Service Fault 65k plus Motor Contributions?

[MarkRobisonPE]

Utilities often give a service fault of 65,000 amps and the plan reviewers require this number be used for the service even though there is a drop in amps due to the service feeders. If you add motor contributions, the number is always over 65,000 amps, which requires heavier duty gear. Is is common to not include motor contributions at the service when the fault is arbitrarily givin as 65,000 amps?

 

[mayanees]

NO

NO

No Mark, you can't arbitrarily neglect motor contributions because the Utility is generalizing about the service rating. Because, of course, all you have to go by is what they're telling you, and if it ends up being close to 65kA, you're going to be over.
You need to qualify the number they're giving you in order to engineer the system for whatever condition it operates in.
That may mean scoping out the distribution system to make your own approximation. But find some way to rationalize a more realistic number, and document it.
It could be that you apply pressure to the Utility perhaps through your client, using the rationale that they're costing them unnecessarily by not doing a little bit of investigation work.
John M

 

[anbm]

Utilities often give a service fault of 65,000 amps and the plan reviewers require this number be used for the service even though there is a drop in amps due to the service feeders. If you add motor contributions, the number is always over 65,000 amps, which requires heavier duty gear. Is is common to not include motor contributions at the service when the fault is arbitrarily givin as 65,000 amps?

I think if you have 480V/3PH main service, depending on your utility xfmr size, impedance, your source fault current could be different.


For quick math, after including all your load + motor contribution, your fault current is likely less than 65kAIC. To be more convervative, contact your utility power rep. and get the actual ground fault # at your xfmr and use that number for your calculation. If you are lazy enough like me, 65kAIC should be a good number to go by (-:

 

[bob]

What size transformer is the utility supplying and what is the voltage?

 

[Jraef]

What size transformer is the utility supplying and what is the voltage?

I think he is speaking hypothetically. I too have seen a couple of "lazy" fault current reports from utilities where they know, for example, the available is over 42kA, so they just say "65kA" because they know that is the next level of gear rating available.

So I sympathize with MarkRobinsonPE; a conscientious engineer would have to take that 65kA at face value, then when the connected load contribution is considered, the user may end up having to buy 100kA rated gear (or put in current limiting main OCPDs). I have always been suspicious of that when I see something like 65kA shown on a drawing, because it rarely comes out to be that exact of a number that corresponds to a gear rating. It's more often something like 48kA, or 62.5kA.

I questioned it once on a project for an Army hospital project that had been engineered by the Corps of Engineers and found that the engineer assigned to the task was indeed "cheating". He thought he was doing us a favor by rounding up to 65kA and thereby picking out the gear rating for us. But when we factored in the motor contributions we were at 71kA. So we had to go to 100kA rated MCCs and had to go back for a change order. It was only when the Procurement Officer questioned it that I was able to get the attention of the COE. Turned out the AFC was really only 48kA.

 

[charlie]

We post ours in our Goldbook and do not go off the deep end in that manner. However, we do use the lowest % impedance of each size transformer we have on our system, assume a bolted fault and infinite bus, plus added 10% variance for the nameplate impedance. If you want it closer, we will provide information for the selection off PPE that may be used for the selection of equipment. :smile:

 

[weressl]

No Mark, you can't arbitrarily neglect motor contributions because the Utility is generalizing about the service rating. Because, of course, all you have to go by is what they're telling you, and if it ends up being close to 65kA, you're going to be over.
You need to qualify the number they're giving you in order to engineer the system for whatever condition it operates in.
That may mean scoping out the distribution system to make your own approximation. But find some way to rationalize a more realistic number, and document it.
It could be that you apply pressure to the Utility perhaps through your client, using the rationale that they're costing them unnecessarily by not doing a little bit of investigation work.
John M

You should also take the tuility to task of supplying accurate data for both maximum and minimum short circuit contribution availablity for your acr-flash calculations.