Irrigation machines, by definition, contain at least one motor [675.2] and thus are special case motor applications. This means Art. 430 applies, except as modified by Art. 675.
Because they operate outdoors, these machines have requirements you don’t see for typical industrial equipment, such as presses and robotic welders. For example, the external plugs and connectors must be of the weatherproof type [675.17].
These machines may be in direct contact with the soil (Photo 1) or they may travel across it on rubber tires (Photo 2). In either case, their conditions of operation mean you really do ground them per the Art. 100 definition of grounding. See what other differences you can spot as we look a little more closely at the requirements of Art. 675.
Irrigation cable
A key component in delivering electricity to the final point of use is the irrigation cable. If you look at an irrigation machine, you’ll see enclosures mounted on it. The irrigation cable is what interconnects these enclosures. A given irrigation cable can contain power, control, and grounding conductors. When determining ampacity, count only the power conductors [675.5].
To be used as an irrigation cable, the assembly must be composed of stranded, insulated conductors [675.4(A)] that are of a type listed in Table 310.104(A) for 75°C operation and listed for use in wet locations. The cable must also have a:
• Nonhygroscopic and nonwicking filler.
• Core of moisture-resistant and flame-resistant material overlaid with a metallic covering.
• Jacket with moisture-, corrosion-, and sunlight-resistant nonmetallic material.
Minimum thicknesses are as follows:
• Core insulation material: 0.76 mm (30mil).
• Metallic overlay: 0.20 mm (8mil).
• Jacketing material: 1.27 mm (50mil).
When running this cable, secure it at intervals of 4 ft or less, using straps, hangers, or similar fittings identified for the purpose [675.4(C)]. Use fittings at all points where the cable terminates [675.4(C)]. But be sure to use only fittings designed for use with the cable and that are suitable for the conditions of service.
Current ratings
As mentioned at the outset, these machines are special case motor applications. Unless the machine is for intermittent duty, use Art. 430 to determine the ratings of controllers, disconnecting means, and conductors [675.7].
So what is intermittent duty, and what are the implications for your calculations? Article 100 defines five types of duty, in sequence, starting with “Duty, Continuous.” Intermittent duty is a term that covers three types of alternating operations:
1. Load and no load.
2. Load and rest.
3. Load, no load, and rest.
Note that “Duty, Periodic” is one type of intermittent duty rather than being something other than intermittent duty. The windshield wiper control on an automobile typically uses this mode.
An intermittent irrigation system (IIS) is an alternative to continuous drip. It’s preferable for particular crops or conditions. For example, some crops need alternating cycles of wet and dry soil; continuous drip doesn’t work for them. With an IIS, the emitters (water nozzles) are larger than the continuous drip versions because they must deliver more water in a given time. The larger emitters mean the intermittent delivery doesn’t necessarily reduce the total amount of water delivered in a day.
Because there are mechanical dimension differences, changing between continuous and intermittent isn’t quick and painless. If it’s an intermittent system, you’re going to wire it as one. But how?
Elsewhere in the Code, we size conductors and overcurrent protection devices (OCPDs) at 125% of the continuous load. But with electric irrigation machines, we pretty much size them for 125% of the intermittent load [675.7(A)].
In doing this, we calculate what the NEC calls “equivalent current ratings.” These are the two current ratings (continuous-current and locked-rotor current) you use for all intermittent duty irrigation equipment. Where non-intermittent duty is involved, apply Article 430 instead.
To calculate the equivalent continuous-current rating [675.7(A)]:
1. Multiply the motor nameplate current rating of the largest motor by 125%.
2. Add to this value the sum of the motor nameplate current ratings of the other motors on the circuit.
3. Multiply the new sum by the maximum % duty cycle at which the motors can continuously operate.
To calculate the equivalent locked-rotor current [675.7(B)]:
1. Add the locked-rotor current rating of the two largest motors on the circuit.
2. Add to this value the sum of the motor nameplate current ratings of the other motors on the circuit.
Now, there’s an interesting twist to this. The title to Section 675.7, to which we’ve been referring, is “Equivalent Current Ratings.” The last Section of Article 675 is 675.22 and it has that same title. The difference is this last Section is for center pivot irrigation machines. And these are inherently intermittent-duty so you must calculate the equivalent current ratings.
To do this, you use the same steps as for other irrigation machines except:
• For the continuous-current rating, use 60% in step 2 and don’t perform step 3.
• For the locked rotor current, multiply the step 2 sum by 80%.
Disconnect
Irrigation machines include three kinds of disconnects:
1. Main controller disconnect [675.8(A)]. If the controller starts and stops the machine, that makes it a disconnect; it must be rated properly for that use. Perform the equivalent rating calculations to get the correct rating.
2. Main disconnect [675.8(B)]. It must provide overcurrent protection, be readily accessible, and be lockable. Its horsepower and current rating must be at least that of the controller disconnect. A common requirement for disconnects is that they are within sight of whatever they are disconnecting power to, but for irrigation machines that is one of two options. The other is to install it at the point of power connection to the machine.
3. Individual disconnect [675.8(C)]. Each motor and each controller must have its own disconnect. A common requirement for disconnects is that they are readily accessible; these disconnects don’t have to meet that requirement.
What if equipment within an enclosure has power from more than once source? If the additional source is 30V or less and complies with the requirements for
Class 2 and Class 3 power supplies (you’ll find those in Art. 725, Part III), you don’t need a disconnect for it [675.16].
Multiple motors
You can run multiple motors on the same circuit, if the circuit OCPD rating is 30A or less and each motor:
• Is 2 hp or less.
• Has a full-load rating of 6A or less.
• Has its own overload protection (e.g., heater strip).
Two final requirements are:
• Taps can be no longer than 25 ft.
• The tap conductor must be at least 14 AWG.
Collector rings
A key feature of an irrigation machine is the collector ring assembly. It’s an assembly of slip rings that transfer electricity from a stationary member to a rotating member [675.2]. This feature allows a single machine to have an irrigation pattern that’s impossible with stationary equipment alone. Common lawn sprinklers illustrate the advantages of rotation in water dispersion.
You must use this method to calculate collector ring control and/or signal current [675.11(B)]:
1. Multiply the full-load current rating of the largest device by 125%.
2. Add to this value the full-load current ratings of the other devices on the circuit.
You can also calculate collector ring transmitting current using that method, or you can use the equivalent current ratings discussed earlier [675.11(A)]. The same applies to determining the current rating of the collector ring used for grounding [675.11(C)]. That brings us to our next topic.
Grounding
Article 100 defines “grounding” as a connection to the earth. This connection facilitates lightning protection. If one of these machines has a stationary point, then the machine must be connected to a grounding electrode system [675.15]. The stated purpose is “for lightning protection.”
Where grounding is required, make sure each cord, cable, or raceway has an integral noncurrent-carrying equipment grounding conductor [675.13].
For the same reason you bond all metallic objects on a roof when installing a lightning protection system (prevent flashover), you bond various metallic objects (e.g., the machine frame), with a metallic path to the equipment grounding conductor [675.14].
Keeping it straight. The biggest challenge in meeting irrigation machine requirements is calculating the correct equivalent current rating. All of the calculations involve multiplying the largest motor current by 125% and adding the sum of the other motor ratings — but the calculation method varies according to the application. And there are other wrinkles.
To avoid making errors when doing electrical calculations, begin by characterizing the load. For irrigation machines, determine if the machine is continuous duty or intermittent duty — and whether it has a center pivot.
Lamendola is an electrical consultant located in Merriam, Kan. He can be reached at [email protected].
