Be careful when matching fuses with circuit breakers. Circuit breakers are thermal magnetic and fuses are thermal. They both have different current clearing characteristics. To do it properly go to the manufacturer and find out what fuse can be matched with the circuit breaker you are using or overlap the current characteristic curves of the two devices.
110.9, 110.10. I've had mixed answers on this topic from both electrical experts and equipment manufacturers. I've run into this problem specifically with HVAC equipment, where the proposed or installed equipment had an interrupting rating that was lower than what was available at the line terminals of the equipment. My first step was to go back to the manufacturer to see if they could make any field modifications. Some manufacturers sell a kit that does not violate the listing and it can be used to increase SCCR. But like I say - I get mixed answers with regard to this topic and from all the big players (Siemens, Bussman, Schneider, and all the rest). I try to catch these issues on plan review. This is a perfect example of why we need plan review.
Hello. Thanks for your responses. I am not sure if my question was completely clear.
CB -------------- Device in branch circuit (5000 SCCR) -- Inline fuse (500A int.) ------------------------------------------------------
l l l l ..... load1 load2 load3 load4 ....
The current flowing in the circuit actually passes through the device and the inline fuse before power is delivered to the loads.
First thing you need to do is find out what the overcurrent device is capable of withstanding in a short circuit condition.
The device should state its maximum withstand rating. If the device is only capable of withstanding 10,000amps sym then the current needs to be limited to a value less then 10,000amps. There is such a thing as series ratings. The manufacturer would have that information. The manufacturer has a schedule that shows what devices can be connected in series to limit fault current to a specific value. If there are no schedules you will need to go to the device trip curves and find someone to teach you how to read them.
I posted an answer but it's locked up (being moderated)
Hi Nick,
As I have had recent experience with your same issue (being moderated) - I believe it has to do with pasting a Link.
I put the Link that I was trying to add as text without http or www and my responses were posted.
I've seen other Links posted?!? https://www.ecmweb.com/electrical-testing/voltage-loss-versus-voltage-drop
There's a Link Icon in the Text Editors above that I used for this one - it worked!
just tried again 2 different ways - post is locked out, "being moderated" I'm not sure why all the problem posting links...
Ok, the original 'reply' flagged as being moderated needs to be deleted, even though you can still make changes to it.
Try deleting the Links, highlight your reply, right click and Copy. Delete the entire Reply thru 'Actions'. Select 'Reply to original question', with the cursor inside, ctrl-V (to paste), add Links, then 'Add Comment' should do it.
I found out another hick-up, the Reply will be flagged yet again if the Link is an "https" (secured) so paste as text without the prefix.
My earlier answer is simple, and absolutely correct in nearly any instance. Manufacturers of the branch circuit panelboards, circuit breakers, and load devices have no say, no impact.
As long as your supplementary overcurrent protection device is placed on the load side of the branch circuit overcurrent protection, your only concerns are wiring that supplemental device in compliance with the code sections covering wiring, enclosures, accessibility, and the like.
Too simple? Apparently so, for this forum. Stand out from the crowd: don't make a mountain out of a molehill.
Perhaps you have some confusion about available fault currents. That's a different question, and a different topic. If so, go Googling, read some Wiki's, or find a textbook which deals with it. You'll get the hang of it.
Disagree with you on this, Gary.
I changed my mind, I may not have understood the original question, Gary. If it's supplementary OC protection, it really doesn't matter now, does it? You are correct, it is too simple, provided the OCPD is properly rated for the available fault current.
I would like to thank everyone for contributing to the discussion. I am afraid that indeed -- a mountain was made out of a molehill. According to my source --- although I don't understand the logic -- only the OCPD protecting the branch circuit has to have an interrupting rating which is capable of interrupting the available fault current. Even though the supplemental fuse is actually connected in the branch circuit, it is still supplemental.
I got confused when you said, "use an inline fuse to protect the device" which somehow made me think this was a question about equipment. Any equipment must be rated for the available fault current at the line terminals of the equipment, was my point. I apologize to Gary that I take part in such a simple and unknowledgeable forum (which I don't believe for a second), and that I can make a mistake.
Yes. It is perplexing to me. The fuse protecting the equipment is actually located in the branch circuit. However, from what I have been told, it is a supplemental device so outside the jurisdiction of the NEC.
Correct, out with the NEC in with NFPA 79, Chapter 7.
{7.2.4.1} … Tapped … in accordance with {7.2.4.2}. Such a tapped control circuit shall not be considered to be a branch circuit … protected by .. supplementary … OCPD.
Bare in mind, the Wire still needs proper Fusing or it becomes the Fuse Link. Example; an in-line 5A (Littlefuse). Wire size to the Fuse sized per the previous OCPD per the Table 12.5.1, after the 5A Fuse cannot be smaller than 20 AWG. Some of the ways around Sensor Cables and Devices - they are all ready self-protected or current-limited.
If you are dealing with branch circuits using no 12AWG conductors short circuit withstand ratings should not be an issue. But its important to work through the math in determining the available fault current anywhere in the AC system.
I don't think there is any way to get 10000amps to a device connected to a 20Amp circuit breaker in a panel having more than 10000amps available. Its the circuit breaker you want to be concerned about not the device at the end of a branch circuit. The wire alone between the circuit breaker and the device should limit the fault current to a value under 5KAIC based on its impedance.
Yes the manufacturers information does matter when using overcurrent protection in that they create the current curves and these curves must be consulted when using them in a circuit. When using overcurrent protective devices in series with one another its critical to consult with the manufacturers data on let through current and clearing times. The manufacturer also has charts for series ratings.
Thank you. I read passages of NFPA 79 this morning, as well as literature published by UL and Rockwell on branch circuit and supplementary protection.
Robert
That is ok for a fuse but what about the circuit breaker? What do you get when you overlap the curves [ fuse and circuit breaker ] under fault conditions?
Thank you Robert for that explanation. You are correct in your graphical presentation. However, I might add that fault current for branch circuits are not as a rule considered in the real world of building design. Placing a 20A circuit breaker in series with a control power fuse of 5amps, for example, is not an issue and not worth the exercise of calculating fault current or determining the clearing time of overcurrent protective devices. My question was merely for clarity. The usual reason you would want to know the clearing times of circuit protective devices is to provide an electrical service that is selective under fault conditions. If there was a fault somewhere in the AC system you would not want to trip the main and take down the entire buildings electrical service if a 20A circuit breaker were to trip under fault conditions.
Selective coordination is an absolute must for hospitals where critical care is an issue. If we were dealing with control circuits critical enough to be concerned about opening under fault or overload conditions we would probably use a technology suitable for the purpose such as and IPS or UPS system.
Thank you. Very clear.
Yes.
Be certain to place it on the load side of the branch circuit protection.