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    Circuit maximum load

    I work in a Computer Center where each cabinet has 2- 20A circuits in each cabinet fed from diverse power sources fed from static switched Power distribution units so that in the event of a power failure to either source the load will be automatically switched to the opposite power source. My question is what is the maximum amperage available to the total circuit ( if one power source failed ) since the load could switch at any time due to a utility outage and therby carry the full load on one circuit. Each piece of equipment is dual power corded for redundancy in the event of a failure. Since this is computer gear does inrush current or steady state load have any bearing on the load since code says 16A for a 20A circuit ( combined ). Head office ( suits ) say we should load up to 10A on each side since it is not truely 20A until switched in an emergency. I disagree due to inrush current and over 16A for a short or possibly longer periods of time. but this is fairly vague in the code ( 96 ). Any help would be appreciated.

    #2
    Re: Circuit maximum load

    I am a little confused :confused: with your wording. A circuit breaker, unless listed for 100% of its rating, can only be loaded to 80% of its rating for continuous loads. This wording is no longer in the 2002 NEC, but is still there in the form of the 125% requirement for branch-circuits ratings. In your case, a 20-amp circuit can only be loaded to 16-amp if used for more than three hours. Total load claculation for your circuits should be at 180va per receptacle. Is this what you are looking for?? :confused:
    Bryan P. Holland, MCP
    NEMA - Codes & Standards

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      #3
      Re: Circuit maximum load

      WAXMANAZ, I believe I understand your question. I assume you have a 2-20 amp dedicated receptacles in each equipment cabinet. If that is correct, then you exceed any code requirement of 180 VA for commercial applications. What I belive you have is a design issue.

      Here is my design advice. 8 amps maximum normal operating load. I assume since you have dual cords, both receptacles are installed on the same phase and evenly share the load.

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        #4
        Re: Circuit maximum load

        I am a little confused with your wiring. Under normal conditions you have a circuit capacity of 16 amps. When there is an outage you are switched to the emergency supply by the static switch. These switches are normally very fast and I doubt that inrush will be a problem. It seems to me that you still have a 16 amp circuit assuming that the emergency source it up to it.

        Comment


          #5
          Charles E. Beck, P.E., Seattle
          Comments based on 2017 NEC unless otherwise noted.

          Comment


            #6
            Re: Circuit maximum load

            continous load: a load where the maximum current is expected to continue for 3 hours or more.

            Many receptacle locations do not meet this definition. The computer may be running for more than 3 hours, but it is not always at max current. A lighting fixture for example will take about the same amount of energy to keep the lamps illuminated for the entire time. But a computer's power usage continues to rise and fall over time, depending on the demands put on it. So you will have to determine if your computers run near max current for more than 3 hours at a time, most likely they do not, although some servers may.

            So the 80% rating of a circuit breaker is for continous loads, which in all likely hood you may not have. Then you could use the full 20 amps of the circuit. It sounds to me like you are doubling up the load by taking the power supplies twice tho. If you have two 350w power supplies for one server, the max load for that server should still be 350w, because the power supplies are redundant. On the servers i have worked on, you can hot swap the power supplies, so each power supply needs to be sized for the full load of the server. But you do not add the (2) 350w to get a 700w load. It sounds like you plug one power supply into one 20 amp circuit and the other power supply into another 20 amp circuit. IF this is the case for all the devices, then you could load each circuit up to 16amps continous or 20amps non-continous and not have a problem if either circuit dies.

            A computer with a 350w power supply does not necessarily draw 350w of power. It simply can handle upto 350w. So a lot depends on how you are coming up with your loads. From experience we have found that computer rooms such as you are talking about consume somewhere between 25% to 40% of nameplate ratings. If you add up all the nameplate ratings from your devices, then look at your UPS for actual load being drawn, you will be able to determine what your Percentage of Nameplate actually is. Once you determine this, you will be able to go to a rack and get a better idea of what the actual draw per rack would be.

            hope this helps, it would be much easier if i could actually see the installation. I am assuming that you have UPS powering the PDU's

            [ April 24, 2003, 11:40 AM: Message edited by: jschultz ]

            Comment


              #7
              Re: Circuit maximum load

              Someone brought up a 100% rated breaker previously. This may be something you may not want to hear but...
              Having a 100% rated breaker has nothing to do with it. It appears to be a belief the breakers that are listed with 100% ratings are better than the standard breaker, They are not. It is all in temperature rise testing and application. To simply replace a standard breaker with one rated 100% gains you nothing because it must still be applied as if it were a standard breaker. Not only does a breaker have to be listed for 100% but it must be cabled correctly and installed in an enclosure that is listed for it. Also, you won't find 100% rated breakers in breaker frame sizes normally 400a and above where solid state trip units can be applied.
              Also, people point a finger to the 80% rated breaker. Don't forget that the cable is sized to carry the load and the breaker sized to protect the cable, 125% of the continuous load plus 100% on the noncontinuous load, the cable must be sized to carry that load, derating factors or temperature, raceways, etc must also be considered of course. That automatically derates the cable 80% to begin with. The breaker is selected to protect the cable, thus applying the breaker at least 80% also. But, if you are lucky you can match the breaker to the ampacity of the cable. If not, you can select the closest rated breaker ampacity above the cable rating up to 800a. Above 800a you must select the next lower breaker rating. Then there is NEC art 240.4(D) for small wire and how it is to be protected.
              Or, am I missing something here or magic involved?

              Comment


                #8
                Re: Circuit maximum load

                I assume that the installation is for information technology. This is covered in Article 645 of the NEC. Section 645.5(A) makes it clear that the branch circuit that supplies data processing equipment must have a rating of not less than 125 percent of the connected load. For example, where you have to supply a 10 ampere load through a single receptacle (not duplex), the branch circuit cannot be less than 1.25 x 10A =12.5Amps. Therefore, No. 14 cu with a 15 ampere overcurrent device is permitted. Where the branch circuit supplies more than one receptacle, duplex, or multiple receptacles, the individual load plugged into any one of the receptacles on the circuit cannot exceed 80 percent of the branch circuit rating. For example 15 amp OCD, maximum load that can be drawn from a receptacle is 12 amps. See 210.23(A)(1).
                Regards, John M. Caloggero

                Comment


                  #9
                  Re: Circuit maximum load

                  so if you use #10 wiring with a 20A breaker, then you are covered for the 125% rating. Some technology rooms do not meet the requirements to be called a "information technology equipment room". although it does appear that this one does.

                  [ April 28, 2003, 04:29 PM: Message edited by: jschultz ]

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                    #10
                    Re: Circuit maximum load

                    Originally posted by templdl:

                    Also, people point a finger to the 80% rated breaker. Don't forget that the cable is sized to carry the load and the breaker sized to protect the cable, 125% of the continuous load plus 100% on the noncontinuous load, the cable must be sized to carry that load, derating factors or temperature, raceways, etc must also be considered of course. That automatically derates the cable 80% to begin with. ..
                    this assumes 100% of the load is continous, which in many non-lighting situations, it is not. All of the breakers are rated for 100% of the demand load (125%continous + 100% noncontinous).

                    Comment


                      #11
                      Re: Circuit maximum load

                      Originally posted by templdl:
                      Someone brought up a 100% rated breaker previously. This may be something you may not want to hear but...
                      Having a 100% rated breaker has nothing to do with it. It appears to be a belief the breakers that are listed with 100% ratings are better than the standard breaker, They are not. It is all in temperature rise testing and application. To simply replace a standard breaker with one rated 100% gains you nothing because it must still be applied as if it were a standard breaker. Not only does a breaker have to be listed for 100% but it must be cabled correctly and installed in an enclosure that is listed for it.
                      are you saying that 100% rated breakers cannot handle 100% continous loads at the rating of breaker(ie a 20 amp 100% rated breaker cannot handle 20 amps worth of lighting?)?

                      Comment


                        #12
                        Re: Circuit maximum load

                        80% and 100% rated breakers

                        Comment


                          #13
                          Re: Circuit maximum load

                          As I stated before: "To simply replace a standard breaker with one rated 100% gains you nothing because it must still be applied as if it were a standard breaker. Not only does a breaker have to be listed for 100% but it must be cabled correctly and installed in an enclosure that is listed for it. Also, you won't find 100% rated breakers in breaker frame sizes normally 400a and above where solid state trip units can be applied."
                          Also, to apply a 20a breaker with #10 wire is OK. However, If one were to load the breaker to 20a you may or may not have a nuisance tripping problem. You could say that you are overprotecting the wire.
                          UL489 tests for a 40degC calibrated breaker states that "All breakers shall be calibrated to carry their continuous rating in an ambient temperature of 40degC." The breaker would take longer to trip at room temperature considering that normal room temperature is about 25degC . Remember that a simple thermal magnetic breaker has a bimetalic element and is affected by the ambient temperature.

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                            #14
                            Re: Circuit maximum load

                            well i thought we were talking about a 20A breaker. Also many people use thhn which is rated for 90C. Also number 12 is rated for 25 amps which is 125% of 20 amps. So a normal installation of a 20 amp breaker would have #12 which would be good for the 125% requirement. So changing out to a 100% breaker would gain you somthing, if the load were a continous load, which it probably is not.

                            Comment


                              #15
                              Re: Circuit maximum load

                              Another point to remeber about 100% rated devices, while they must be wired with 90C insulation, there is no UL Listed OCPD rated for 90C temperature rise at it's termination. The result is, the ampacity of a conductor directly connected to any OCPD, even 100% rated, cannot be greater than the amount in the 75C column of any NEC table.
                              Just because you can, doesn't mean you should.

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