Electrical – Rated power vs power output

A resistor will reach a thermal equilibrium when the dissipated energy equals the energy drained to the environment. To drain heat to the environment the resistor's temperature has to be higher than the environment's; the higher the temperature difference the more heat will flow. So the resistor can dissipate more power at low environment temperatures. Rated power may be specified at 25°C and derate at higher temperatures. These thick film chip resistors derate only from 70°C, as the following graph shows:

So a 100mW 0603 can still dissipate these 100mW at 70°C, but shouldn't dissipate more than 50mW at 100°C environment temperature.

The datasheet doesn't give suggestions towards copper layout (land patterns and trace widths) which will influence conducted heat. (Convection will be low for SMDs, and radiation almost zero; the temperature is too low for that.) It may be tempting to have lots of copper connecting to a pad, but make sure that this doesn't cause trouble in soldering.

I found this excellent blog from Schneider Electric on the differance between 100% rated and 80% rated breakers. This is a link to the Podcast mentioned in the blog.

Clearing up Confusion over 80% vs. 100%-rated Circuit Breakers

Paul Desmond | June 12, 2014

1. 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.

2. 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.

3. 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 continuous 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.

4. 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.

5. 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.

6. 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.

7. 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.

8. 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.

9. 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.