ROOF TOP UNIT SCCR

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petersonra said:
There is nothing in the code that says you can add fuses to the system to reduce available short circuit current.
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It is my understanding that current limiting devices that act within their current limiting range can be used to lower the short circuit current to "passive" equipment. If there are OCPDs in the equipment, the equipment is not passive.
 
don_resqcapt19 said:
It is my understanding that current limiting devices that act within their current limiting range can be used to lower the short circuit current to "passive" equipment. If there are OCPDs in the equipment, the equipment is not passive.
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that only applies to UL508a evaluations. nothing in the code allows for it EXCEPT for some provisions for existing systems where a PE is willing to put his license on the line.
 
Section 240.86 Series Ratings applies. Section (A) is the part where a PE can make the analysis and the downstream device can't be a dynamic impedance, but it's for existing installations so that doesn't apply here. Section (B) looks for Tested Combinations. I think section B is all you can apply for this so you'd need to know what components are in the RTU and find an upstream fuse that's listed with it and install that fuse in the disconnect.
 
I noted
Jraef said:
In my experience, if a manufacturer ships something with a 5kA SCCR, it's BECAUSE they did not want to bother with even the simplest of engineering principles it would have takne to get a higher value, such as asking the component mfr for the series listings. What I see happen is that they used XYZ brand of circuit breaker, ABC brand of contactor and FGH brand of overload relay, all because each individual component was cheapest they could find; engineering by procurement department. Competitors don't typically spend the money to have their devices series listed with competitive products, they are going to test them with their own. So all it really takes is for the mfr of the panel to chose components that have been series tested and listed for a higher SCCR. It's not in the LEAST BIT difficult. So if they didn't bother with that tiny tiny amount of effort to click some links from a supplier, they most likely are not going to give you an engineered fuse selection to go ahead of it. But good luck...
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I just noted that the equipment didn't meet the criteria on the study and that the contractor needed to have a discussion with the consulting engineer and manufacturer before coming to a final decision. I don't feel too bad for the contractor because they ordered the equipment against what the engineer had specified and before I was even hired to do this study. They just messed up plain and simple.
 
petersonra said:
that only applies to UL508a evaluations. nothing in the code allows for it EXCEPT for some provisions for existing systems where a PE is willing to put his license on the line.
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I thought the requirement for a PE or a listed combination only applied to series rated OCPDs.
 
mayanees said:
Section 240.86 Series Ratings applies. Section (A) is the part where a PE can make the analysis and the downstream device can't be a dynamic impedance, but it's for existing installations so that doesn't apply here. Section (B) looks for Tested Combinations. I think section B is all you can apply for this so you'd need to know what components are in the RTU and find an upstream fuse that's listed with it and install that fuse in the disconnect.
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both sections only apply to circuit breakers not passive components.
 
Some of the many previous discussions on this subject in this forum.

Current Limiting Fuses to address SCCR Underrated Panels

Can current limiting fuses, upstream of panel, be used to reduce available fault current to the panel? My research seems to indicate that UL does not recommend this. This rather old article from EC&M also suggests it's not recommended. Unless of course the fuse has a series rating with the...
forums.mikeholt.com

Available fault current vs. SCCR

I'm doing an inspection on a piece of equipment with a nameplate SCCR of 5 kA but the calculated available fault current at the panel is 14 kA. They contest that since they're using 150 Amp RK1 fuses as feeder protection, the let-through current is only 3 kA. Is this valid code-wise? Thanks!
forums.mikeholt.com

limiting short circuit current at chiller

I am the engineer on a project currently under construction. We have a chiller being installed in the vicinity of a 208V utility transformer. The chiller is fed directly from the transformer, with a fused switch at the chiller. The electrical inspector noted that the nameplate data shows the...
forums.mikeholt.com

SCCR of control & starter panels

We are paying attention to the SCCR (short circuit current rating) of equipment purchased by our clients as we design the power supply to these panels....such as Air Compressors, HVAC equipment, etc. We want to ensure that the panels are rated for the available short circuit curent from the...
forums.mikeholt.com
 
petersonra said:
true.

but the code no where gives you a pass on supplying equipment with a SCCR of less than the available SCC just because you used current limiting fuses.
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So a current limiting device does not change the available fault current on the load side of the device?
 
petersonra said:
The code just does not allow for it. Maybe it should.
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I am sorta agreeing with Don though. Where does the code state I can't use an appropriate CL fuse? I need a certain SCC, a listed CL fuse provides that. Isn't the general rule that the code defaults to telling us what we can't do and does not have to give us permission? Series rated OCPD's are different because the code gives us specific options and a CL fuse is no one (unless a tested combination).

If I used a CL fuse properly in such a situation, what would you cite as a violation?
 
The code says you have to have equipment with a sccr exceeding the maximum available short circuit current. The only exception is for series rated breakers.
 
The way I see it is that the CONTROL PANEL must be marked with an SCCR and cannot be installed where the Available Fault Current is greater than that marked amount. Nowhere in any calculations of Available Fault Current is there any mention of using Current Limiting Fuses to alter that value; impedance, resistance, yes, fuse clearing time is not mentioned. Case in point; try to find a place to load a fuse current limitation value into SKM or ETAP engineering software for determining AFC... it's not there. BUT, if I'm wrong about this last point, someone tell me, I'm all ears.

The PANEL MANUFACTURER can use CL fuses as part of their methodology in determining the SCCR of the panel, because they can carefully coordinate the let-through of the fuses under the right circumstances against the withstand ratings of the components used on the power train. That is their job here. Those fuses CAN be located outside of the panel now, so long as the label on the panel, WHERE THE HIGHER SCCR IS LISTED, clearly dictates that those specific fuses MUST be used. But that is still the responsibility of the panel mfr. A contractor in the field is not changing the marked SCCR of the panel by adding CL fuses (that the panel mfr didn't specify) and the contractor in the field is not able to change the AFC by adding CL fuses either. There just is no provision in the NEC that allows this to happen. Maybe nobody thought it necessary because, again, attaining a higher SCCR is not that difficult, but maybe nobody realized just how lazy some mfrs would be with this.

Should there be a way to use CL fuses in the field? Maybe. One place to start is to add a definition of "Available Fault Current" to the next NEC. As of the 2017 code there isn't one, yet the term "available fault current" is used many times throughout the Code. It might be time to define it more clearly. If that definition allows for the use of current limiting fuses or breakers to be used in that calculation, so be it. But until that happens, it's not there.
 
Jraef said:
Should there be a way to use CL fuses in the field? Maybe. One place to start is to add a definition of "Available Fault Current" to the next NEC. As of the 2017 code there isn't one, yet the term "available fault current" is used many times throughout the Code. It might be time to define it more clearly. If that definition allows for the use of current limiting fuses or breakers to be used in that calculation, so be it. But until that happens, it's not there.
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110.10 seems to allow it, IMO.
 
electrofelon said:
110.10 seems to allow it, IMO.
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Your are correct about some wording in 110.10 that mentions clearing a fault without extensive damage.
But the last sentence clearly says the equipment must be applied per its listing. If upstream fuses were used in the testing you are good to go, if the manufacturer has information showing that. If fuses were not part of the test, then you are on your own.
If there are breaker style protection devices internal to the equipment, you fall into the issue with series ratings.
Do you know the Ipeak and the I²t of your proposed fuse and if the equipment can tolerate those values? What if the fault current is less than your calculated 8kA but still above the 5kA on the label, how does the fuse operate then?
 
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