for instance, if power had just been restored after a failure, or if several machines were power cycled simultaneously).
- cagenut 14y ago
  Its this. You need the headroom for power-on/reboots. Blade chassis are smart enough to stagger starting up each blade for this reason. 1Us don't know about each other.
  - kika 14y ago
    1U also "know" as long as you use proper PDUs (Power Distribution Units, basically power strips with some tiny brain). PDU's will also measure the amps for you and you call poll them for numbers and you can powercycle servers remotely. They're usually worth every penny.
    - deleted 14y ago
      [deleted]
nodesocket 14y ago
Metered PDU's are awesome, they have a nice LED display on the front showing the current used amps. We use Tripp lite, cheaper than APC, and hold up solid.
- kika 14y ago
  I have nothing good to say about our Tripplites. After 2 years in service their brains start to hang rapidly.
  - brandon 14y ago
    I can corroborate this for Tripplite switched PDUs, but their dumb metered PDUs always held up for me.
wmf 14y ago
http://en.wikipedia.org/wiki/National_Electrical_Code#Actual_vs_maximum_current_rating http://en.wikipedia.org/wiki/National_Electrical_Code#Actual... I'm not sure why this came about in the first place, though. AFAIK in Europe breakers are rated at 100%.
mmt 14y ago
The article says that you're are not allowed to use the full rated capacity of a circuit. Can anyone tell me why? This seems weird to me. I think this is an NFPA/NEC[1] standard. You could look at it another way: circuits are breakered such that you can temporarily peak 25% above their normal capacity without tripping the breaker. I don't know why the standard isn't more conservative, except to guess that it's because typical household/office uses are likely to have low continuous and high intermittent loads (e.g. vacuum cleaner), making it a good trade-off. [1] USA (and Canada?) only. Anyone know how it is in other countries?
- mindslight 14y ago
  Yes, this is a requirement of the NEC. Branch circuits need to be rated at 125% of the continuous load. In general, wires are sized not only on their overcurrent protection, but also (overbuilt) for the planned load. Unplanned "noncontinuous" loads get a free pass as the only thing that really keeps them in check is the overcurrent protection. But kika's downthread comment is most on the mark - of course data centers would rather advertise the less relevant "circuit rating" as the number is higher. The load calculations are the same either way. (btw speaking of the NEC, http://bulk.resource.org/codes.gov/ http://bulk.resource.org/codes.gov/ is a fantastic resource)
rdl 14y ago
There is also power factor, as well as surge capacity.

tptacek 14y ago

The concern is probably mostly about the power grid. Power problems have caused outages at large colo centers in the last 5 years.

kintamanimatt 14y ago
Why wouldn't they just rate a 20A circuit as 16A if that's all that's usable? What's the benefit to rating it as a 20A circuit if 4A are unusable? I feel like I'm missing something.
- kika 14y ago
  Because it looks better in the pricelist? 20A for $500 is better than 15A for $500.
- tedunangst 14y ago
  Peak draw is less than typical draw. If the fuse blew at 16A, you'd only be able to use 12A. So they give you some headroom. But if everybody draws at 20A all the time, the building melts down. Soft quotas vs hard quotas.
- Daniel_Newby 14y ago
  1. As mmt mentions in another comment, that's the way the U.S. safety codes rate circuit capacity. 2. A 20 A circuit breaker is allowed to trip at 20.000001 A. Derating the continuous load provides the safety margin for reliable operation, even using inexpensive circuit breakers.
- manuscreationis 14y ago
  Believe it or not, this is standard electrical engineering. The breakers in your home breaker box operate the exact same way. A 20A breaker will trip once you exceed a certain threshold, which I believe is 16 or 17. They're rated to go to 20 by the factory, but to prevent problems with overloading and causing fires/explosions/etc, they purposefully break at a lower threshold. So this isn't just a colo price scam thing, this is the way the world works.

duskwuff 14y ago

Most electrical equipment, including computers, draw significantly more power as they're starting up than they do at steady-state. If everyone's server were pulling the maximum rated capacity at steady-state, you'd probably trip a breaker when everything started up (for instance, if power had just been restored after a failure, or if several machines were power cycled simultaneously).

cagenut 14y ago
Its this. You need the headroom for power-on/reboots. Blade chassis are smart enough to stagger starting up each blade for this reason. 1Us don't know about each other.
- kika 14y ago
  1U also "know" as long as you use proper PDUs (Power Distribution Units, basically power strips with some tiny brain). PDU's will also measure the amps for you and you call poll them for numbers and you can powercycle servers remotely. They're usually worth every penny.
  - deleted 14y ago
    [deleted]

nodesocket 14y ago

Metered PDU's are awesome, they have a nice LED display on the front showing the current used amps. We use Tripp lite, cheaper than APC, and hold up solid.

kika 14y ago
I have nothing good to say about our Tripplites. After 2 years in service their brains start to hang rapidly.
- brandon 14y ago
  I can corroborate this for Tripplite switched PDUs, but their dumb metered PDUs always held up for me.

wmf 14y ago

http://en.wikipedia.org/wiki/National_Electrical_Code#Actual_vs_maximum_current_rating http://en.wikipedia.org/wiki/National_Electrical_Code#Actual... I'm not sure why this came about in the first place, though. AFAIK in Europe breakers are rated at 100%.

mmt 14y ago

The article says that you're are not allowed to use the full rated capacity of a circuit. Can anyone tell me why? This seems weird to me. I think this is an NFPA/NEC[1] standard. You could look at it another way: circuits are breakered such that you can temporarily peak 25% above their normal capacity without tripping the breaker. I don't know why the standard isn't more conservative, except to guess that it's because typical household/office uses are likely to have low continuous and high intermittent loads (e.g. vacuum cleaner), making it a good trade-off. [1] USA (and Canada?) only. Anyone know how it is in other countries?

mindslight 14y ago
Yes, this is a requirement of the NEC. Branch circuits need to be rated at 125% of the continuous load. In general, wires are sized not only on their overcurrent protection, but also (overbuilt) for the planned load. Unplanned "noncontinuous" loads get a free pass as the only thing that really keeps them in check is the overcurrent protection. But kika's downthread comment is most on the mark - of course data centers would rather advertise the less relevant "circuit rating" as the number is higher. The load calculations are the same either way. (btw speaking of the NEC, http://bulk.resource.org/codes.gov/ http://bulk.resource.org/codes.gov/ is a fantastic resource)

rdl 14y ago

There is also power factor, as well as surge capacity.