AIC for main disconnect

[MFS-PDX-313]

I've got an inspector from TUV telling me that I need to provide the AIC (not SCCR) for the main disconnect and the main breaker. From what I can find AIC is only for overcurrent protection devices (OCPD) such as breakers or fuses. The inspector is saying that the main disconnect is also an OCPD. Where am I getting confused here?

 

[jim dungar]

The NEC now requires you to provide a label, in part, stating the available short circuit current.
The NEC has required that all equipment be applied correctly 110.9 and 110.10, for several decades.

Yes, fusible disconnect switches have an AIC (depends on the fuse clips and the installed fuse), while non-fusible switches have just an SCCR.

 

[Jraef]

Do you have a Non-Fused Disconnect Switch as the Main then? If so that may be a problem. There is no "AIC" (Amps Interrupting Capacity) rating on a device that is not an "Interrupting" component. There is a "withstand" rating, which is now what we call the "SCCR" (Short Circuit Current Rating" and in the case of many Non-Fused Disconnects, it is 10kA or less without fuses, basically useless in most industrial applications. The way around that is to use a Moded Case Switch (an MCCB with no trip elements) which often have the same SCCR as if they were breaker (but don't assume anything). You may find though that because of volume, you can buy the full blown MCCB for less than a MCS.

 

[MFS-PDX-313]

Thanks, folks!

The disconnects we are using are non-fused, which explains why wasn't finding anything on the cut sheet for AIC. Unfortunately, the cut sheet only lists SCCR values when used with corresponding fuse sizes. Without that, I am left determining the value at 5kA per table SB4.1 in UL508A. As we have to reach 10kA for SEMI S2/S22, we'll likely have to add a type J or T fuse. Changing the disconnect is not an option, as it would require modifying the enclosure, which we do not have time for (changes from this TUV inspection is already making the ship date riskier).

Jraef, can you suggest a panel Non-Fused Disconnect that has a 10kA rating without fuses? I've searched in the past and was unable to find anything. If not, no worries. When I saw 10kA in your comment, my spidey-sense was tingling.

 

[texie]

If this is a control panel just getting your disconnect to 10K AIC is not going to solve your problem. The entire panel will need a 10K rating if that is the available fault current. Current limiting fuses will not solve this.

 

[electrofelon]

Thanks, folks!

The disconnects we are using are non-fused, which explains why wasn't finding anything on the cut sheet for AIC. Unfortunately, the cut sheet only lists SCCR values when used with corresponding fuse sizes. Without that, I am left determining the value at 5kA per table SB4.1 in UL508A. As we have to reach 10kA for SEMI S2/S22, we'll likely have to add a type J or T fuse. Changing the disconnect is not an option, as it would require modifying the enclosure, which we do not have time for (changes from this TUV inspection is already making the ship date riskier).

Jraef, can you suggest a panel Non-Fused Disconnect that has a 10kA rating without fuses? I've searched in the past and was unable to find anything. If not, no worries. When I saw 10kA in your comment, my spidey-sense was tingling.

There is some ambiguity about the SCCR of non fused safety switches. It is generally accepted knowledge that NF safety switches have 10k SCCR, however some manufacturers are saying that they have NO SCCR without an OCPD ahead of them. Here is a thread you can check out, note that I was complaining about some of the language not making sense.

PV utility non fuse disco

Attached sketch shows non fuse PV utility disco that is required by utility sp says the plans. The AC disco upstream is fused. I have heard but not sure that non fuse short circuit rating is 10kA and their is no interrup rating. I was wondering do these utility non fuse disco for PV have to...

forums.mikeholt.com

 

[Dennis Alwon]

Is this for a dwelling. If so then this is what he is reading from the 2020-- if you are using the 2020, One can argue that the service conductors don't end in a disconnect switch with no fuses so you don't need an aic rating

230.85 Emergency Disconnects.
For one- and two-family dwelling units, all service conductors shall terminate in disconnecting means having a short-circuit current rating equal to or greater than the available fault current, installed in a readily accessible outdoor location. If more than one disconnect is provided, they shall be grouped. Each disconnect shall be one of the following:

If you mean 110.9 then the rating is only required for equipment that interrupts current

110.9 Interrupting Rating.
Equipment intended to interrupt current at fault levels shall have an interrupting rating at nominal circuit voltage at least equal to the current that is available at the line terminals of the equipment.
Equipment intended to interrupt current at other than fault levels shall have an interrupting rating at nominal circuit voltage at least equal to the current that must be interrupted.

 

[jim dungar]

There is some ambiguity about the SCCR of non fused safety switches.

For a control panel like the OP has, the terminology becomes kind of critical. Service disconnect is a very subjective term.

'Safety switches' which can be wall mounted, have an external operating means, and often some type of quick break mechanism. These types of devices are typically 10kA SCCR.
'AC disconnect' type of device, which are often a pullout style, although some are molded case switches. These could have an SCCR as low as 5kA
Control panel style disconnects are often internal to an enclosure with an externally operated mechanism. NEMA/UL style devices can have SCCRs comparable to those of safety switches. IEC style control panel/motor switches are often relatively small in size, Din rail mountable, have an external knob connected by a 6-8mm square shaft, thier SCCR may be as low as 5kA for non-fused version.

Almost all non-fusible disconnects will have a series rating with a fuse mounted somewhere upstream. However, as mentioned above, the SCCR of the disconnect may not be the deciding factor for the whole panel.

 

[Jraef]

What do you have in there now? Make and model would be good, but is it a flange mount or a rotary thru-the-door type disconnect? With rotary type, I’ve seen some very compact fusible designs that take up almost no additional room compared to non-fused types if you use Class J fuses. But it’s hard to make recommendations in the dark.