Residential wiring and available fault current ratings

[electrofelon]

How does the series rating work?

You can Google it and I'm sure pull up a bunch of explanations. Basically the two devices working/opening together can result in the downstream device not needing to be fully rates for the available fault current. You can Google "Siemens series ratings" (or whatever manufacturer) and see the charts. These have to be tested to confirm they work by the manufacturer. Many/most common beakers will have series ratings. There is usually a limitation in how different the frame sizes of the two circuit breakers can be. For example it is very common to have a series rating with a 200/250 amp frame breaker with a AFC of even 65k, with a standard off the shelf 10k breaker, but go up to 400 amp frame for the first device and there are few or none - you would likely need to go up to 22k branches in that case.

 

[Dsg319]

You can Google it and I'm sure pull up a bunch of explanations. Basically the two devices working/opening together can result in the downstream device not needing to be fully rates for the available fault current. You can Google "Siemens series ratings" (or whatever manufacturer) and see the charts. These have to be tested to confirm they work by the manufacturer. Many/most common beakers will have series ratings. There is usually a limitation in how different the frame sizes of the two circuit breakers can be. For example it is very common to have a series rating with a 200/250 amp frame breaker with a AFC of even 65k, with a standard off the shelf 10k breaker, but go up to 400 amp frame for the first device and there are few or none - you would likely need to go up to 22k branches in that case.

Thanks I will have to research it some!

 

[Dsg319]

Often AIC calculations are omitted in residential, and frankly, seldom necessary. But, they are also extremely easy. The Bussman app is great, I use it a lot, but you don’t even need that for residential. Most utilities have a fault current chart in their service manuals that will give you the fault current at the transformer secondary, and with various combinations of service drop sizes/lengths. All you normally have to do is take a quick peek at the utilities chart to verify that you are under 10k, and you’re done. In a few rare circumstances you may need to make a quick call to the utility.

Pertaining to the app..... I see to find the short circuit available at the transformer, than if you want to add conductors say to the service disconnect is that where you just add the conductors size and length on the app to get the shirt circuit rating for the service disconnect?

 

[kwired]

My understanding is due to typical single phase transformer construction, the L-N is typically higher. There is a somewhat common rule of thumb to use 1.5 times the L-L for the L-N for single phase

L-N is higher at the transformer and for very short distances if you have larger conductors, otherwise conductor impedance usually results in L-L being higher by the time you get to your service disconnect unless it is a very short, high ampacity run.

Just 25 feet of ~200 amp conductor has often has enough impedance to make L-L the higher number, but also will reduce a pretty high available current to less than 10K but even if not that low likely less than the 22 or 25 K main breaker and the 10 K branch breakers are series rated anyway.

 

[Dsg319]

Often AIC calculations are omitted in residential, and frankly, seldom necessary. But, they are also extremely easy. The Bussman app is great, I use it a lot, but you don’t even need that for residential. Most utilities have a fault current chart in their service manuals that will give you the fault current at the transformer secondary, and with various combinations of service drop sizes/lengths. All you normally have to do is take a quick peek at the utilities chart to verify that you are under 10k, and you’re done. In a few rare circumstances you may need to make a quick call to the utility.

Also with this app. Would would be the most common and accurate percentage to use for the tolerance?

 

[kwired]

Pertaining to the app..... I see to find the short circuit available at the transformer, than if you want to add conductors say to the service disconnect is that where you just add the conductors size and length on the app to get the shirt circuit rating for the service disconnect?

Transformer output has a certain amount of current that can flow in a short circuit, based on impedance and VA rating of the transformer, something that is a factor but often unknown is how much can the primary deliver? If primary has too much impedance then secondary will not be able to draw what it would if primary had indefinite ability - but often we assume an indefinite primary for worst possible case scenario anyway, plus if POCO changes things over the years these primary conditions may change anyway.

Once you know what the transformer can put out, next step is find out how much limitation exists in conductors between transformer and first point of interest (usually the service equipment) This varies because of conductor size, type and length, plus whether run in magnetic conduit or not. Picture a 2 inch water main with a certain GPM at a certain pressure - tap onto it with only a 1/2 inch line and you will not get that same GPM at same pressure. Conductors have resistance also, smaller they are the more resistance they have. And one that can carry 30KA when a short circuit occurs is probably rather large and/or short length. One that is only rated for 200 amp or less has too much resistance to carry 30 KA even at only 10-15 feet of length, though I plugged some numbers into a fault current calculator and 15 feet or 3/0 copper was still giving about 25KA when starting with 30, but increase to 50 feet and it drops to 16K.

Then for a sub panel, you typically have even smaller conductor than the service had, and you start calculation all over with whatever was available at service as the starting point when figuring what is available at the load end of the feeder.

In dwellings you often have at least 50 maybe 100 feet of service drop or lateral, often the transformer is no larger than 50 KVA which maybe only has around 10KA available line to line at the transformer terminals to begin with. It will only be less in the dwelling. Large McMansion or large multifamily units are main cases where available current maybe is something that needs serious attention, but usually only at the service equipment or a very nearby distribution panel.

 

[Dsg319]

Transformer output has a certain amount of current that can flow in a short circuit, based on impedance and VA rating of the transformer, something that is a factor but often unknown is how much can the primary deliver? If primary has too much impedance then secondary will not be able to draw what it would if primary had indefinite ability - but often we assume an indefinite primary for worst possible case scenario anyway, plus if POCO changes things over the years these primary conditions may change anyway.

Once you know what the transformer can put out, next step is find out how much limitation exists in conductors between transformer and first point of interest (usually the service equipment) This varies because of conductor size, type and length, plus whether run in magnetic conduit or not. Picture a 2 inch water main with a certain GPM at a certain pressure - tap onto it with only a 1/2 inch line and you will not get that same GPM at same pressure. Conductors have resistance also, smaller they are the more resistance they have. And one that can carry 30KA when a short circuit occurs is probably rather large and/or short length. One that is only rated for 200 amp or less has too much resistance to carry 30 KA even at only 10-15 feet of length, though I plugged some numbers into a fault current calculator and 15 feet or 3/0 copper was still giving about 25KA when starting with 30, but increase to 50 feet and it drops to 16K.

Then for a sub panel, you typically have even smaller conductor than the service had, and you start calculation all over with whatever was available at service as the starting point when figuring what is available at the load end of the feeder.

In dwellings you often have at least 50 maybe 100 feet of service drop or lateral, often the transformer is no larger than 50 KVA which maybe only has around 10KA available line to line at the transformer terminals to begin with. It will only be less in the dwelling. Large McMansion or large multifamily units are main cases where available current maybe is something that needs serious attention, but usually only at the service equipment or a very nearby distribution panel.

Awesome info thanks! Do you also have the bussman app? I’ve had it for a while never had to use it but been wanting to learn about available fault current and short circuit ratings so I started toying around with it.

 

[Dsg319]

Also, how does one go about doing a transformer bank? Such as 3 single phase transformers used in a 3phase operation. Is impedance added up or averaged?

 

[kwired]

Also, how does one go about doing a transformer bank? Such as 3 single phase transformers used in a 3phase operation. Is impedance added up or averaged?

I'm not altogether certain on that one. If all you are interested in is knowing maximum fault current that is available then just using the impedance of marked on one unit should give you a worst case result though, actual values are likely lower.

If it is a 120/240 delta I sort of always assume you treat it like single phase 120/240, and don't see why it should give you anything different other than on the high leg.

A 208/120 wye I would think would give you same result on a single line to ground fault as if you were just using just one of those units as a single two wire source. For a line to line fault between just two lines I would think gives you 2 times individual transformer impedance, three phase bolted fault not for certain but seems you likely need to multiply individual transformer impedance by square root of three.

Different size transformers (like is common with delta systems sometimes) probably gets more complicated.

 

[electrofelon]

Another comment on this topic: one common issue is you do not know the transformer impedance and or size. Either the transformer is on a pole, not on site yet, or you may not even know where the serving transformer is. Sometimes it's a hassle to get the data from the POCO and when you do, they usually provide a generic figure that is not very accurate. For the few cases I have run across where I got both actual nameplate data and Poco data, POCOs figure was 150-200% higher than actual. There are times where you want to consider the value the POCO gives, it depends on the situation. Knowing ballpark/typical impedance values definitely can come in handy.

A few examples.

Recent job is a 4 unit apartment Reno. Meter center with 5 meters and disconnects. That area is funky and has several larger banks and a larger one serving multiple houses. I told the gear guy to just sent me qo2100VH's (22kaic) for the mains instead of the regular 10k ones because I didn't want to spend the time trying to track down the specifics. Actually I asked for the price first and there was no additional charge to supply the VH's.

Another one. Commercial job, 1000 amp 120/208 existing but need to upgrade equipment. POCO says 110k AFC. Vault very close to service equipment with big wire so not much lower at gear terminals. I call BS on it and want to get below 65k, so i go in the vault and look at the data plate. Turns out it's about 60k at the gear, good. I snap a photo of the data plate for the plan reviewer. In this case the transformers were dedicated to this client, very large (exceeding the service size, previous owner was a printing company, guessing they had some large motors), and had some of the lowest impedance I have ever seen, so I had no issue with using actual value over the POCO value.

 

[Dsg319]

Another comment on this topic: one common issue is you do not know the transformer impedance and or size. Either the transformer is on a pole, not on site yet, or you may not even know where the serving transformer is. Sometimes it's a hassle to get the data from the POCO and when you do, they usually provide a generic figure that is not very accurate. For the few cases I have run across where I got both actual nameplate data and Poco data, POCOs figure was 150-200% higher than actual. There are times where you want to consider the value the POCO gives, it depends on the situation. Knowing ballpark/typical impedance values definitely can come in handy.

A few examples.

Recent job is a 4 unit apartment Reno. Meter center with 5 meters and disconnects. That area is funky and has several larger banks and a larger one serving multiple houses. I told the gear guy to just sent me qo2100VH's (22kaic) for the mains instead of the regular 10k ones because I didn't want to spend the time trying to track down the specifics. Actually I asked for the price first and there was no additional charge to supply the VH's.

Another one. Commercial job, 1000 amp 120/208 existing but need to upgrade equipment. POCO says 110k AFC. Vault very close to service equipment with big wire so not much lower at gear terminals. I call BS on it and want to get below 65k, so i go in the vault and look at the data plate. Turns out it's about 60k at the gear, good. I snap a photo of the data plate for the plan reviewer. In this case the transformers were dedicated to this client, very large (exceeding the service size, previous owner was a printing company, guessing they had some large motors), and had some of the lowest impedance I have ever seen, so I had no issue with using actual value over the POCO value.

Yeah I can see it being a hassle trying to receive info from other people and POCO. Thanks for the information.
I’ve never yet personally had to deal with it or even seen somebody do the calculations. Not saying they didn’t just never seen it.
Still wanted learn how to do it myself.

Havnt yet done anything in paper. Only the fault current app I have which I will just continue to use. Not sure if there are fault current calculations on a master test in West Virginia or not but probably wouldn’t hurt to learn how to do a few on paper.

 

[jim dungar]

Also, how does one go about doing a transformer bank? Such as 3 single phase transformers used in a 3phase operation. Is impedance added up or averaged?

Unless there are huge differences you can simply take 3x the lowest single value for worst case 3 phase result.

 

[Dsg319]

Unless there are huge differences you can simply take 3x the lowest single value for worst case 3 phase result.

Thanks

 

[kwired]

Yeah I can see it being a hassle trying to receive info from other people and POCO. Thanks for the information.
I’ve never yet personally had to deal with it or even seen somebody do the calculations. Not saying they didn’t just never seen it.
Still wanted learn how to do it myself.

Havnt yet done anything in paper. Only the fault current app I have which I will just continue to use. Not sure if there are fault current calculations on a master test in West Virginia or not but probably wouldn’t hurt to learn how to do a few on paper.

Don't know what app you have, probably ok though. There is a downloadable spreadsheet on Mike Holts main website under "free stuff" IIRC. I think that is somewhat widespread recognized tool to use. You can rename and save for each project you do for your records. If you don't have Microsoft Office it does work in the free "Open office"

Below are screen snips of it using figures I mentioned in a previous post in this thread. It does allow you to figure available current from the source (assuming infinite primary ability) by entering kVA and impedance, or if you already have a known starting fault current level you can plug it in instead.

It has spaces on the sheet for next two levels downstream, say a subpanel then a branch circuit. Need more levels start a new sheet with available current at starting point at the utility fault current portion at top of the page.

Notice how on this one L-N available current was higher than L_L at the service (mostly because of short length of conductors) but as we got further away not only did both L-N and L-L get lower but L-N did become less than L-L.

2% impedance I have there probably typically will be higher in the field on a 50 KVA transformer, but not necessarily a lot higher. 25's and 37.5's is somewhat safe to assume they will be near 2.0% worst case some will be higher.

 

[Dsg319]

Don't know what app you have, probably ok though. There is a downloadable spreadsheet on Mike Holts main website under "free stuff" IIRC. I think that is somewhat widespread recognized tool to use. You can rename and save for each project you do for your records. If you don't have Microsoft Office it does work in the free "Open office"

Below are screen snips of it using figures I mentioned in a previous post in this thread. It does allow you to figure available current from the source (assuming infinite primary ability) by entering kVA and impedance, or if you already have a known starting fault current level you can plug it in instead.

It has spaces on the sheet for next two levels downstream, say a subpanel then a branch circuit. Need more levels start a new sheet with available current at starting point at the utility fault current portion at top of the page.View attachment 2554622View attachment 2554623View attachment 2554624

Notice how on this one L-N available current was higher than L_L at the service (mostly because of short length of conductors) but as we got further away not only did both L-N and L-L get lower but L-N did become less than L-L.

2% impedance I have there probably typically will be higher in the field on a 50 KVA transformer, but not necessarily a lot higher. 25's and 37.5's is somewhat safe to assume they will be near 2.0% worst case some will be higher.

Thanks I’ll have to check those charts out on his website.
I still need to work on (continue researching)understanding the series rating and current limiting devices so can have lower rated branch circuits below.

May never be in the situation to have to but I’d like to learn it for my own sake. I have the EATON BUSSMAN app. It’s pretty simple.

 

[jim dungar]

I still need to work on (continue researching)understanding the series rating and current limiting devices so can have lower rated branch circuits below.

IMO. For series ratings concentrate on the literature from breaker manufacturers as they have a vested interest in presenting unbiased information concerning breaker-breaker options.
Series ratings have been in the industry for almost 40 years, but I am amazed how much misinformation is still being passed around.

 

[Carultch]

How does the series rating work?

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.

Also doing a short circuit calculation for a sub panel. Would it be done the same way? Or anything different.

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.

 

[kwired]

Thanks I’ll have to check those charts out on his website.
I still need to work on (continue researching)understanding the series rating and current limiting devices so can have lower rated branch circuits below.

May never be in the situation to have to but I’d like to learn it for me sake. I have the EATON BUSSMAN app. It’s pretty simple.

Don't get too hung up on series ratings and current limiting devices, they aren't always intended for what people think they are.

Bottom line is something like a standard 10kA breaker is only series rated with devices the manufacturer has tested and listed them for.

Often one manufacturer will test them with certain common fuse types as well as what they feel would be somewhat typical feeder device that they happen to manufacture supplying it.

Typical loadcenter with main breaker as already mentioned earlier is usually 22 or 25kA main with standard branch breakers for that panel being series rated for use with that main.

Where you are maybe more likely to get into trouble in a dwelling setting is in multifamily with a large service entrance, and maybe exceeding the AIC of feeder breakers that may not be series rated with the service main, or also in same applications the dwelling units that are closest to the electrical distribution could also have some situations where not high enough rating or not series rated for what is there.

But do take note a breaker doesn't need to be series rated with whatever is ahead of it if there is small enough conductor and/or enough length that there isn't more available current at the device then it is rated for.

 

[jim dungar]

It has to do with the relative timing time-current curves of the OCPD's.

No, it primarily has to do with the dynamic impedance of the arc being interrupted. The testing procedure also comes into play.
Series ratings and selective coordination are not always mutually exclusive. Often the downstream breaker is the device clearing the fault.

For example if the standard UL test ratings are 10kAIC or 22kAIC I cannot list my breaker at 18kAIC, even though it passes at that level. However if I go to a series combination test I could probably get an18kAIC rating regardless of the upstream device performance.

 

[kwired]

No, it primarily has to do with the dynamic impedance of the arc being interrupted.
Series ratings and selective coordination are not always mutually exclusive. Often the downstream breaker is the device clearing the fault.

Also remember if that fault is a hundred feet downstream from the breaker in question the available current at that point in the circuit is reduced even further by the characteristics of conductors of that circuit, and that is a major factor in limiting how much current flows in the entire path from the fault back to the source. should the fault occur right near the breaker in question the current may be a lot higher.