Sc18g Danfoss Compressor Wiring Diagram

Understanding the Sc18g Danfoss Compressor Wiring Diagram

The SC18G is one of Danfoss's scroll compressors, part of their commercial refrigeration lineup. Getting the wiring right matters because these units run on single-phase or three-phase power depending on the model variation, and getting the connections wrong can damage the startup components or trip protection circuits. The wiring diagram for this compressor isn't particularly complicated, but there are enough specifics that guessing won't work. Before touching any wires, you need the correct diagram for your specific variant. Danfoss uses suffix letters and numbers after the SC18G designation that indicate voltage, phase, and oil type. A SC18G-0452 runs differently from a SC18G-5451, even though the terminal layout looks the same. The terminal block on top of the compressor has three main terminals: Common (C), Run (R), and Start (S). Between R and S you'll measure the highest resistance. Between C and R you'll get medium resistance. Between C and S you'll get the lowest reading. This is standard across almost all Danfoss scroll compressors and it holds true regardless of voltage rating. I spent an afternoon on a job last year where a technician had replaced a SC18G compressor but wired it as if it were a three-phase unit when the nameplate clearly said 230/1/60. The start relay was humming, the capacitor was warm to the touch, and the compressor wasn't pulling any amperage. The resistance readings between the terminals checked out fine, which should have been the first clue that something was off with the external circuit, not the compressor itself. Once I traced the wiring back to the relay board, I found the common and run leads had been swapped relative to the diagram on the unit's junction box cover. It took about twenty minutes to reterminate everything correctly and the compressor pulled normal running amps right away.

What the Wiring Actually Looks Like

The terminal configuration on the SC18G follows a flat arrangement with three studs on the top plate. Terminal C is Common, terminal R is Run, and terminal S is Start. The internal windings connect these three points, and your external components attach here. You will typically need a start relay, a startup capacitor in most single-phase applications, and an overload protector. The overload is usually wired in series with the common terminal and breaks power to the entire compressor if current gets too high or the windings overheat. For three-phase versions, you don't need a start relay or capacitor at all. Three-phase power provides the rotating magnetic field automatically, so you only connect L1, L2, and L3 to terminals R, S, and C respectively. Phase rotation doesn't matter on scroll compressors the way it does on some other motor types, but you should still verify that all three phases are present and balanced before energizing the unit. I've seen cases where one phase was dropping to around 180 volts while the other two stayed near 230, and the compressor would run but draw excessive current and trip the overload within minutes. Checking voltage at the terminals under load catches problems like that faster than anything else. The internal thermal overload on the SC18G is a dual-purpose device. It monitors both winding temperature and current. If either gets too high, it opens the circuit to the common lead and de-energizes the compressor. This is not user-serviceable. When it trips and stays tripped after cooldown, the usual suspects are a bad start component, a failed run capacitor, a blocked refrigerant circuit causing high head pressure, or a failing windings situation. The resistance test between terminals is your first diagnostic step, but it only tells you so much. A compressor can show good resistance across all three pairs and still have an internal short that only manifests under load.

Wiring Steps and Common Mistakes

Start by confirming the voltage rating on the nameplate. Then verify your power supply matches that rating. A 230-volt compressor on 460-volt supply will fail fast. A 460-volt compressor on 230 volts will pull excessive current and trip protection. Both scenarios are common in retrofit situations where someone pulls a compressor from a different system without checking the label. Connect the overload in series with the common terminal. The overload has two prongs that plug directly into the terminal studs on the compressor. Most Danfoss compressors use a proprietary connector that snaps on. If yours doesn't have one, you'll need to use spade connectors rated for the current, but factory connectors are more reliable because they maintain proper contact pressure and don't loosen from vibration over time. I usually inspect the terminal studs for corrosion before installing anything. A green or white residue on the copper means moisture got in, and cleaning it with contact cleaner and a brass brush is worth five minutes of your time. Dirty terminals cause voltage drop and heat buildup that mimics electrical faults. For single-phase wiring, the run capacitor connects between the start and run terminals. The start relay switches power to the start terminal only during startup and then drops out once the compressor reaches speed. On permanent split-capacitor designs, the capacitor stays in circuit during operation. Danfoss compressors are typically designed for a start-capacitor configuration, so confirm which approach your system uses before jumping to conclusions. I ran into a situation where a tech wired a PSC setup on a compressor that required a start relay because the system manual showed a PSC diagram from a different Danfoss model family. The compressor ran but sounded abnormal and the capacitor heated up beyond its rating within an hour. The correct fix was swapping to a start relay with the proper capacitor value and disconnecting the run capacitor entirely.

Where to Find the Diagram

Danfoss publishes wiring diagrams in their official documentation. The SC18G series documentation is available through the Danfoss website under the compressor product section. You can also find it in the FC-Refrigeration catalog or the online technical database at danfoss.com. Some HVAC suppliers carry printed wiring guides for popular Danfoss models. If you're working on a specific unit, the diagram is often printed on a tag attached to the compressor housing or on the system's electrical panel. Always cross-reference the tag number with the official documentation rather than relying solely on field markings, which can get mislabeled during previous service visits. The most common mistake I see is assuming the terminal layout is universal. It isn't. While most Danfoss scroll compressors follow the same C-R-S pattern, some variants have different terminal configurations or include additional pins for sensors and auxiliary connections. Check the terminal diagram on the unit before assuming standard wiring applies. Another frequent issue is using the wrong overload rating. The SC18G draws different full-load amps depending on the model and operating conditions. Installing an overload rated too low causes nuisance tripping. Installing one rated too high removes proper protection. The factory-specified overload for most SC18G variants is in the 12 to 18 amp range, but you should verify this against the specific model's data sheet. Capacitor selection also causes problems. The start capacitor on a SC18G is typically in the 15 to 30 microfarad range for single-phase models, but the exact value depends on the horsepower and voltage rating. Using a capacitor that's too large increases start torque but stresses the windings. Using one that's too small causes slow starts and high current draw. The run capacitor, if your system uses one, should match the manufacturer's specification exactly. Mismatched capacitors are harder to diagnose than most people expect because the compressor appears to run normally during a brief test but degrades over hours of operation.

If you need a copy of the Sc18g Danfoss Compressor Wiring Diagram, the most reliable source is the Danfoss technical documentation portal. Search for your exact model number including the full suffix code. The diagram will be included in the product datasheet or the installation and service manual. Some third-party HVAC supply websites also host these diagrams, but verify they match your specific model variant before using them as a reference.