VFD Brake Resistor Wire Size

[metal_pro_SAF]

When selecting VFD Brake resistor cables the ABB VFD manufacturer manual states "Use the same cable type for the resistor cabling as for the drive input cabling to
ensure that the input fuses also protect the resistor cable" However several Brake resistor manufactures only size the wires (and connecting terminals) on the amperage of the resistor. So if I have a 200 amp VFD with #4/0 wire input cabling and try and connect the VFD to a 52 amp brake resistor with #8 wiring there is a problem even if you use UL508a table 36.1 Conductor rating factor for power resistors. The Brake Resistor say that "Majority of our customers size the wire based on the internal wire size of the resistor". I do not think that in doing so would meet the UL508a requirements?

 

[Jraef]

Be careful here. Most VFD mfrs with a built-in braking transistor size the chopper transistor for the same current rating as the motor transistors, so they don’t bother to put any additional protection on the braking circuit, which is the basis of that statement. If you use a much smaller resistor like that, there is nothing protecting it (unless you add it). That means the drive could theoretically try to dump 240A (assuming 480V) of current into a resistor only rated for 52A. So if you fuse that resistor circuit based on the 52A, your smaller wire size is also covered.