Residential wiring and available fault current ratings

[jim dungar]

Also remember if that fault is a hundred feet downstream...

Conductor length,past 4 feet, is not part of the series rating test.
What your are describing impacts the available short circuit current which is used when applying the series rating.

 

[Dsg319]

It has to do with the relative timing time-current curves of the OCPD's. A listed series combination of OCPD's, will mean that the upstream OCPD will trip before the downstream OCPD with less KAIC or SCCR will catastrophically fail. A combination only gets to take credit for series ratings, if the products are documented as having passed the series rating tests.

You will often see part of the part numbers in the chart, telling you that certain product families of breakers are series rated with a related product family of breakers. For instance, Square D QO breakers, are series rated with Square D MG breakers. It is rare (if ever) that breakers of different manufacturers will carry a series rating. Fuses can be series rated with breakers, and it is often identified by the fuse class, so that it is agnostic to the manufacturer of the fuses. For instance, class J fuses are series rated with Square D QO breakers, and it wouldn't matter if it is a fuse from Bussmann, Mersen, or Littelfuse.

As an example. Suppose you have a main breaker at 22 kaic, and series-rated branch breakers at 10 kaic. For faults up to 10 kA, either the branch breakers or the main breakers can interrupt the fault safely. For faults between 10kA and 22 kA, the listing as a series rated combination means that the main breaker will safely interrupt the fault, before it has a chance to damage the 10 kA breaker. Faults above 22kA could either damage one breaker or both breakers.

While not strictly a requirement to comply with the NEC, ideally you would want the branch breaker to be more likely to trip first, so that only the faulted branch circuit gets shut off. This is called selective coordination. You want a cross-over in the time-current curves overlaid on each other, so that the main breaker trips first on high current faults, and the branch breaker trips first on low current faults.



You would start with the available fault current at the main panel, and then apply the calculation to account for the feeder's impedance. This assumes no loads within the subpanel are significant sources of fault current, such as motors. You could either use a subpanel rated "in its own right" for the available fault current at its position, or you could use a subpanel with its breakers series rated with any of the upstream breakers that supply the feeder.

Just to put into easy terms I’ll create and example. Just off my head to make sense.....
say I have a shop, do the calculations and come up with a available fault current of 24ka.

Obviously I get a main breaker rated above that......But what do I have to do in order to have the branch circuits to just have a rating of 10ka and not have to go through the trouble of getting different breakers.

I know that’s a common set up for many load centers. Just wondering how it’s done and how it actually works.Is the main also current limiting so the rest of the branch breakers are covered and can’t see that amount of fault current more than there rated for?

 

[Dsg319]

I’ll try not and confuse anyone by this question lol. But see the 200amp breaker, I suppose it has a rating of 22kia. As for the smaller breakers such as 15amp, 20 and so forth. Are they also rated at 22K? Or smaller and compatible with the 200amp main because of the series rating?

 

[Dsg319]

 

[Dsg319]

^^^^^
In the above photos I believe I answered some of mine own confusion questions lol. Cant believe I’ve went this long and never really thought about available fault current and short circuit ratings. I’ve seen the numbers just never really thought much more into it..

But just to clear my head, in most residential settings with pole mount trannys and unknown specs on them. I imagine you just use standard 10ka branch circuit breakers and larger on the main breakers such as 22ka or whatever is typical for 100,200 and possibly 400amp main breakers.

 

[Carultch]

Just to put into easy terms I’ll create and example. Just off my head to make sense.....
say I have a shop, do the calculations and come up with a available fault current of 24ka.

Obviously I get a main breaker rated above that......But what do I have to do in order to have the branch circuits to just have a rating of 10ka and not have to go through the trouble of getting different breakers.

I know that’s a common set up for many load centers. Just wondering how it’s done and how it actually works.Is the main also current limiting so the rest of the branch breakers are covered and can’t see that amount of fault current more than there rated for?

Put the word "series rated" and the main breaker kaic rating in your description when you request your quote. The vendor will produce a quote within the brand, of corresponding breakers. If you already have a main panel, and you plan on adding a subpanel, specify something such as "series rated with Square D LPJ breaker" if LPJ breakers are in the main panel. You may find there are series ratings that apply at <= 240V, but not at 480V, so specify the system voltage as well

 

[Carultch]

I’ll try not and confuse anyone by this question lol. But see the 200amp breaker, I suppose it has a rating of 22kia. As for the smaller breakers such as 15amp, 20 and so forth. Are they also rated at 22K? Or smaller and compatible with the 200amp main because of the series rating?

If it is fully rated, as opposed to series-rated, both the branch breakers and main breakers would have to be rated for the available fault current independently of other breakers in the system. Fully-rated means both the main and branch breakers would need to be identified for 22 kaic on the device markings, if you want to use them with up to 22kA of fault current. If you can't produce documentation that it is a listed combination, that means you need a fully-rated system. In otherwords, fully-rated means all devices have to withstand the available fault current at their position in the circuit, as if the other devices didn't exist.

If it is series rated, then you *get to* have branch breakers downstream of the main, of less kaic rating, provided that they are a listed series rated combination. This is an advantage, because lower kaic ratings are generally cheaper, and you'll have a lot more branch breakers than main breakers. Given a 22 kaic main, and 10 kaic branch breakers, this means the following. The main can withstand up to 22 kA of available fault current, as a stand-alone breaker. The branch breakers can withstand up to 10 kA of available fault current as stand-alone breakers. As a series-rated combination, the 10 kaic breakers can be downstream of the 22 kaic main breaker, and this combination can be used in an application where the available fault current is up to 22 kA.

 

[Carultch]

But just to clear my head, in most residential settings with pole mount trannys and unknown specs on them. I imagine you just use standard 10ka branch circuit breakers and larger on the main breakers such as 22ka or whatever is typical for 100,200 and possibly 400amp main breakers.

The utility has the records of what transformer they provide, and they can provide the available fault current to the customer when requested. It can be like pulling teeth to get this question answered, but if you provide the address and any administrative numbers associated with the service, the information is available, and they can provide it.

Sometimes you can see the KVA rating with binoculars, and you can look up typical impedance ratings for the type of transformer to get a good estimate. Unfortunately also, on pad-mount transformers, this information is covered by a dead front. Some utilities spraypaint the kVA as a big number on the outside of transformers. You can also find a job number from the original manufacturer labeled on the equipment, and request the nameplate as well.

 

[Dsg319]

If it is fully rated, as opposed to series-rated, both the branch breakers and main breakers would have to be rated for the available fault current independently of other breakers in the system. Fully-rated means both the main and branch breakers would need to be identified for 22 kaic on the device markings, if you want to use them with up to 22kA of fault current. If you can't produce documentation that it is a listed combination, that means you need a fully-rated system. In otherwords, fully-rated means all devices have to withstand the available fault current at their position in the circuit, as if the other devices didn't exist.

If it is series rated, then you *get to* have branch breakers downstream of the main, of less kaic rating, provided that they are a listed series rated combination. This is an advantage, because lower kaic ratings are generally cheaper, and you'll have a lot more branch breakers than main breakers. Given a 22 kaic main, and 10 kaic branch breakers, this means the following. The main can withstand up to 22 kA of available fault current, as a stand-alone breaker. The branch breakers can withstand up to 10 kA of available fault current as stand-alone breakers. As a series-rated combination, the 10 kaic breakers can be downstream of the 22 kaic main breaker, and this combination can be used in an application where the available fault current is up to 22 kA.

Awesome thank you. Really cleared up the questions in my head!

 

[Dsg319]

The utility has the records of what transformer they provide, and they can provide the available fault current to the customer when requested. It can be like pulling teeth to get this question answered, but if you provide the address and any administrative numbers associated with the service, the information is available, and they can provide it.

Sometimes you can see the KVA rating with binoculars, and you can look up typical impedance ratings for the type of transformer to get a good estimate. Unfortunately also, on pad-mount transformers, this information is covered by a dead front. Some utilities spraypaint the kVA as a big number on the outside of transformers. You can also find a job number from the original manufacturer labeled on the equipment, and request the nameplate as well.

Thank you!

 

[Dsg319]

Thank you! I just wonder how much this is taken into consideration in the residential setting. I’d imagine not much considering there would have to be some really big factors in getting the available fault current above and beyond the standard rating of a 200amp(main) and series rated panel.

In my setting of work (industrial) almost everything is calculated and drawn from an engineering firm.[/QUOTE]

 

[Fred B]

Thank you! I just wonder how much this is taken into consideration in the residential setting. I’d imagine not much considering there would have to be some really big factors in getting the available fault current above and beyond the standard rating of a 200amp(main) and series rated panel.

In my setting of work (industrial) almost everything is calculated and drawn from an engineering firm.

[/QUOTE]
This discussion been very enlightening, thanks. From a residential perspective most jobs I've been on I've never heard a discussion of fault rating. Most just grab a 200 amp box off shelf and put it on. Haven't even seen the Ka nor looked for it on a res panel. Always thought it was more a industrial or commercial concern, now have to look into it.
For residential service does POCO already accommodate for Ka for residential, perhaps why residential panels seem to be not prominently addressing the info? Never even been asked at supply house when asking for ie a 20A single pole what Ka rating.

 

[Dsg319]

This discussion been very enlightening, thanks. From a residential perspective most jobs I've been on I've never heard a discussion of fault rating. Most just grab a 200 amp box off shelf and put it on. Haven't even seen the Ka nor looked for it on a res panel. Always thought it was more a industrial or commercial concern, now have to look into it.
For residential service does POCO already accommodate for Ka for residential, perhaps why residential panels seem to be not prominently addressing the info? Never even been asked at supply house when asking for ie a 20A single pole what Ka rating.
[/QUOTE]
I never really thought much about it until I watched a YouTube video of Mikes talking about the subject.

I’ve never been taught it in the field(maybe just never caught it), nor learned about it in journeyman study materials when I was studying for my test.

As for your other questions I am also curious myself. As earlier in the thread some guys mentioned it was rarely necessary during residential on single family dwellings unless it’s a “Biggen” or multi family living complex.

But still would be probably a good idea if you had available info to plug in the numbers and see if they actually work with standard off the shelf 200amp panels and breakers.

 

[kwired]

This discussion been very enlightening, thanks. From a residential perspective most jobs I've been on I've never heard a discussion of fault rating. Most just grab a 200 amp box off shelf and put it on. Haven't even seen the Ka nor looked for it on a res panel. Always thought it was more a industrial or commercial concern, now have to look into it.
For residential service does POCO already accommodate for Ka for residential, perhaps why residential panels seem to be not prominently addressing the info? Never even been asked at supply house when asking for ie a 20A single pole what Ka rating.
[/QUOTE]
Residential applications simply seldom ever have more than 22-25kA available, which is what the main breakers will likely be rated for in a typical loadcenter. Also not too likely to find plug on miniature breakers in stock that are over 10kA - they just don't move that inventory. Might be a little more common to find a 100 amp two or three pole in say 22-25kA versions mostly to be used as backfed mains.