When building or upgrading a data center, at some point you need to make a decision about which circuit breakers to use. While on the face of it that may seem to be a simple decision – use the one that’s the best fit for your load – in fact it can become significantly more complicated if you don’t have a thorough understanding of breaker ratings and what they mean.
In the circuit breaker world there’s been some misunderstanding about the terms “100%-rated” and “80%-rated” circuit breakers. To dispel the confusion, Schneider Electric’s Mohamed Shishani put together a short (less than 10 minutes) podcast that does a nice job explaining the issue.
Understanding the difference between the two begins with a reading of the 2011 National Electric Code. Section 210.20(A) of the code basically says that a circuit breaker for a branch circuit must be rated such that it can handle the noncontinuous load plus 125% of the continuous load. (A continous load is one where the maximum current is expected to continue for 3 hours or more.) In other words, the breaker needs an extra 25% capacity of the continuous load for headroom. That, of course, means you need a larger, more expensive breaker.
There is, however, an exception. When the circuit breaker is listed for operation at 100% of its rating, the additional 25% requirement goes away. Instead, the device simply has to be able to handle the sum of the continuous load and the noncontinuous load.
Now, in practice, you may think it will nearly always make sense to buy 100%-rated breakers and call it a day. But as the podcast points out, it’s not quite that simple.
You need to do some load calculations to determine if your loads are primarily continuous or noncontinuous. If all your loads are non-continuous, you don’t have to worry about the 125% requirement so you can just size your breakers for 100% of your load. In that case, standard, 80%-rated breakers will be more economical.
If you do have some continuous loads, Shishani says it’s best to segment your circuits so they’re all the same flavor, either continuous or noncontinuous. Then the choice of breaker will become clear.
Where you can’t do that, you need to determine the load on each branch circuit, then calculate the required ampere rating you need for each circuit breaker. The rating will be higher for the standard, 80%-rated breakers because you need to allow for an extra 25% capacity on the continuous loads. That may make the 100% breakers the more economical choice. On the other hand, if you need room for growth, that may also play into the equation.
The podcast goes through a few sample calculations to help you understand all the tradeoffs. Check it out to see if you can save some money the next time you need to buy circuit breakers.