370 Volt Capacitor: The Complete North American HVAC Standard Guide

Walk into any HVAC supply house in the United States or Canada and you’ll find row after row of run capacitors at two voltage ratings: 370V and 440V. Ask an experienced tech which one they stock on their service truck and most will say 440V. Ask them why equipment still ships from the factory with 370 volt capacitors and you’ll get a combination of answers ranging from cost to engineering history — and all of them are partially right.

The 370V rating traces directly to North American single-phase power supply characteristics. On a standard 240V residential or light-commercial supply, the theoretical peak voltage is 340V (240 × √2). Assuming a maximum voltage of 250 volts, the minimum capacitor rating should be 354 volts — so 370 volts provides adequate margin. Trouble Free Pool Equipment manufacturers adopted 370V as the minimum sensible rating that clears this peak under normal supply conditions, and it stuck as the cost-optimized standard for OEM equipment specification.

Understanding the full picture — where 370V works, where it doesn’t, and when to upgrade — is something every engineer or technician working on single-phase motor systems in North America needs to have straight. This guide covers all of it.

What a 370 Volt Capacitor Actually Does in an HVAC System

Before getting into specifications and selection, it’s worth being precise about the electrical function. A lot of confusion in the field comes from people treating the voltage rating as a functional parameter rather than an insulation specification.

A typical HVAC run capacitor is two long sheets of thin metal, insulated with a barrier of very thin plastic and immersed in oil to help dissipate heat. On a properly wired PSC (permanent split capacitor) motor, the start winding can only have current flow through it if the capacitor stores and discharges. The higher the MFD of the capacitor, the greater the stored energy and the greater the start winding amperage. ACHR News

The 370V rating has nothing to do with the function of phase-shifting the auxiliary winding current. The voltage rating on a capacitor is the voltage limit of the capacitor — exceeding it may cause the dielectric to break down and the capacitor to short out. A motor acts like a transformer, with the run winding acting as the primary and the start winding acting as the secondary. This is why the capacitor has a much higher voltage rating than the supply voltage. HVAC-Talk

That back-EMF (counter-electromotive force) generated by the motor windings is what pushes actual operating voltage across a run capacitor well above the supply voltage — which is why a 240V supply system requires a 370V or higher rated capacitor.

Single Run vs. Dual Run 370 Volt Capacitors

Single Run Capacitors

Single run capacitors serve one motor load — typically either the compressor or the condenser fan motor, but not both simultaneously. They are the standard configuration on older residential equipment, simpler commercial systems, and virtually all fan motors operating as standalone loads.

Most run capacitors are rated for 2.5–100 µF with voltage ratings of 370 or 440 VAC, are usually 50/60Hz rated, and come in round or oval case designs using either a steel or aluminum shell. Terminations are usually ¼” push-on style. TEMCo Industrial

Single run capacitors at 370V are found across a wide MFD range. Condenser fan motors typically need 5–15 µF, while compressor motors use anywhere from 20 to 80 µF depending on tonnage and motor design.

Dual Run Capacitors: The Modern Standard

The dual run capacitor became the dominant configuration in North American central air conditioning and heat pump equipment because it combines both the compressor run capacitor and the fan motor run capacitor in a single hermetically sealed unit. This simplifies the equipment layout and reduces the number of components in the control compartment.

A dual run capacitor has three terminals: HERM (hermetic compressor winding), FAN (condenser fan motor), and C (common). The two capacitance values printed on the label — for example, 35+5 MFD 370VAC — refer to the compressor section (35 µF) and the fan section (5 µF) respectively. These sections are electrically independent inside the same housing, sharing only the common terminal.

A typical run capacitor rating ranges from 2 µF to 80 µF and is rated at either 370 VAC or 440 VAC. Industrial Equipment News In dual configuration, the combined unit at 370V covers the majority of residential and light commercial HVAC equipment from about 1.5 tons through 5 tons cooling capacity.

370V Capacitor Specifications Reference Table

Capacitor Type	Typical MFD Range	Voltage	Terminals	Common Applications
Single Run (round)	2.5 – 80 µF	370 VAC	2	Fan motors, small compressors
Single Run (oval)	2.5 – 60 µF	370 VAC	2	OEM equipment, replacement
Dual Run (round)	15+5 to 60+10 µF	370 VAC	3 (HERM/FAN/C)	Residential split systems
Dual Run (oval)	25+5 to 50+7.5 µF	370 VAC	3 (HERM/FAN/C)	Heat pumps, package units

370 Volt vs. 440 Volt Capacitors: Getting the Substitution Rules Right

This is the most common source of confusion in field replacement work, and there is a definitive answer that every tech and engineer should know cold.

Many techs think they must replace a 370V capacitor with a 370V capacitor. The voltage rating displays the “not to exceed” rating, which means you can replace a 370V with a 440V but you cannot replace a 440V with a 370V. This misconception is so common that many capacitor manufacturers began stamping 440V capacitors with 370/440V just to eliminate the confusion. ACHR News

The voltage rating on a capacitor is its “breakdown voltage” — the maximum pressure it can withstand before the insulation inside fails and arcs. If your AC system runs at 370V and you install a 440V capacitor, the result is that the capacitor is underworked — it can handle 440V but only sees 370V. It will run cooler and likely last longer. This is an upgrade. If your system needs a 440V capacitor and you install a 370V one, the voltage spikes in the system exceed the capacitor’s limit, the dielectric punches through, and the capacitor fails or explodes. Specap, Inc.

The one consistent rule: MFD must match exactly. You cannot substitute the capacitance value — replacing a 35 µF with a 45 µF will cause the motor to draw more current than it’s designed for, shortening its life. Specap, Inc.

370V vs. 440V Substitution Rules Quick Reference

Situation	Acceptable?	Notes
Replace 370V with 370V (same MFD)	✅ Yes	Exact OEM match
Replace 370V with 440V (same MFD)	✅ Yes	Upgrade — runs cooler, lasts longer
Replace 440V with 370V (same MFD)	❌ No	Risk of dielectric failure
Replace 35 µF with 40 µF (same voltage)	❌ No	Wrong — MFD must match exactly
Replace 370V with 370/440V dual-rated	✅ Yes	Fully compatible — same component

Common 370 Volt Capacitor Applications in North American HVAC

Residential Central Air Conditioning

The compressor in a residential split system is almost always a single-phase hermetic compressor using a PSC (Permanent Split Capacitor) motor design. The run capacitor stays in circuit continuously during compressor operation, maintaining the phase relationship between the run winding and the start winding that keeps the motor running efficiently.

A failing run capacitor on a residential condenser doesn’t always cause immediate failure — the compressor may limp along, drawing higher current, running hotter, and wearing prematurely for weeks or months before it finally trips the overload or burns a winding. By the time a homeowner notices the system isn’t cooling, there may already be secondary damage building up.

Typical residential split system capacitor values at 370V run from 30+5 µF dual run for a 1.5–2 ton unit up to 45+7.5 µF or 50+5 µF for a 3–4 ton system. Five-ton residential systems often use 60+5 µF or require separate single-run capacitors.

Heat Pumps

Heat pump systems have the same capacitor requirements as central AC during cooling operation, but they add the complexity of heating-season operation in colder ambient temperatures. Cold weather reduces the viscosity of the dielectric oil inside run capacitors, and capacitance values can drift slightly in very cold conditions. For heat pumps in northern climates, the 370V/440V dual-rated capacitors are particularly common as they offer a slightly higher insulation margin during those high-demand cold-weather startup cycles.

Package Units and Commercial Rooftop Equipment

Commercial package units — the horizontal and vertical rooftop systems found on small commercial buildings — use 370V capacitors for both compressor and fan motor applications in smaller tonnage ranges (up to about 10 tons). Above that, three-phase motor designs that don’t require run capacitors become more common.

Condenser Fan Motors

Every single-phase condenser fan motor on a residential or light commercial system uses a run capacitor. These are almost always the smaller value in a dual run capacitor combination — typically 3, 5, 7.5, or 10 µF at 370V. The fan motor section is less demanding than the compressor section in terms of peak current, which is why these values are standardized at the lower end of the range.

How to Select and Size a 370 Volt Replacement Capacitor

Reading the Existing Capacitor Label

Before touching anything else on a service call, read the existing capacitor label completely. The four parameters you must record are: voltage rating, MFD value (or dual values), physical case type (round or oval), and terminal count. On a dual run capacitor the label will show something like 35+5 MFD 370VAC 50/60Hz with terminal markings HERM, FAN, and C.

Sizing Reference by System Type

System Type	Tonnage	Compressor MFD	Fan MFD	Voltage
Mini-split / ductless	0.75–1.5T	15–25 µF	2.5–5 µF	370V
Residential split AC	1.5–2.5T	25–35 µF	5–7.5 µF	370V
Residential split AC	2.5–4T	35–50 µF	5–10 µF	370V
Residential split AC	4–5T	45–70 µF	7.5–10 µF	370V
Light commercial RTU	5–10T	40–80 µF	5–15 µF	370V / 440V
Heat pump (all sizes)	Any	As above	As above	370V / 440V

The most important rule: always match the MFD value to the OEM specification. Never substitute a different capacitance value without verifying the motor manufacturer’s specifications, and never go lower on voltage rating.

UL 810: The North American Certification Standard

Available in versions meeting UL810 and IEC 60252-1 or IEC 60252-2 standards, quality AC film capacitors carry national and international third-party approvals including UL, VDE, CQC, ISI, and CE. TDK Electronics

UL 810 is the primary North American standard for capacitors connected to the power mains. For HVAC service work in the United States and Canada, always verify that replacement capacitors carry UL listing. CSA (Canadian Standards Association) certification is the equivalent requirement for Canadian installations. Components without these markings may not be acceptable for permitted HVAC work and may void equipment warranties or homeowner’s insurance.

Why 370 Volt Capacitors Fail: The Real Causes

Understanding failure modes saves time on service calls and helps you set realistic expectations with customers about replacement intervals.

Thermal stress is the single biggest killer of 370V run capacitors. The condenser compartment of a rooftop or outdoor unit in a hot climate can easily reach 70–80°C on a summer afternoon. Capacitor dielectric life roughly halves for every 10°C above the rated temperature. An OEM-spec 370V capacitor that’s fine at 55°C may survive only two or three summers of operation in a Phoenix, Arizona installation.

Back-EMF voltage stress is the other major factor. The potential measured between capacitor terminals on a properly operating 440V rated compressor capacitor can read 377–385V during normal operation MikeHolt — uncomfortably close to the 370V rated limit. In systems with supply voltage on the high end of tolerance (up to 252V on a nominally 240V circuit), the capacitor sees proportionally higher back-EMF.

Capacitance drift is how most run capacitors die gradually rather than suddenly. A capacitor that reads 28 µF instead of the rated 35 µF isn’t providing enough phase shift for the motor, causing higher winding current and heat even though the system appears to be running. Testing with a true RMS capacitance meter on every service call is best practice.

Useful Resources for 370 Volt Capacitor Selection and Service

Standards and Technical References

• UL 810 — Capacitors standard for North American AC power applications: ul.com/standards
• EIA-456-A — Electronic Industries Alliance motor run capacitor reliability standard
• ACHR News Capacitor Facts Guide: achrnews.com

Manufacturer and Distributor Technical Resources

• Genteq (Regal Beloit) Capacitor Catalog — Major OEM supplier to Trane, Carrier, Lennox: regalbeloit.com
• TemcoIndustrial Motor Capacitor FAQ: temcoindustrial.com
• Specap 370V vs 440V Replacement Guide: specap.com
• ASC Capacitors Technical Series: ascapacitor.com

Component Search and Databases

• Digi-Key Parametric Capacitor Search (filter 370VAC, film type): digikey.com
• Mouser Electronics Run Capacitor Selection: mouser.com
• Johnstone Supply HVAC Parts (trade distributor with UL-listed stock): johnstonesupply.com
• Grainger Industrial Run Capacitors: grainger.com
• North America HVAC Capacitor Database: northamericahvac.com

Frequently Asked Questions About 370 Volt Capacitors

Can I use a 370/440V dual-rated capacitor in place of a plain 370V unit?

Yes, completely. 440V capacitors are considered upgrades for 370V capacitors North America HVAC, and the 370/440V dual-stamping that many manufacturers now use is simply acknowledging this compatibility explicitly. The component is built to 440V specification and will operate at or below 370V with additional margin. Same MFD value, no other changes required.

Why does my system’s capacitor read a higher voltage than the 370V rating during operation?

This is normal and expected. We often see higher voltage across a capacitor than the line voltage, but this is due to the back-EMF generated by the motor, not the capacitor itself. The motor acts inductively, and the start winding can develop voltages considerably above supply voltage. ACHR News Measuring 340–370V across a capacitor in a 240V system during operation is completely normal — which is exactly why the 370V rating is the minimum acceptable for these applications.

How do I test if my 370 volt capacitor is failing?

The most reliable method is a true RMS capacitance meter. Measure the actual µF value and compare it to the rated value. You can also calculate capacitance by measuring the current in amps flowing through the motor start winding, multiplying by 2652 (for 60Hz), and dividing by the voltage measured across the capacitor terminals. ACHR News A reading more than 10% below the rated value indicates a weak capacitor that needs replacement. Visible signs of failure include a bulged or deformed top on the capacitor (the pressure relief vent has activated), oil leakage, or a burnt smell.

Is there a performance difference between a 370V and 440V capacitor at the same MFD?

Electrically, they perform identically in a 370V application — the phase shift provided to the motor winding is determined entirely by the MFD value, not the voltage rating. A 440V capacitor installed in a 370V application runs cooler because it is operating well below its insulation limits, which translates to longer service life. Specap, Inc. The practical difference is longevity: in hot climates or high-duty-cycle applications, a 440V unit installed where a 370V is specified will typically outlast the original by a meaningful margin.

What happens if I install the wrong MFD value in a 370 volt capacitor slot?

If the MFD value is too high, the motor or compressor will draw more current than it normally would and its life will be shortened. The higher the cap value above spec, the worse the effect — too high a value effectively emulates a hard start cap that doesn’t switch out and stays in circuit. HVAC-Talk An undersized MFD reduces starting torque, causes the motor to run at elevated temperature, and in worst cases prevents startup entirely under load. Always match the OEM MFD value exactly — voltage is the one parameter where you have substitution flexibility, capacitance is not.