Electrical Isolator Size Calculator

The Electrical Isolator Size Calculator calculates the correct isolator rating for electrical circuits using load, supply voltage, fault level, and installation conditions.

About the Electrical Isolator Size Calculator

An electrical isolator, also called a disconnector, is a switch used to de-energize a circuit for maintenance. A load-break switch-disconnector can also switch currents under load; its duty is defined by utilization categories such as AC-22A or AC-23A. Selecting the right size is essential so the device carries normal current, withstands short-circuit stresses, and meets code.

This calculator focuses on practical sizing for construction projects. It converts your load details into a recommended isolator current rating and duty category. It also checks short-circuit withstand needs and prompts for pole count, enclosure impacts, and ambient temperature derating. The result is a defensible estimate that helps control materials and avoid oversizing wastage.

You still need to confirm your selection against manufacturer data and local standards. The tool points you to the right range and category, then you match an exact catalog number.

How the Electrical Isolator Size Method Works

The method starts by calculating the load current from known power, voltage, and power factor. It then applies factors for duty (continuous or not), ambient temperature, and utilization category. Finally, it verifies whether the device’s short-time withstand suits the site fault level.

• Find full-load current from power and voltage (single-phase or three-phase).
• Apply a continuous-load factor where required by code or design policy.
• Correct for ambient temperature and enclosure effects using derating factors.
• Choose a utilization category that matches the load type and switching duty.
• Check short-circuit parameters: short-time withstand current and making capacity.

The output is a current rating and category, plus notes on poles, neutral switching, and fault withstand. That information narrows your product shortlist and supports a compliant installation.

Equations Used by the Electrical Isolator Size Calculator

The calculator uses standard electrical relationships and simple derating logic. It does not replace the manufacturer’s performance curves, but it sets a safe baseline for device selection.

• Single-phase current: I = P / (V × PF × η), where P is power, PF is power factor, and η is efficiency.
• Three-phase current: I = P / (√3 × V × PF × η). For kW, convert to watts before using.
• Continuous-load adjustment: Icont = I × 1.25 for loads ≥ 3 hours (typical NEC practice; confirm your code).
• Temperature/enclosure derating: Inom ≥ Iadjusted / Kt, where Kt is the manufacturer’s derating factor.
• Fault current from fault level: If using MVA, Ik = (MVA × 10^6) / (√3 × V). If fault level is in kA, use that directly.
• Short-time withstand check: Icw(device) at the specified time (often 1 s) ≥ Ik at the installation point.

These relationships size current capacity first, then confirm fault withstand. The utilization category (for example, AC-22A vs AC-23A) refines the choice based on how “tough” the switching duty is.

Inputs, Assumptions & Parameters

Provide realistic data so the estimate aligns with site conditions. Where you do not know an exact value, enter a conservative figure and note it for follow-up.

• System voltage and phase: single-phase or three-phase nominal voltage.
• Load power, power factor, and efficiency: kW (or kVA), PF, and η used to compute current.
• Duty: continuous (≥ 3 hours) or non-continuous; pick utilization category (AC-20A/22A/23A or DC rating).
• Ambient temperature and enclosure: affects Kt derating; higher temperature or tighter enclosure reduces rating.
• Fault level at the point of installation: in kA or MVA, for Icw/Icm checks.
• Poles and earthing: 2P/3P/4P and whether neutral switching is required.

The calculator flags edge cases, such as very low PF, high altitude, or harmonics. In those cases, it advises higher margins or manufacturer consultation. Enter ranges if unsure, and select the highest resulting size to minimize rework and materials wastage.

Using the Electrical Isolator Size Calculator: A Walkthrough

Here’s a concise overview before we dive into the key points:

1. Select AC or DC, then choose single-phase or three-phase.
2. Enter voltage, load power (kW or kVA), power factor, and efficiency.
3. Indicate whether the load is continuous and select a utilization category that suits the load type.
4. Provide ambient temperature and enclosure type so the tool can apply derating.
5. Enter the site fault level (kA or MVA) at the isolator’s location.
6. Choose required pole count and whether the neutral must be switched.

These points provide quick orientation—use them alongside the full explanations in this page.

Worked Examples

Three-phase motor, 30 kW at 400 V, PF 0.86, efficiency 0.92, continuous duty, ambient 50 °C with Kt = 0.90, fault level 25 kA. Current I = 30,000 W / (√3 × 400 V × 0.86 × 0.92) ≈ 54.7 A. Continuous-load factor: 54.7 A × 1.25 ≈ 68.4 A. Temperature derating: Inom ≥ 68.4 A / 0.90 ≈ 76.0 A. Choose standard size ≥ 80 A, utilization category AC-23A, 3P or 4P as required. Verify Icw ≥ 25 kA for the specified duration or pair with coordinated fuses. What this means: Select an 80 A AC-23A switch-disconnector with adequate short-circuit coordination for a robust installation.

Single-phase HVAC unit, 6 kW at 230 V, PF 0.95, efficiency 0.97, non-continuous duty, ambient 35 °C with Kt = 0.95, fault level 6 kA. Current I = 6,000 W / (230 V × 0.95 × 0.97) ≈ 28.3 A. No continuous factor applied. Temperature derating: Inom ≥ 28.3 A / 0.95 ≈ 29.8 A. Choose standard size ≥ 32 A, utilization category AC-22A, 2-pole to switch line and neutral if required by local practice. Check Icw ≥ 6 kA or use an associated protective device. What this means: A 32 A AC-22A isolator suits this load with reasonable margin and proper coordination.

Assumptions, Caveats & Edge Cases

This tool guides initial selection. Always confirm with device datasheets and local codes such as IEC 60947-3, IEC 60269, or NEC/CEC rules. Fault withstand depends on protective device clearing time, not just current magnitude.

• Variable frequency drives can increase peak and harmonic currents; use AC-23A or consult the drive vendor.
• Photovoltaic and battery systems require DC-rated isolators with polarity, arc, and voltage-specific ratings.
• High altitude and high ambient reduce ratings; apply both altitude and temperature derating if applicable.
• Neutral switching may be mandatory in certain systems; verify earthing and code requirements.
• Isolators are not overcurrent protective devices; pair with breakers or fuses for fault interruption.

If inputs are uncertain, use conservative assumptions and re-check during detailed design. Building teams can update the estimate later, avoiding rework and minimizing materials wastage.

Units Reference

Units are central to correct sizing. A mismatch between kW and kVA, or between kA and MVA, can skew results. Use consistent units and convert where necessary before comparing with catalog data.

Read the table left to right. If your site gives fault level in MVA, convert to kA before comparing with a device’s kA withstand rating using the equation above.

Common Issues & Fixes

Most sizing issues stem from incomplete inputs or confusing category definitions. Another frequent trap is ignoring the effect of enclosure heating on derating.

• Problem: Isolator trips or overheats. Fix: Increase rating or improve enclosure ventilation; verify Kt.
• Problem: Device damaged on fault. Fix: Select higher Icw or coordinate with fuses/breakers for faster clearing.
• Problem: Motor stalling on switching. Fix: Use AC-23A category suited to motor loads.
• Problem: Neutral not isolated where required. Fix: Choose 2P/4P models per system earthing rules.

Document your assumptions. When in doubt, select the next standard size and confirm with the manufacturer’s application notes.

FAQ about Electrical Isolator Size Calculator

What is the difference between an isolator and a load-break switch?

An isolator (AC-20A) is for off-load disconnection, while a switch-disconnector (AC-22A/AC-23A) is designed to open and close current under load. The calculator supports both, prompting you to choose the correct utilization category.

How do I handle continuous loads in sizing?

For loads operating three hours or more, apply a 125% factor unless your standard states otherwise. The tool includes this step and folds it into the nominal rating after derating.

Can I use the same approach for DC systems?

Yes, but only DC-rated isolators should be used on DC circuits. Provide DC voltage, current, and duty. Pay attention to polarity, arc suppression, and the device’s DC utilization rating.

Do I still need fuses or circuit breakers?

Yes. Isolators do not provide overcurrent protection. Use fuses or breakers to interrupt faults and to limit energy so the isolator’s short-time withstand is not exceeded.

Key Terms in Electrical Isolator Size

Utilization category

A duty rating that defines the kind of load a switch can make or break, such as AC-22A for mixed loads or AC-23A for motor loads.

Short-time withstand current (Icw)

The RMS current a device can endure for a specified time, often one second, without damage during a short circuit.

Making capacity (Icm)

The peak current a switch can close onto at the instant of a fault, typically expressed as a high multiple of Icw.

Continuous load

A load expected to run for three hours or more. Many codes require sizing at 125% of calculated current for these circuits.

Derating factor (Kt)

A multiplier less than one that reduces the usable rating to account for ambient temperature, enclosure heating, or altitude.

Switch-disconnector

A device combining switching under load with isolation in the open position, compliant with isolation requirements.

Diversity factor

A planning factor recognizing not all loads peak simultaneously; used at panel level but applied cautiously for isolator sizing.

Fault level

The prospective short-circuit current at a location, derived from system impedance or MVA, used to check Icw and coordination.

References

Here’s a concise overview before we dive into the key points:

• IEC 60947-3: Low-voltage switchgear and controlgear – Switches, disconnectors, switch-disconnectors and fuse-combination units
• NFPA 70 (NEC): National Electrical Code
• Electrical Engineering Portal: Prospective short-circuit current (PSC) basics
• ABB Low Voltage: Switches and disconnectors product information
• Schneider Electric: Electrical Installation Guide

These points provide quick orientation—use them alongside the full explanations in this page.