I am ordering a VFD for a 3 Phase 20 HP motor to drive an HVAC fan, and I want to know what the AIC rating should be. Is it simply looking at the wire resistance and using Ohms law? I asked for advice from a colleague and he thought I just needed to look at the let through current for the fuses before the Drive and make sure the VFD AIC rating was greater than that, this doesn't seem right.
Minimum KAIC rating for a VFD
- Thread starter ps172011
- Start date
- Status
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
The VFD needs to have a SCCR rating (AIC ratings are only for protective devices) that is greater than the available Short Circuit Amps (SCA) at its line side terminals.
It is often more complicated than simply looking at fuse let-through values only.
Most VFD manufacturers have 'listed' series ratings with specific protective devices.
skeshesh
Senior Member
- Location
- Los Angeles, Ca
As jim said you need to use ratings exceeding the available short circuit current on the line side of the unit. If you want to value engineer it to be the smallest rating possible then you need proper short circuit analysis - if not you may have to side with caution and match the rating of the transformer or other source upstream of the VFD.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
You may be technically correct but using the wrong terminology, yet you also may be missing an important point. "AIC" means "Amps Interrupting Capacity" and is a term reserved for interrupting circuit protective devices such as circuit breakers or fuses. SCCR (Short Circuit Current Rating) is the new terminology to be used for control assemblies such as VFDs and it is well documented as to how it is to be determined. You can't have a VFD with a "30kAIC" rating, but you can have a VFD with an SCCR of 30kA. There is a difference. Some people expressing otherwise may not yet be aware of the recent changes, so I'll outline them below for everyone, it's good to know if you are installing VFDs in areas that are following the 2005 NEC (or later).
A VFD has typically been only a control device, not a fault interrupt device. The only thing a VFD could "officially" protect for was overload on the motor. More recently however because of the changes in Article 409 of the 2005 NEC, VFD manufacturers got a "special consideration" of their devices as a kind of fault interrupt device. Some of them can (if tested) protect the LOAD from short circuits and therefore were given high SCCR ratings all by themselves. But they still are NOT considered to be Branch Circuit Protection (BCP) all by themselves. They must have BCP ahead of them. The special SCCR rating given to VFDs now simply allows them to be used in circuits up to the SCCR rating of the BCP ahead of it without further testing, as long as it does not exceed the SCCR rating assigned to the VFD.
Confusing :-? I know, but an example will help.
You (probably) have a VFD with an SCCR rating of 30kA. If you have an up stream Feeder Circuit Breaker that is rated 30kAIC, then you can install that drive and have an SCCR rating of 30kA for the circuit (assuming nothing else in the box with the VFD). But if you have a CB rated only 14kAIC, then your overall SCCR rating of the circuit becomes 14kA, even though the VFD is rated higher. If on the other hand your CB was rated 50kAIC, the overall SCCR rating is still only 30kA because the VFD is only rated for that.
But this belies the actual acceptability of the VFD in the circuit, because you STILL need for someone to tell you what the AFC (Available Fault Current) is at the site where you want to install this. You cannot ass-u-me it or guess at it no matter what. It is determined by a lot of factors, including but not limited to the utility supply capacity, transformer impedance, wire length, contributing loads etc. etc. Someone will have had to have done this at some point, at the very least the Utility can tell you what it is at their terminals which is a big start. Once you know that, then you will need to check the BCP device ahead of the VFD to know the overall SCCR rating of the circuit.
And yes, for the inquisitive among us, Series Ratings will make this a little more complicated.:roll:
Last edited:
- Status