I have the 2005 NEC here (it's the one I was tested to on my P.E. in 2008), some of the language changed in the 2008 and 2011 but they don't change fundamentally. You want a floating ground path unless the system you are connecting to is earth grounded, then you have to reference the generator to earth.
Don't ground.
I don't have the NEC to quote from, so I won't (should be in section 250). Basically, it's safer to not run a separate ground on portable gensets. I know, I used to do it too.
If I can find the specific reference tomorrow at work, I'll post it up.
Good luck.
250.34 Generators-Portable and Vehicle-Mounted
(A) Portable Generators. The frame of a portable generator is not be required to be grounded to the earth if:
(1) The generator only supplies equipment or cord-and-plug-connected equipment through receptacles mounted on the generator, or both, and
(2) The metal parts of generator and the grounding terminals of the receptacles are bonded to the generator frame.
(B) Vehicle-Mounted Generators. The frame of a portable generator is not required to be grounded to the earth if:
(1) The generator frame is bonded to the vehicle frame.
(2) The generator only supplies equipment or cord-and-plug-connected equipment through receptacles mounted on the generator, or both, and
(3) The metal parts of generator and the grounding terminals of the receptacles are bonded to the generator frame.
(C) Grounded Conductor Bonding. A system conductor that is required to be grounded by 250.26 shall be bonded to the generator frame where the generator is a component of a separately derived system.
FPN: For grounding portable generators supplying fixed wiring systems, see 250.20(D).
250.20 Alternating-Current Circuits and Systems
System grounding is the intentional connection of one conductor of an alternating-current system to the earth so as to limit elevated voltage on conductors from high voltage surges imposed by lightning, line surges, or unintentional contact with higher voltage lines and to stabilize the phase-to-ground voltage during normal operation [250.4(A)(1)].
(A) AC Systems of Less Than 50 V. Alternating-current systems that operate at less than 50 V are not required to be grounded unless:
(1) The primary exceeds 150 V to ground
(2) The primary is ungrounded
(B) AC Systems Not Over 600 V. Alternating current systems of the following types must have the neutral (X0) terminal of the power supply bonded to a suitable grounding electrode (earth).
Single-phase, 2- or 3-wire, 120 V or 120/240 V system
3-phase, 4-wire, 208Y/120 V or 480Y/277V wye-connected system
3-phase, 4-wire, 120/240 V delta-connected system (high-leg)
(D) Separately Derived Systems. Separately derived systems, which are required to be grounded by 250.20(A) or (B), must be grounded in accordance with the requirements of 250.30.
250.30. Grounding Separately Derived Systems
(A) Grounded Systems. Separately derived systems that operate at over 50 volts [250.20(A) and 250.112(I)] must be grounded to an effective fault current path to ensure that dangerous voltage, which can create a shock and/or fires from ground-faults, will not remain [250.2(A)(3)]. In addition, separately derived systems must be grounded to the earth to stabilizing the voltage during normal operation [250.2(A)(1)].
(1) Grounding - Effective Fault Current Path. To provide the low impedance path necessary to clear a ground-fault (line-to-ground fault), the metal parts of the separately derived system (equipment grounding conductor) must be bonded to the system grounded conductor (X0 Terminal). The bonding jumper used for this purpose must be sized in accordance with Table 250.66, based on the total area of the largest ungrounded (hot) conductor.
The neutral-to-case bond can be made at the source of a separately derived system or to the first system disconnecting means or overcurrent device, or it shall be made at the source of a separately derived system that has no disconnecting means or overcurrent devices.
(2) Grounding - To the Earth. A grounding electrode conductor, that connects the separately derived system grounded (neutral) conductor to a suitable grounding electrode [250.30(A)(4)], shall be installed in accordance with 250.30(A)(2)(a) or (b). Grounding electrode conductor taps as permitted by 250.30(A)(2)(b), shall be installed in accordance with the requirements contained in 250.30(A)(3).
Exception: The size of the bonding jumper for a system that supplies a Class 1 circuit from a transformer rated not more than 1000 volt-amperes, shall not be smaller than the derived phase conductors and shall not be smaller than 14 AWG copper.
(a) Single Separately Derived System. A grounding electrode conductor for a single separately derived system must be sized in accordance with 250.66, based on the total area of the largest ungrounded (hot) conductor. This conductor shall connect the grounded conductor of the derived system to the grounding electrode as specified in 250.30(A)(4). The grounding electrode conductor must terminate at the same point on the separately derived system where the neutral-to-case bonding jumper is installed [250.30(A)(1)].
Exception: A grounding electrode conductor is required for a system that supplies a Class 1 circuit from a transformer rated not more than 1000 volt-amperes. However, the system grounded conductor must be bonded to the transformer frame or enclosure in accordance with 250.30(A)(1).
(b) Multiple Separately Derived Systems. Where more than one separately derived system is connected to a common grounding electrode conductor as provided in 250.30(A)(3), the common grounding electrode conductor shall be sized in accordance with Table 250.66 based on the total circular mil area of the derived phase conductor from all separately derived systems.
(3) Grounding Electrode Taps. Grounding electrode taps from a separately derived system to a common grounding electrode conductor must connect the grounded conductor of the separately derived system to the common grounding electrode conductor and must be installed in accordance with (a) through (d) below.
(a) Tap Conductor Size. Each tap conductor must be sized in accordance with 250.66 for the derived phase conductors of the separately derived system it serves.
(b) Connections. All connections must be made at an accessible location by an irreversible compression connector listed for the purpose, listed connections to copper busbars not less than 1/4 in. x 2 in., or by the exothermic welding process. Tap conductors must be connected to the common grounding electrode conductor as specified in 250.30(A)(2)(b) in such a manner that the common grounding electrode conductor is not spliced.
(c) Installation. The common grounding electrode conductor and the taps to each separately derived system must comply with 250.64(A), (B), (C) and (E).
(d) Bonding. Exposed structural steel that forms the building frame or the interior metal piping in the area served by the separately derived system must be bonded to the grounding electrode conductor in accordance with 250.104(A)(4).
(4) Grounding Electrode. The grounding electrode conductor must terminate to a grounding electrode that is located as close as practicable, and preferably in the same area of the neutral-to-ground termination of the derived system. The grounding electrode shall be the nearest one of the following:
(1) Effectively grounded metal member of the building structure.
(2) Effectively grounded metal water pipe, within 5 ft from the point of entrance into the building.
Exception: The grounding electrode conductor can terminate any point on the water pipe system for industrial and commercial buildings where, (1) conditions of maintenance and supervision ensure that only qualified persons service the installation, and (2) the entire length of the interior metal water pipe that is being used for the grounding electrode is exposed.
Where effectively grounded metal member of the building structure or effectively grounded metal water pipe is not available, then one of the following electrodes must be used:
The metal frame of the building or structure, where effectively grounded, see 250.52(A)(2).
An electrode encased by at least 2 in. of concrete, located within and near the bottom of a concrete foundation or footing that is in direct contact with the earth, consisting of at least 20 ft of one or more bare or zinc galvanized or other electrically conductive coated steel reinforcing bars or rods of not less than ½ in. in diameter, or consisting of at least 20 ft of bare copper conductor not smaller than 4 AWG, see 250.52(A)(3).
A ground ring encircling the building or structure, in direct contact with the earth, consisting of at least 20 ft of bare copper conductor not smaller than 2 AWG, see 250.52(A)(4).
Rod and pipe electrodes not less than 8 ft in length, see 250.52(A)(5).
Plate electrode expose not less than 2 ft2 of surface to exterior soil, see 250.52(A)(6)
Other local metal underground systems or structures such as piping systems and underground tanks, see 250.52(A)(7).
FPN: Interior metal water piping in the area served by a separately derived system must be bonded to the grounded (neutral) conductor at the separately derived system in accordance with the requirements of 250.104(A)(4).
(5) Equipment Bonding Jumper Size. Where an equipment bonding jumper is run with the derived phase conductors from the source of a separately derived system to the first disconnecting means, it shall be sized in accordance with Table 250.66, based on the total area of the largest derived ungrounded (hot) conductors.
(6) Grounded (neutral) Conductor. Where a grounded (neutral) conductor is installed and the neutral-to-case bond is not located at the source of the separately derived system, the following must apply:
(a) Routing and Sizing. The grounded (neutral) conductor must be routed with the secondary conductors, and it cannot be smaller than the required grounding electrode conductor as specified in Table 250.66 based on the largest ungrounded supply conductor, but it is not required to be larger than the ungrounded derived phase conductor. For secondary phase conductors larger than 1100 kcmil copper or 1750 kcmil aluminum, the grounded (neutral) conductor is not permitted to be smaller than 121/2 percent of the area of the largest derived phase conductor.
(b) Parallel Conductors. If the secondary conductors are in parallel, the grounded (neutral) secondary conductor must be sized based on the total circular mil area of all of the parallel conductors per phase. Where installed in two or more raceways, the size of the grounded (neutral) conductor in each raceway must be, based on the total area of the largest derived ungrounded (hot) conductors, but in no case smaller than 1/0 AWG are required by 310.4.