Paralleling diesel banks — two engines, one bus, one show.

Sets, paired

The shop standard pair is two Multiquip DCA-150 diesel-towables, each rated 113 kW continuous at 0.8 PF, with John Deere 4045HF285 engines and Mecc Alte ECP32-2L alternators. Both sets carry a Woodward EGM-1100 governor and an SR7 voltage regulator from the factory; we replace the SR7 with a DECS-150 from Basler Electric on every paralleling-eligible set because the SR7’s VAR-share droop is too coarse for a synchronous parallel.

When the load demands more than 226 kW continuous, we add the Cat XQ230 as the third leg. Cat’s 230 kVA prime-rated set comes from the factory with paralleling switchgear available; we use a portable Woodward easYgen-3500 with paralleling breakers brought in from a separate Pelican case so the same crew can swap between two-set and three-set configurations. The XQ230 has a sound-attenuated enclosure that reads 71 dB at 23 ft per the spec — meaningful at fairground events with audience proximity to the gen bank.

The easYgen, in plain English

Paralleling is a control-system problem more than a wiring problem. Two engines on a common bus must agree on three things: frequency, voltage magnitude, and voltage phase angle. The Woodward easYgen-3500 is the brain that makes them agree.

The easYgen reads frequency on each set, looks at the bus frequency, and adjusts the governor through a 4-20 mA output until the two match within a 0.2 Hz tolerance. It does the same with voltage magnitude through the DECS-150 voltage regulator. Finally, it monitors phase angle on the synchronizer page — a vector display showing the live machine’s angle relative to the bus. When the angle is within ±5° and closing toward zero (the machine is sweeping into phase rather than away from it), the easYgen closes the synchronizing breaker. The whole sequence runs in 30 to 90 seconds depending on how cold the second set is at startup.

Once the sets are paralleled, the controller transitions to load-share mode. kW share is set on the engine governors via a droop curve — every 1 Hz of governor droop equals 100% of the machine’s rated load. We set both sets to 3% droop, which means the load shares within 1% of nameplate when both sets are at thermal equilibrium. kVAR share uses a similar droop on the voltage regulators, 3% as well. The easYgen reads the share imbalance and trims the governor command up or down to compensate — the result is two engines that run hot together, neither one carrying more than 55% of nameplate on a typical festival load.

Grounding — switched neutral, single bond

The most consequential decision in a paralleled bank is where the neutral-to-ground bond lives. NEC 250.30 governs separately derived systems; IEEE 142 (the Green Book) walks the engineering. We bond the neutral to ground at the ATS, not at either set. The neutral is switched at the ATS so that during paralleling, only one bond exists — the gen bank’s combined neutral comes through the ATS to the grounding electrode at the venue’s ground rod array.

Each Multiquip set leaves the factory with a neutral-to-ground bond inside the cabinet; we cut that bond before the set enters paralleling service. (We do not modify the factory wiring without recording the change on the set’s service binder and stamping a NEUTRAL BOND REMOVED placard on the cabinet door per NFPA 110. We restore the bond when the set returns to non-paralleled service.) The Cat XQ230 has a factory-installed switchable bond; we throw the switch to remove for parallel and restore for stand-alone.

Failure to remove a parallel bond results in circulating ground-currents that can cause nuisance trips on the ATS, false ground-fault alarms on the easYgen, and in extreme cases, neutral-conductor heating sufficient to set off thermal protection. We have seen it once, in 2020, on a job we inherited — a previous shop had not removed the bond on the second set, and the ATS tripped on ground-fault during the first cue change. We caught it on the walkthrough; the show went without further trouble. The set went back to the previous shop with a note.

Load shed and the cue list

The easYgen runs a load-shed program against the show’s cue list. We program four shed tiers:

1. Tier A — life safety: egress lighting, exit signs, PA announcement, dressing-room lights. Never shed.
2. Tier B — primary show: console feeds, FOH amplifier racks, video switcher, primary lighting truss. Shed only if both sets fail.
3. Tier C — secondary show: backup truss, secondary video walls, audio iso bus, hospitality. Shed if one set fails and we drop below 75% of nameplate.
4. Tier D — utility & convenience: backstage Edison receptacles, dressing-room amenities, video-village ancillaries. Shed first if any set wobbles.

The cue list goes into the easYgen as a sequence of bus-shed contacts; when the genset hits 90% load on combined nameplate, Tier D drops; at 95%, Tier C drops; at 100%, the controller alarms and dispatch is called. We have shed Tier D twice in nine years: once at CMA Fest 2023 when a third stage’s lighting truss was activated without notice, and once at Bonnaroo Day-Stage Loadout Support 2024 when an unannounced trade-vendor truck plugged into a backstage Edison strip and pulled 90 A from a 60 A circuit.

Fuel discipline

Diesel generators run out of fuel. The DCA-150 has a 110-gallon main tank that gives 12 hours runtime at 75% load; the Cat XQ230 has a 132-gallon tank that gives 14 hours at 75%. We carry a subbase auxiliary tank that adds 280 gallons per set and a fuel polishing pump (Algae-X Tank Cleaner) we run during runs longer than 36 hours. Fuel quality matters more than fuel quantity in summer; ULSD with stale-fuel additive in a tank that has sat fueled for 90 days is the most common cause of a set running rough.

Fuel is delivered by Mansfield Energy on the day of load-in for tours that run beyond a single shift. We require a copy of the BOL with sulfur content (15 ppm max for ULSD) and a fuel sample drawn before each fill; the sample sits in a labeled jar in the truck and is dated. We have rejected one fuel delivery in nine years.

Lead time, price, and the way to book

Lead time on a paralleled bank is five working days for the standard two-set DCA-150 configuration; eight working days if the Cat XQ230 is on the load list (it lives in a separate yard and must be delivered with the operator). Price banding starts at $3,800 for a single-event two-set parallel and runs to $24,000 for a multi-day, three-set, with full fuel logistics included.

For booking and the advance form, see /advance/. The walkthrough at the site with the AHJ and the load survey is included; we will not parallel a bank without the walkthrough.