Hello everyone, we recently imported a batch of Japanese equipment with a specified power requirement of 480V 60Hz. The FLA nameplate on the equipment indicates 32A. However, upon inspecting the control panel, I found that there may be violations of NEC regulations regarding circuit breakers and wire gauge. The main breaker is 40A and the main feeder size is 14mm2. I need to make changes to comply with NEC regulations with minimal modifications. The panel includes 11 servo drives, a 1HP water pump, and a controller. The supplier mentioned that certain servo drives do not operate simultaneously. Servo drives #1-8 have a rated input current of 8.5A, while drives #9-11 have a rated input current of 4.6A. The selection of circuit breakers and wire gauge was based on duty cycle considerations. NEC requires considering all continuous and non-continuous loads when selecting OCPD and conductors, regardless of logic interlocks. Are servo drives considered continuous or non-continuous loads? Most servos operate below 50% load and do not run continuously, so they may be classified as non-continuous loads. The calculations for selecting the appropriate breaker size can be done using different methods prescribed by NEC and NFPA 79. In terms of changing conductor size without changing the breaker, it may be feasible as long as the current carrying capacity requirements are met. NEC specifies current carrying capacity for AWG wire sizes, but for IEC standard wire sizes, manufacturer specifications should be followed.
When choosing Overcurrent Protection Devices (OCPD) and conductors, the NEC mandates considering both continuous and non-continuous loads, regardless of any logic interlocks. It is important to note that in the case of servo drives, it is more accurate to describe them as non-coincident rather than non-continuous. This distinction is crucial because non-continuous implies that all motors could potentially run simultaneously, even if only for short durations. However, in your machine, certain motors are designed to prevent running at the same time as others. According to NEC 220.60 on Noncoincident Loads, if it is unlikely for two or more non-coincident loads to be in use simultaneously, only the largest load(s) being used at one time should be considered when calculating the total load of a feeder or service. Additionally, Exception #3 of NEC 430.24 states that if circuitry is interlocked to prevent simultaneous operation of specific motors or loads, the conductor ampacity can be based on the sum of currents resulting in the highest total current when operated simultaneously. There is no restriction on the use of software-based interlocking systems, such as a PLC program. Regarding servo drives, which may present a gray area, it is unclear whether non-running motors are completely disabled or still exerting holding torque. NFPA 79 Section 19.3.2 may offer some clarity as it specifically addresses servos: "Motor circuit conductors for motors operating in non-continuous mode can have reduced ampacity based on the design load and duty cycle." This guideline may provide some helpful insight into addressing the unique considerations of servo drives in electrical systems.
When considering demand factors, both the National Electrical Code (NEC) and NFPA 79 offer provisions to account for them. However, NEC tends to be more cautious in this aspect. For instance, NEC paragraph 430.26 allows the Authority Having Jurisdiction (AHJ) to approve feeder conductors with a lower ampacity than specified in 430.24. This decision will ultimately be in the hands of the AHJ. In comparison, NFPA 79 paragraph 19.3.2 permits motor circuit conductors to have reduced ampacity based on the design load and duty cycle. It seems from your profile that you are based in Taiwan. Will this equipment be installed in the US? It would be advisable to consult with the AHJ at the installation site for further clarification and guidance.
Hello Kolyur and PaulB, thank you for your response! The machine will be installed in the United States and is just one of many machines we have. I am concerned about getting approval from the AHJ due to past cases. We have a different type of machine that requires a 200A breaker according to NEC guidelines, even though the actual working current is only around 50A due to the number of servos in the system. This presents a challenge in terms of cable size in the cabinet. I am curious if this situation is common in the US. It seems like a waste to size the breaker according to NEC standards when the actual current is much lower. We are using the appropriate cable size based on the breaker size, but I question the necessity for the breaker to be so large. While I understand the safety reasons behind NEC regulations for wire size limitations, I am puzzled by the requirement for a breaker of a certain size. Wouldn't a smaller breaker with a larger cable still provide protection in case of overcurrent? I have noticed a similar design approach in Germany where calculations are based on the actual load requirements for the feeder, which contrasts with practices in the US. This discrepancy seems puzzling.
In my understanding, based on NEC paragraph 430.62, a feeder providing power to specific fixed motor loads must have a protective device with a rating or setting that does not exceed the rating or setting of the branch circuit's short-circuit and ground fault protective device for any motor connected to the feeder. It is important to verify if the circuit breakers shown in the initial post are accurate. According to information from the Schneider website, the circuit breaker in question has a rating of 6-10A and a magnetic tripping current of 138A. As per NEC guidelines, the main circuit breaker should be rated lower than 138A to comply with the specification of "not greater" in the NEC.
kolyur suggested that your servo drive groups may be considered non-coincident rather than non-continuous. Non-continuous loads imply that all motors could potentially run simultaneously, which is not the case for your machine as certain motors are prevented from running at the same time. Refer to code 220.60 for Noncoincident Loads, which allows for the calculation of total load based on the largest load used at one time. Exception #3 of 430.24 permits conductor ampacity based on interlocked circuitry preventing simultaneous operation of motors or loads. This interlocking can potentially be software-based, like a PLC program. As for the use of servo drives, NFPA 79 section 19.3.2 allows for reduced ampacity based on design load and duty cycle for motors operating in other than continuous mode. While this may help avoid code violations, the issue remains a gray area, especially for a new facility where local inspectors may conduct detailed checks. This uncertainty could not only impact your panel but also affect the upstream PDP due to potential breaker changes.
You're correct in assuming that for NEC regulations, continuous and non-continuous loads are considered when selecting OCPD and conductors. It does seem like servo drives can be classified as non-continuous loads, especially if they run below 50% capacity and not continuously. However, I would still reach out to a professional for an assessment just to be certain in case there are nuances I might be overlooking. As for the conductor size, check the specifications from the manufacturer, keeping in mind the potential for changes in resistance due to temperature, or the skin effect due to high frequencies. It's always better to be safe than sorry, especially when it comes to electrical safety.
You're correct that it's important to respect the NEC regulations to ensure both safety and the longevity of your equipment. When it comes to servo drives, they can be considered either continuous or non-continuous loads, depending on their actual operation. If they operate for three hours or more continuously, they would be classified as continuous loads according to the NEC. If they operate less than that, they can be considered non-continuous. To be on the safe side, it's always a good idea to plan for a continuous load scenario, particularly when it comes down to OCPD and conductor sizing. Also note that NEC 210.19(a)(1) requires conductors for continuous loads to be rated at not less than 125% of the load. Therefore, make sure that the conductor size is chosen accordingly.
Hi there, I understand your concern and dilemma here. You're right about NEC and NFPA 79 guidelines, they mandate considering the full load of all parts of the system for safety's sake, regardless of if they function simultaneously or not. While your supplier might have designed the system with efficiency in mind, NEC prioritizes worst-case scenarios for, well, worst-case scenarios. As for the servo drives, commonly they are classified as non-continuous loads since they often operate below 50% load and do not run continuously. However, you might want to double-check in this scenario, mainly because these are the major parts of the current draw, and it might change your calculations significantly. Regarding the wire size, you should consider checking the manufacturer-specific specs for the IEC standard wire you mentioned (14mm2). Usually, IEC wire sizes can handle more current than similar AWG sizes, so you might not need to change it, but again it's worth checking. And remember, safety is always paramount!
It sounds like you have a twofold issue - complying with NEC regulations and efficiently integrating your equipment. Servo drives are often classified as non-continuous loads, particularly given your statement that most are operating under 50% load. Dependent upon your individual duty cycles and expanses of operation, you might be able to arrange for select servo drives to run during off-peak times, further cementing their classification as non-continuous loads. As for conductor sizes, you're correct that you can likely avoid changing the breaker size if the wire used has sufficient current carrying capacity to satisfy the requirements of NEC. Moreover, when considering the IEC standard wire sizes, I agree that adhering to manufacturer specifications is crucial to ensure proper operation. The key here is to maintain efficient operation while adhering to safety regulations, so a careful study of both your equipment and the regulations is definitely vital in making the necessary modifications.
It sounds like you’re navigating a bit of a tricky situation with that equipment! Regarding your question about the classification of servo drives, it’s true that the NEC typically distinguishes between continuous and non-continuous loads, and since those drives might often run below 50% load, they could potentially be treated as non-continuous. Still, it's best to err on the side of caution and consider their maximum ratings when sizing your breakers and conductors. Given the fixed size of your feeders, you might want to explore adjusting the circuit breakers for those specific loads while ensuring all connections adhere to the NEC guidelines. It might also be worth checking with a local electrical inspector to ensure compliance, especially if modifications could impact safety. Good luck with the adjustments!
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Answer: - Servo drives may be classified as non-continuous loads based on the fact that most servos operate below 50% load and do not run continuously.
Answer: - NEC requires considering all continuous and non-continuous loads when selecting Overcurrent Protection Devices (OCPD) and conductors. Different methods prescribed by NEC and NFPA 79 can be used for calculating the appropriate breaker size.
Answer: - It may be feasible to change conductor size without changing the breaker as long as the current carrying capacity requirements are met. Manufacturer specifications should be followed for IEC standard wire sizes, while NEC specifies current carrying capacity for AWG wire sizes.
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