Do you match Available Fault Current to AIC?

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egurdian3

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What is the recommended practice for installing equipment at the fault location? i.e.: If you have a Fault Current = 24,571A, do you install a panelboard rated @ 25kAIC or do you go to the next AIC rating? say, 42kAIC @ 240V .
 
Keep in mind that fault current can change over time (Adding motors, changes in utility vaules, etc..) so going up to the next rating is smart.
 
egurdian3 said:
What is the recommended practice for installing equipment at the fault location? i.e.: If you have a Fault Current = 24,571A, do you install a panelboard rated @ 25kAIC or do you go to the next AIC rating? say, 42kAIC @ 240V .
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Technically you need to compare the actual fault current and X/R ratio against the protective device's tested AIC and X/R ratio. An adjustment must be made depending on the difference between the X/R ratios. Short Circuit studies usually take this into account, but manufacturer's quick selectors tend to 'gloss over' it.

Some experienced engineers will specify a margin between SCA and AIC (e.g. 10-25%).
 
You can also make the supply conductors longer to get a lower available fault current at the load end. It is done sometimes. If transformer is not set yet, moving it farther away if you can may be easiest way to deal with this.
 
egurdian3 said:
What is the recommended practice for installing equipment at the fault location? i.e.: If you have a Fault Current = 24,571A, do you install a panelboard rated @ 25kAIC or do you go to the next AIC rating? say, 42kAIC @ 240V .
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IMO, it is up to the guy designing the system to specify an AIC rating, and not the contractor.

My personal opinion is that if the numbers are that close it is a wise design choice to go up to the next size. but it is still a design choice.
 
petersonra said:
IMO, it is up to the guy designing the system to specify an AIC rating, and not the contractor.

My personal opinion is that if the numbers are that close it is a wise design choice to go up to the next size. but it is still a design choice.
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Sometimes the designer and installer are the same person.
 
If you have your signing supervisors license in Oregon, the state allows you design anything you install, legally. You're on the same level as an engineer. The only difference, from a legal standpoint, is you aren't allowed to design for 3rd parties where an engineer can. Since we do a ton of ag, my bosses do A LOT of designing.
 
Cow said:
If you have your signing supervisors license in Oregon, the state allows you design anything you install, legally. You're on the same level as an engineer. The only difference, from a legal standpoint, is you aren't allowed to design for 3rd parties where an engineer can. Since we do a ton of ag, my bosses do A LOT of designing.
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Simple job of adding one general use receptacle, still has a design phase. May seem like it is an automatic thing, but someone has to decide how it is going to be done no matter how simple it may seem. That is still designing, just not on the same level as what engineers are commonly called for.

That same receptacle has many choices of wiring methods that could be used, decisions can be made as to whether or not to use AFCI, GFCI, TR, WR, low grade receptacle, spec grade, other premium grade, finish color, conductor sizing, overcurrent device selection....
 
Cow said:
If you have your signing supervisors license in Oregon, the state allows you design anything you install, legally. You're on the same level as an engineer. The only difference, from a legal standpoint, is you aren't allowed to design for 3rd parties where an engineer can. Since we do a ton of ag, my bosses do A LOT of designing.
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This is also the case for California.
 
I am not suggesting you need a PE and a fancy design to add a few switches, or a motor here and there, but there does come a point where a close look at the system from the outside is warranted.

An EC is not a good choice IMO for an unbiased look at the overall situation. They will naturally have a bias toward what they want to install that may not be the best option for the end user.
 
petersonra said:
I am not suggesting you need a PE and a fancy design to add a few switches, or a motor here and there, but there does come a point where a close look at the system from the outside is warranted.

An EC is not a good choice IMO for an unbiased look at the overall situation. They will naturally have a bias toward what they want to install that may not be the best option for the end user.
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The same goes to an EE. They have certain brands that they like. Besides, nothing is taught in Univ that even remoatly looks like what we do. Unless you get an AE. But we are beating a dead horse, this has been discussed to the point of kneling in fron of the porcilin god
 
petersonra said:
I am not suggesting you need a PE and a fancy design to add a few switches, or a motor here and there, but there does come a point where a close look at the system from the outside is warranted.

An EC is not a good choice IMO for an unbiased look at the overall situation. They will naturally have a bias toward what they want to install that may not be the best option for the end user.
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A PE is just as likely to have his preferred way of designing things or preferred manufacturers of equipment - even though there is more than one way to get it done.
 
kwired said:
A PE is just as likely to have his preferred way of designing things or preferred manufacturers of equipment - even though there is more than one way to get it done.
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Yes, but the difference is that the owner will usually go with the contractor that gives the lowest bid. And if a contractor ignores code requirements or good design practices, they will have the lowest bid.

But inspections level the playing field somewhat. And I'm not against contractors doing design for smaller or simpler projects. It would be crazy to have an engineer design the addition of a simple receptacle.

But I am amazed at the number of contractors that are working on things like new health care facilities (or even surgery centers) and asking things like : Do I need a backup generator? Or 3 transfer swtiches? Or can I use Romex? :? Or does this building need emergency lights or exit signs?

Maybe I'm reading more into these questions than I should, but I can't imagine there is an engineer involved when the contractor is turning to an online forum to answer these questions. And I can't imagine how these projects ever get through the code review process or inspections and state licensing.
 
steve66 said:
Yes, but the difference is that the owner will usually go with the contractor that gives the lowest bid. And if a contractor ignores code requirements or good design practices, they will have the lowest bid.

But inspections level the playing field somewhat. And I'm not against contractors doing design for smaller or simpler projects. It would be crazy to have an engineer design the addition of a simple receptacle.

But I am amazed at the number of contractors that are working on things like new health care facilities (or even surgery centers) and asking things like : Do I need a backup generator? Or 3 transfer swtiches? Or can I use Romex? :? Or does this building need emergency lights or exit signs?

Maybe I'm reading more into these questions than I should, but I can't imagine there is an engineer involved when the contractor is turning to an online forum to answer these questions. And I can't imagine how these projects ever get through the code review process or inspections and state licensing.
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It is a little hard to believe there is no PE involved with the design of a NEW hospital. I do some work at a small community hospital. Over the past few years we have made a lot of upgrades of old equipment, remodeling of patient rooms, addition of CT scanner, installation of fire pump, replacement of old HVAC. Some day the generator that is at least 30 yrs old is going to need replaced. I will likely have a lot of input into what happens then. When it comes to remodeling patient rooms we have been bringing in required emergency system circuits, and of course installing redundant grounding, neither of which existed in this 60 year old building. Some of it has been done one room at a time with provisions to incorporate other rooms when the time comes to work on those rooms. I would almost guess many PE don't even want to get involved in this project that is happening over several years.

As far as owners going for the lowest bid - that is just the way things often work.

A similar analogy - I can go to specialty stores and buy clothing, household goods, electronics, etc., or I can go to Wal Mart and buy similar items usually at lower price. The items purchased at Wal Mart are not necessarily the same quality, and is reason they cost less. They do have some quality items there also, but not quite the bargain price as the other stuff, it is only less because of purchasing power that Wal Mart has.
 
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