Data CenterPower and Cooling

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

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.


18 Responses
  1. Mohamed Shishani

    @Mike Ahern, 100% breakers are identified with a “C” suffix in the part numbers in the online digest. For 100% rated circuit breakers add a “C” in the 9th character place (for example, HDL26015C or JDL26150C)

    Reply
  2. Lu

    What are the design difference between 1000A 80% rated circuit breaker and 800A 100% rated circuit breaker? in terms of frame size and contact size? They can both be used for 800A continuously, but the cable size for 1000A 80% rated breaker is bigger than 800A 100% rated breaker, right?

    Reply
  3. D.M.

    Am I correct in assuming that the 80% vs. 100% only applies above a certain voltage and or amperage level? I have to install some “smaller” breakers and want to know if I need the extra 125% head room.

    Reply
  4. Ahmed Musa

    Sir,
    I wondering if we have an equivalent background for the 80% CBs in IEC or BS standards.

    Reply
    • Mohamed Shishani

      Hello Ahmed, this is a National Electrical Code compliance. I am not sure that this requirement is valid in IEC

      Reply
    • Mohamed Shishani

      Hello Ahmed, This is a requirment to meet the National Electrical Code ( NEC) I am not sure this is valid in IEC

      Reply
  5. jared1089

    I believe that overload devices rated to U.S. (North American ?) standards are rated for continuous load of no more than 80% of rating and that such devices rated to IEC standards are rated for use on loads to 100% of rating. Schneider sells both products as a global company, and a “hot-shot” engineer may find he can shave some cost by designing with one or the other, but I think the customer is best serve by using what is common to the area of the application. It will typically not be maintained by hot-shot engineers. The IEC standards will likely take-over one day, but for now people (customers) need to work with what’s at hand.

    Reply
  6. Daniel Manosalva

    What is the main difference between a standard and a 100% rated circuit breaker? I have read only about when to use either of them but not about their physical difference. Is it only the enclosure? What would happen if I use a standard CB in a 100% rated enclosure and make a 100% current flow through it?

    Reply
    • Kurt

      Great question of which the manufactures will not answer. I heard that the lugs are the only difference. Seems strange Daniel, but not really. Anyone willing to reply must include references (i.e. prove it!)
      Kurt

      Reply
  7. ronald hansen

    The 100% verses 80% rated devices is a topic that comes up from time to time especially in how some AHJ’s are enforcing the rule. The City of Orlando Florida for example mandates that all main circuit breakers be rated at 100% regardless. In most applications the load at the service entrance is mostly noncontinuous with continuous lighting loads at 125%. The way I apply the code is this. If I have a 1550 amp calculated load the circuit breaker will be 1600Amp. There is no need to use a 2000amp circuit breaker for your service unless the load is mostly continuous. Service demands on commercial buildings in many cases are less than 50% of connected with power factors ranging anywhere from 50 to 70% sometimes more. The code typically forces the design professional to size his power distribution system based on connected load rather then demand load [ with some exceptions ] and the electrical system in reality is oversized by a factor of three or more.

    Reply

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