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Thread: proper sizing of a branch circuit with single receptacle in a data room

  1. #11
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    Quote Originally Posted by roverted View Post
    These go on large server racks for a surveillance system that records video 24/7/365, these units have never been powered down in the 8 years they have been in existence. These racks are also backed up with very large UPS units and multiple generator backup. I don't see how these could not be considered continuous load....My dilemma would be easy (non existent) if they indeed were not continuous. My feeling is to just put the 30 amp receptacles on the 8ga conductor and a 40 amp breaker and call it good. I feel like im violating code no matter what.
    This just crossed my mind, some of these PDU units (not the ones Im dealing with) are hardwired rather than cord and plug connected...If I weren't dealing with a receptacle, I wouldn't be violating 210.21 (B)(1)...correct? In this scenario my 8ga and 40 OCPD is just fine
    did you look at the code definition I posted for what a continuous load is? not likely that the load runs at full current for 3 consecutive hours.
    Bob

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    Quote Originally Posted by petersonra View Post
    did you look at the code definition I posted for what a continuous load is? not likely that the load runs at full current for 3 consecutive hours.

    Hmm..it does appear from doing a search in these Forums that im not the first to be faced with the conundrum of whether data servers are considered continuous. I do question why the code would mandate the conductors to be sized 125% if they were not implying these as being continuous. (they don't come out and actually call it a continuous load but they treat it that way). I guess I could run my 8ga conductors and put in a 30amp OCPD and call it a day........Im just anal and like to do things correctly.....I do appreciate your input!!

  3. #13
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    Quote Originally Posted by roverted View Post
    Hmm..it does appear from doing a search in these Forums that im not the first to be faced with the conundrum of whether data servers are considered continuous. I do question why the code would mandate the conductors to be sized 125% if they were not implying these as being continuous. (they don't come out and actually call it a continuous load but they treat it that way). I guess I could run my 8ga conductors and put in a 30amp OCPD and call it a day........Im just anal and like to do things correctly.....I do appreciate your input!!
    It is just a rule. It may have been put in place because the loads are non-linear in nature.
    Bob

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    Quote Originally Posted by roverted View Post
    I'm being tasked with designing and installing the branchcircuit wiring for the following scenario:
    I am installing 4power distribution units (PDU) (which is IT lingo for fancy power strips) ratedat 30 amps each in a data server room. The total load being served currently is 19600watts but I want to size everything for the full 30 amps and I'm going to divide this up between the 4 PDU units on 4 separate circuits.I'm going to supply this load with 208 volts from a 120/208 volt 3-phase panel.Which calculates to 23.6 amps per PDU.

    My confusion isthis: These PDU units come with a NEMA 30amp plug to go into a standard 30 ampNEMA receptacle. The code is clear that I have to upsize both my conductor andbreaker 125% because it’s a continuous load and because its IT equipment645.5(A)….which I interpret as meaning conductor capable of 37.5amps (8 gauge)and a breaker rated at 40 amps….but the code also states at 210.21 (B) (1) thata single receptacle on an individual branch circuit cannot have an amp ratingless than the circuit. In addition Table 210.21 (B)(3) seems to back this up that a 30 amp receptacle must be installed on a 30 amp circuit.

    How can I upsize125%? And still meet the requirements of 210.21 (B)(1)

    I'm sure this is a probably a dumb question and an easy answer and I'm just not grasping the whole concept, but if you could set me straight I would sure appreciate it!!

    There are no such things as dumb questions only dumb answers.

    But I'll give it a shot.

    You are asked to wire a 30Amp PDU --which requires a 30 Amp protective device as well as appropriately sized conductors. Why would it be appropriate [in your opinion] to have the branch circuit protected by an oversized breaker that is rated 40 Amp.

    These PDUs are manufactured to handle a max 30 Amp load. These are common in data centers and in hospitals and laboratories for their lab research and development and for use with their special equipment.

    They are usually equipped with 30 Amp twist lock plug for US customers [NEMA L5-30 P.]. The maximum number of outlets of a 30 Amp PDU is 24. Most I have installed is 12 outlets per PDU.

    The biggest power supply for a server is 400 watts. There are some rated at a max 1800 watts, but those are for die-hard gamers with high performing video cards.

    Now, if you count the number of clients [users] that will plug into a PDU will use the remaining PDU units that you said will be supplied by the 120/208 “feeder” or homerun whatever you call it.

    So, this will relieve the possible overloading of one PDU. Not to mention servers and workstations that are connected.

    As had been pointed out, the system has already been engineered. . . you just provide the branch circuit and take your money.

    You don't have to worry about upsizing or demand/load factors.

    The determination of these factors is the responsibility of the designing engineer since it requires detailed knowledge of the installation and the conditions in which the individual circuits are to be exploited.”

    If you think, wiring these PDUs is what you consider "design"-- it would need more basic knowledge-- more specifically engineering knowledge.
    You already know the capacity of the PDU. . . what more is there to figure out?


    There are several demand factors that can be used in different in complex situations such as yours. The electrical engineer who designed it could have used one of these factors. One or more of these are beyond pay grade for most regular joes and even EEs who are not regularly involved in data centers.

    1. Demand factor (this can be based on NEC)
    2. Load factor
    3. Diversity factor.

    The demand factors that are outside the NEC tables are more discreetly applied depending on the type of facility-- whether it is commercial or industrial or the use of a particular facility.

    When applied to industrial installations. . . the demand factor is often based on the kind of machinery the power is intended to provide for.
    Steel mills for example have different loading demand than an ice cream manufacturing facility.

    So, in conclusion, your 30 Amp PDU requires a 30 Amp CB breaker. . . not a 40 Amp CB.

  5. #15
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    These racks are also backed up with very large UPS units and multiple generator backups.


    IMHO you want to approach this differently. Especially if you are looking to establish yourself in this area. Not everyone understands how things "could and should" be wired for best reliability.

    You mention multiple generators and UPS's. If that includes multiple ATS's then you can offer your client a far superior wiring methodology.

    Most of the stuff you are talking about comes with dual redundant hot swappable power supplies. The "best" way is to run two PDU's to each rack. Each PDU comes from a different generator/ATS/UPS.

    Initial costs will be higher but the customer needs to see added reliability as a selling point. Nothing like have a rack full of servers running mission critical stuff and killing a UPS to prove the added reliability of your wiring methods. The IT guys love knowing that they can replace a given PDU with no downtime.

    Number 10 on a 30 amp breaker is fine. I'd pull a 3 phase 5 wire circuit to cover future changes. Better yet, get customer to commit to 3 phase PDU everywhere.

  6. #16
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    Quote Originally Posted by myspark View Post
    There are no such things as dumb questions only dumb answers.

    But I'll give it a shot. . . . the demand factor is often based on the kind of machinery the power is intended to provide for.[/FONT][/COLOR]
    Steel mills for example have different loading demand than an ice cream manufacturing facility.

    So, in conclusion, your 30 Amp PDU requires a 30 Amp CB breaker. . . not a 40 Amp CB.
    I agree with you and Bob. I worked in a data center for a few years. The 30A PDUs (we called them racks) got #10 attached to 30A receptacles and 30A breakers (QOBs and THQBs).

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