Aug 20, 2026
Motor & VFD Control Panel Spec Checklist: What to Send Your Supplier | UniRegal
A motor control panel with VFDs fails on heat, harmonics, or the wrong enclosure — not on the motor. Use this 5-step spec checklist before you send an RFQ to a China control panel supplier.
A motor control panel is a box that switches, protects, and (with a VFD) regulates one or more motors. Most RFQ problems are not electrical — they are heat, enclosure, and documentation problems that show up weeks after installation. The short answer: lock the starter type first, size the enclosure for the drives' heat, spec protection and EMC filtering, then ask for the wiring diagrams and certificates in writing before you order. This checklist walks through those decisions so your quote is comparable across suppliers.
What a motor / VFD control panel contains
A typical panel breaks down into four layers:
- Incoming protection — main isolator, MCCB or fuse switch
- Motor circuit — contactor, overload relay (or soft starter / VFD), motor protection breaker
- Control circuit — 24 V DC power supply, PLC or relay logic, HMI, terminal blocks
- Auxiliaries — cooling, lighting, metering, surge protection
The expensive decisions are the starter type and the enclosure, so start there.
Step 1 — Lock the starter type first
Every other spec depends on how the motor starts. There are four common answers:
Starter | Inrush | Cost | Best for |
|---|---|---|---|
DOL (direct-on-line) | ~6–8× full load | Lowest | Small motors, occasional starts |
Star-delta | ~2–3× | Low | Medium motors, long run times |
Soft starter | ~2–4× | Medium | Pumps, conveyors needing gentle ramps |
VFD (variable frequency drive) | ~1–1.5× | Highest | Speed control, energy savings, soft starts |
If the application needs variable speed — pumps, fans, mixers — a VFD is the only real choice. If it only needs a soft start, a soft starter costs less. The full comparison of starting methods is in our DOL vs star-delta vs soft starter vs VFD guide; your panel supplier needs this decision before they can size the enclosure, the protection, and the cooling.
Step 2 — Size the enclosure around the drives, not the motor
The enclosure is sized by what is inside — mainly the VFDs — not by the motor kW on the nameplate. A 30 kW drive needs roughly double the volume of a 30 kW contactor-only circuit, because the drive wastes 3–5% of its power as heat and needs clearance around its heatsink.
Practical sizing rules:
- Allow 15–20 cm clearance above and below each VFD for airflow
- Keep VFDs on the same plane, not stacked directly above each other
- Leave 30% spare mounting space for future circuits
- For dusty or outdoor sites, choose a sealed box with a filtered fan or heat exchanger instead of a louvered vent
We build cabinets from carbon steel, 304, or 316 stainless with IP65 / NEMA 4X ratings in our Foshan workshops — for wash-down areas, spec 316 and sealed glands. The drive-heat trade-offs are compared in our best cooling options for outdoor electrical cabinets guide.
Step 3 — Manage VFD heat before you buy
Heat is the number one cause of premature VFD failure inside a panel. A drive's data sheet states a derating curve: above ~40 °C ambient inside the box, it must be derated — meaning you pay for a bigger drive or it trips on hot days.
Three ways to keep the box cool:
- Filtered fan — cheapest; fine for indoor, dry rooms
- Air-to-air heat exchanger — sealed; no ambient dust or humidity enters
- Cabinet air conditioner — for hot climates or tightly packed drives
Whatever you choose, calculate the heat load from all drives and transformers, then add margin. If you skip this step, the panel will come back to you at the first summer.
Step 4 — Specify protection, EMC, and component brands
The motor circuit needs coordinated protection, not just any parts:
- Short-circuit — MCCB or fuse switch rated for the drive's prospective fault current
- Overload — thermal overload relay for contactor circuits; VFDs protect themselves but still need input fuses or a breaker sized to the drive manual
- EMC — VFDs emit harmonics and radio-frequency noise. Ask for input line reactors or DC chokes, screened motor cables, and proper earthing. This is where "cheap panel" usually cuts corners
- Control supply — a reliable 24 V DC PSU with enough margin for all I/O
On brands: we source Mitsubishi, Siemens, ABB, Omron, and Schneider drives and components — tell us your installed base and we build the panel around it, so you keep one spares strategy instead of mixing vendors.
Step 5 — Ask for the documents that prove the build
A panel is only as good as its paperwork. Put these in your RFQ so they arrive with the shipment:
- Wiring diagram (as-built, with component references)
- Bill of materials — brand and part number per component
- Test certificate / FAT report — megger test, continuity, and a logged run test of the VFDs
- Enclosure rating certificate — IP or NEMA test report
- Market compliance — UL 508A panel certification or UL-listed enclosures for US-bound projects, CE for EU-bound
Our control panel build spec checklist is the full document template to send, and the control panel FAQ on lead times and testing covers the testing questions buyers ask most.
What to send with your RFQ
Six answers are enough to get an accurate quote:
- Motor list: kW, voltage, phase, and whether each needs speed control
- Starter type per motor (DOL / star-delta / soft starter / VFD)
- Enclosure: indoor or outdoor, material, IP/NEMA rating
- Ambient temperature at the site
- Component brands you prefer
- Certifications required (UL, CE) and target lead time
As a China sourcing supplier rather than a brand-locked builder, we quote the components you already run, with enclosure lead times around 15 days and stock components shipping in 3–5 days. The panel and the box are built in one place, so one supplier owns the whole responsibility — and our electrical enclosure supplier guide explains how we qualify enclosures for overseas buyers. If you are still comparing builders, our motor control panel supplier shortlist shows what to look for.
FAQ
Can one panel handle both fixed-speed and VFD motors? Yes — mixed panels are common. Each motor gets its own starter and protection circuit; only the incoming and control sections are shared. Tell the supplier which motors need speed control so the panel is sized correctly.
How much bigger is a VFD panel than a contactor-only panel? Roughly 30–50% more volume for the same motor kW, mostly for drive clearance and heat dissipation. A 30 kW contactor circuit fits in a 600×400 mm box; a 30 kW VFD usually needs 800×600 mm or larger.
What happens if I skip input reactors on the VFDs? The drives still run, but you risk nuisance trips on the incoming breaker, harmonics that upset other equipment, and a bigger EMC headache at commissioning. Line reactors are cheap insurance on any panel with multiple drives.
Do you build around the VFD brand we already use? Yes. Mitsubishi, Siemens, ABB, Omron, Schneider — we source the drives and build the panel around them. You keep one installed base instead of mixing vendors across machines.
What is the lead time and MOQ? Custom enclosures start at 5 cabinets and take about 15 days; stock components ship in 3–5 days. Full panel builds are quoted per project with the documents above included.
When your six answers are ready, submit your requirements with the motor list and we will quote the panel and enclosure together from our Foshan supplier network. For a full checklist of what to include, see our RFQ guide for outdoor electrical cabinets.