[bxk730]

Applicable code is 2011 ICC.

Trying to correctly set up a portable generator inlet box (4 wire twistlock) and run almost 20 ft through a finished garage to the transfer switch (which is mounted next to the main service panel). The intended genset is 7kw, 110/220v, and has integrated 30A breakers.

Q1: Must I use SE for the run from inlet box to xfer switch, or is 10-3 NM acceptable?

Q2: If I enclose a portion of this run in conduit for mechanical protection, do I size conduit like pulling THHN, or did I miss a derating factor?

Q3: What is the proper grounding arrangement? (I got conflicting advice locally) The generator's frame is bonded to neutral. The manufacturer does not require (but does recommend) a ground rod be used.

Q4: Is labeling for multiple power sources required on the main panel, and if so, may it be inside the access door with the manufacturer's label? The xfer switch powered circuits will pass through the enclosure, with wirenutted connections.

Thank you!!

[tkrussell]

Good questions.

Answers as follows:

#1. NM cable is fine, assuming you have the typical transfer switch that does not switch the neutral. If the neutral is switched, then NM cable is not allowed, as the genset would be considered as a separately derived system.

#2. Referring to Table 1 in Chapter 9, one cable shall be considered as 1 conductor in the conduit, and is allowed to fill the conduit up to 53%. You will need to find the cross sectional area of the cable, and find a conduit size that will only be filled 53%.

Checking the specs of a popular brand of #10-3 NM cable, I find the diameter is .442 in, and will have a cross sectional area of .14 in sq. Checking Table 4 of Chapter 9 for Sch 40 PVC, I find that 1/2 In conduit can be filled with a cable up to .151 In Sq, so 1/2 in PVC is the minimum size conduit for a particular brand #10-3 NM cable.

Keep in mind this is minimum, and need to consider the cable is solid wire, and consider friction of plastic against plastic.

For a conduit sleeve for physical protection of a #10-3 NM cable, I would not use a conduit smaller than 3/4 or even 1 inch, depending on length of conduit and amount of bends in the conduit. #10-3 NM cable will not pull easy through conduit.

#3. Grounding the genset with a rod is only required by NEC if the manufacture requires it. From the genset on, since it has a four wire receptacle, the neutral and equipment ground is separated and treated so through the transfer switch and each the neutral and equipment ground conductor simply connects at the panel's neutral and equipment ground bar.

#4. I cannot find a specific code to address this at the moment, but considering that,assuming your using something similar to a GenTran transfer switch that only powers a few circuits, it is conceivable that both the generator and when the utility power is restored, the panel can have both power sources in the panel, and the panel should be labeled that there are two power sources available in the panel.