Xfrmr Brain Fart

[Smart $]

Only if its primary

What does it being primary, or secondary for that matter, have to do with it? Since when does the ampacity of a conductor depend on being primary or secondary.

 

[Smart $]

Per my Ugly's, you can fit four #3 copper Teflon (TFE) conductors in a 1" PVC. Now you can pull from end to end, have 100A rating at 75C. It is probably not cost-effective though!

ehh!... the secondary is a 4-wire system. Where ya gonna put the neutral and ground?

 

[tryinghard]

What does it being secondary have to do with it? It's a 3? 4-wire [assuming separately derived] system. I do not think the load on the system compels the neutral to be counted as a CCC. So it's 3 CCC's, a neutral, and a ground. No multiconductor derating necessary.

Kool, I was counting the neutral. This is why I spoke of primary vs secondary

 

[electriciansnet]

PS: The tap boxes should be as far away as possible from the disconnect and the panelboard.[/QUOTE]

Yes I was figuring on 3 CCC's and never counted the nuetral. And this is all hypothetical now but why as far away as possible.

Its out of my hands now but bringing the issue to the owner's PM has got us talking and won me the project and another just like it in another mall so I'll let you guys know the outcome.

p.s. Had 95 amps been adequate I probably wouldn't have thought to consider the 75 deg. rating of the switch gear itself and might have gotten a RED TAG. Thanks to you guys I will pay more attention to the whole circuit for now on. Thanks again!

 

[don_resqcapt19]

Per my Ugly's, you can fit four #3 copper Teflon (TFE) conductors in a 1" PVC. Now you can pull from end to end, have 100A rating at 75C. It is probably not cost-effective though!

Take a look at Table 310.13. You can't use TFE for this application.

 

[Smart $]

PS:

The tap boxes should be as far away as possible from the disconnect and the panelboard.[/QUOTE]

...[/QUOTE]
While it's not written anywhere in the code book, any one who knows a little more than "if you say so" joe on the matter knows that it has to do with the original reason why you are otherwise limited to the 75?C termination rating. The closer the tap boxes are to the 75?C rated gear, the higher conductor temperature of the "90? ampacity" conductors is transferred through the "75?C ampacity" towards the 75?C equipment. Making the transition as far away as possible just reduces the likelihood of the 75? gear's terminations reaching 75?C. IIRC, Jim Dungar said termination ratings are tested out to ten feet. Doesn't mean you have to go to ten feet but it does demonstrate further is better.

 

[electriciansnet]

Oh. Okay well that does makes sense. The architect has sent the PM an email stating that there are pleanty of EC,s on the mall's prefered list that say they can get a 100/3 circuit in a 1" PVC with no problem! I'm challenging them to show me in 310.16.... Unless they plan on using a type Z

 

[tryinghard]

They can, 3 #3's & 1 #8; oops no neutral. 3 #3's 1#14 & 1#8

 

[Smart $]

...Unless they plan on using a type Z

Wire type doesn't relinquish termination limitations on conductor ampacity.

Is this job subject to plan review and inspection? ...because a violation is the only way to get around it, without using the tap box method.

They can, 3 #3's & 1 #8; oops no neutral. 3 #3's 1#14 & 1#8

For a 100A feeder, the minimum size grounded conductor is #8... same as the minimum size EGC. Refer to 215.2(A)(1)... after exceptions... second to last paragraph.

 

[tryinghard]

You really think "plenty of the other electrical contractors" are using a tap box arrangement in their design. They could be I suppose but not likely.

I think the engineer is doing a disservice to their client by not designing this installation, but who knows what their relationship is anyway. The contractor really doesn?t have to remain in business after installing and journeymen can leave the trade meaning the responsibility is lose to leave the design up to the contractors, especially while the engineer is adamantly pressing the limits. This is a design build, but probably lacking submitting the design so its more just do what your supposed to do and don?t be wrong.

 

[tryinghard]

Give them multiple value engineering options in your bid. One of these can include the XFMR located at the kiosk with: 50A breaker to 3 #8?s 1#10 (in ex 1?) to 30KVA 4:1 XFMR to 100A disconnect (for their desire to have 100A circuit). I suggest you also offer one for the circuit sized per ?calculated load?, if they like money this will be the one they sign a contract with you on. Make sure you include one with their exact design for comparison.

If you already have a contract you can still offer value engineering, most often this is greatly appreciated by the client. It appears any application possibility is different then the design criteria in your original post so make the way you want to install it the most attractive.

 

[Smart $]

You really think "plenty of the other electrical contractors" are using a tap box arrangement in their design. They could be I suppose but not likely.

No, I think they are not... they are likely proposing to install a non-compliant system. Whether knowingly or not, is anybody's guess.

I think the engineer is doing a disservice to their client by not designing this installation, but who knows what their relationship is anyway. The contractor really doesn’t have to remain in business after installing and journeymen can leave the trade meaning the responsibility is lose to leave the design up to the contractors, especially while the engineer is adamantly pressing the limits. This is a design build, but probably lacking submitting the design so its more just do what your supposed to do and don’t be wrong.

That's why I asked if this job is subject to a plan review and/or inspection.

 

[electriciansnet]

Xfrmr in Kiosk

Xfrmr in Kiosk

I think I have them sold on the xfrmr in the kiosk but what is the smallest KVA transformer and feeders I can use for a 100 amp panel? According to the calcs I have a con. load of 67.29/ dem. load 58.32. Remember- the primary has to fit in a 1" PVC and we will want the secondary as big as possible but there is a serious space issue in this little kiosk. Can you guys give me your opinion on what is the most ideal app. for this situation? Thanks

 

[Smart $]

I think I have them sold on the xfrmr in the kiosk but what is the smallest KVA transformer and feeders I can use for a 100 amp panel? According to the calcs I have a con. load of 67.29/ dem. load 58.32. Remember- the primary has to fit in a 1" PVC and we will want the secondary as big as possible but there is a serious space issue in this little kiosk. Can you guys give me your opinion on what is the most ideal app. for this situation? Thanks

Questions:

1. Is this going to be or have to be a 100A MB panel, or a 100A-rated panel with a lower rated MB?
2. What is the value unit of your connected/demand load? It certainly can't be in kVA units because you'd need a panel rated higher than 100A?which has a maximum 36 kVA demand.

If the panelboard has a 100A MB which also serves as transformer secondary and secondary tap protection, your transformer output can go as low 80% of the MB rating, or 80A per line... 80A ? 120V ? 3? = 28.8kVA... round up to next higher standard transformer rating of 30kVA.

As such you'd be looking at 4 #3 Cu minimum 75?C rated or better secondary conductors and a #8 EGC/EBJ. Initial calculation for the primary conductors' minimum ampacity puts them at 30kVA/480/1.732=36A, or 3 #8 Cu 75?C+ w/#8 Cu GEC.

 

[electriciansnet]

Answers:

Answers:

I'm pretty sure I'm gona have to go with a 30kva because I think it sizes out to about 25kva which is not available in 3 phase. But, as I posted above, the connected and demand loads are: "con. load of 67.29/ dem. load 58.32.".
(I believe this means I will have a 36a primary which I can feed with #8s easily in the existing 1" PVC under the mall's marble floor.)

They are calling for a 30 circuit MLO panel with a bus rating of 100a.. BUT I know that I will have to provide a disconnect means to the xfrmr in the kiosk because the existing disc. is in another tenant's space 150' away. This is going to require even more room.

 

[electriciansnet]

Hey- can't I cord and plug connect the xfrmr to save space?

 

[electriciansnet]

Oh I forgot to factor in the 1.732 so I guess my primary will be 62.35A, right?

 

[winnie]

But, as I posted above, the connected and demand loads are: "con. load of 67.29/ dem. load 58.32.".

I presume by this that you mean 67/58 amps 3 phase at 208V, rather than single phase load, 480V load, or kva? I belive that was the crux of Smart$'s question.

They are calling for a 30 circuit MLO panel with a bus rating of 100a.. BUT I know that I will have to provide a disconnect means to the xfrmr in the kiosk because the existing disc. is in another tenant's space 150' away. This is going to require even more room.

Your transformer requires secondary protection.

-Jon

 

[electriciansnet]

Offered them to alternates. #1 was as suggested with 30Kva xfrmr re-using just about all the primary and secondary. Opt.2 Re-use primary, install 25Kva xfrmr with 240v single phase secondary. 2-#3, 1#6, 1#8 in exist. underground to single phase, MB breaker in kiosk. I would like to show you drawings but even after reducing, both files are too big to upload.

 

[electriciansnet]

I presume by this that you mean 67/58 amps 3 phase at 208V, rather than single phase load, 480V load, or kva? I belive that was the crux of Smart$'s question.



Your transformer requires secondary protection.

-Jon

Yes my drawing shows it on the kiosk for the otp that includes leaving xfrmr in storeroom.