Xfrmr Brain Fart

Status
Not open for further replies.
E

electriciansnet

Member
I'm bidding a kiosk. The architect is showing a 100 amp/ 3 phase/ 208-120 panel in the kiosk. Well the existing conduit is only 1" and its in the slab under marble floors. This is the International Plaza in Tampa and it is very high-end. There is no way trenching is an option and the mall has expressed they will never allow it but he is insisting "Replace the pipe!". The demand load is 58.32.
So I started looking a little harder at his riser just to get all the options for the owner.
He has this 100 amp panel fed from a 45KVA transformer and has us ripping out a perfectly good 35kva transformer and it's breaker and set of 60 amp feeders and disconnect to be replaced by: Primary- a very expensive Square D 'KCL' 80A breaker, 3-#3s & 1-#8 in 1-1/4C , 80a disconnect to a 45kva transformer. Secondary- Xfrmr to 100a disconnect (necessary for remote room), 4-#3 & #8 in 1-1/4 (replaces the 1" under marble) to 100 amp kiosk panel.

Q1
Now since power in = power out and I take 100 amp secondary x 208v (s) / 480v (p) I come up with 43.33... do I have to multiply by 1.732 bringing it to 75.05? No right?
The 60 amp primary should work, right?
Q2
What would you guys do to solve the pipe fill problem? My recommendation was to move a smaller transformer to the kiosk and send 480 out to it.
 
George Stolz

George Stolz

Moderator
Staff member
Location
Windsor, CO NEC: 2017
Occupation
Service Manager
electriciansnet said:
There is no way trenching is an option and the mall has expressed they will never allow it but he is insisting "Replace the pipe!".
Click to expand...
Sounds like the mall and the engineer need to come up with the resolution, based on what you've said, but you haven't said much about the contractual particulars. You can't obey two different parties directly telling you to do two opposite tasks.

But, playing along, how big is this kiosk that you're going to hide a transformer in it? :-?
 
tryinghard

tryinghard

Senior Member
Location
California
electriciansnet said:
...Q2
What would you guys do to solve the pipe fill problem? My recommendation was to move a smaller transformer to the kiosk and send 480 out to it.
Click to expand...

This could work 30KVA fed by 50A OCPD & 4-6's THHN 1-8 all cu, if the kiosk calcs less than 80A. The kiosk will need a main breaker no larger than 100A in this scenario.

It looks like the 45KVA can work the same but with 60A OCPD (same feed), this will enable the kiosk to calc right up to 124A
 
Last edited:
E

electriciansnet

Member
George- I agree but I'm sure I'm not the only bidder that sees no good reason to replace an adequate primary and most time these guys don't compare apples for apples meaning that if I simply bid to the drawing I will just be passed over and the other guy who took the liberty to adjust wins the bid. The secondary will not work because of the pipe so im thinking I can provide the most cost affective resolution (including leaving primary alone) and qualify it.
The kiosk is very space limited to answer your question though.
______________
Tryinghard- Can you show me your calculation? Because I'm missing how the kva sizing is derived. (ExIx1.73/1000)
(208x100=20800*1.73=35984/1000=36A).
Also here is his load calcs: Con. amps 67.29, Dem. amps 58.32.
 
tryinghard

tryinghard

Senior Member
Location
California
electriciansnet said:
George- I agree but I'm sure I'm not the only bidder that sees no good reason to replace an adequate primary and most time these guys don't compare apples for apples meaning that if I simply bid to the drawing I will just be passed over and the other guy who took the liberty to adjust wins the bid. The secondary will not work because of the pipe so im thinking I can provide the most cost affective resolution (including leaving primary alone) and qualify it.
The kiosk is very space limited to answer your question though.
______________
Tryinghard- Can you show me your calculation? Because I'm missing how the kva sizing is derived. (ExIx1.73/1000)
(208x100=20800*1.73=35984/1000=36A).
Also here is his load calcs: Con. amps 67.29, Dem. amps 58.32.
Click to expand...

electriciansnet, I recommend an RFI if this project is in the formal bid stage, if done right it will be included with the bid docs. Remember to make your RFI a question not a statement.

Line current can be determined by I = VA / (E x 1.732), your formula can be used to determine apparent power and you are using a projected (unknown) ampere. In the light of 450.3(B) the transformers overcurrent protection should be sized at 125%, you need to be mindful of this for the feeder to fit in the 1?, but I guess the transformer can be oversized as long as the OCP and feeders are sized at least per calculated load.

I don?t know what you mean with ?Dem. Amps 58.32? but if ?Con. Amps 67.29? are the calculated amperes from Article 220 use this as the minimum base limit meaning: 69.39A = 25KVA / (208V x 1.732), revealing a 25KVA transformer could be used again at minimum. In this scenario the kiosk main breaker in the sub will be a 90A and the primary 480V circuit to the kiosk would be 40A with 3-#8?s and 1-#8 grnd all cu THWN minimum.

Due to the 1? conduit size your limited up to 3-#4?s, 1-#8 (neutral) & 1-8 grnd all cu THWN at 70 amperes maximum for the secondary. In summary the conduit will limit the secondary to 70 amperes due to its size unless they move the transformer to the kiosk area. Though the 45KVA is rated for a 70A primary circuit you can undersize the circuit to 50A and obtain the 70A OCP at the kiosk but only if the neutral is calced and downsized otherwise its all 6?s with #8 grnd and 60A circuit. The 1? conduit is kinda driving the application.
 
Smart $

Smart $

Esteemed Member
Location
Ohio
tryinghard said:
...

Due to the 1? conduit size your limited up to 3-#4?s, 1-#8 (neutral) & 1-8 grnd all cu THWN at 70 amperes maximum for the secondary. In summary the conduit will limit the secondary to 70 amperes due to its size unless they move the transformer to the kiosk area. Though the 45KVA is rated for a 70A primary circuit you can undersize the circuit to 50A and obtain the 70A OCP at the kiosk but only if the neutral is calced and downsized otherwise its all 6?s with #8 grnd and 60A circuit. The 1? conduit is kinda driving the application.
Click to expand...
This is one area where using full 90?C ampacity of conductors can be advantageous...

90Cexample.gif
 
E

electriciansnet

Member
I'm sorry. I think see where your going with this and how using the tap box may work in my favor and I'm in the table which is about adjustment factors for >3 conductors but I'm not picking up on 90 deg. wire based on 75 deg. ampacity.

Even using THHN, I still dont see how I can get a 100A circuit in a 1"
 
beanland

beanland

Senior Member
Location
Vancouver, WA
600V Option

600V Option

If the problem is conduit size, in order to use the smallest conductors possible, use the highest voltage possible. SquareD and others make 600V transformers. Raise the voltage to 600V and then run a 600V feeder to a 600V-208Y/120V transformer at the kiosk.

I have done this where I need to move 10kVA 3000 feet. Raise the voltage to 600V. Smaller wire, lower losses, reasonable cost.
 
E

electriciansnet

Member
Beanland- that was my 1st recommendation (top thread) even though it is a kiosk and very space-limited.

My 2nd was to go to a single phase secondary since there is only 1 circuit @ 274 watts on c phase, space and no 3 phase loads.

But, as George said, it's really up to the architect and owner and not me. I'm pretty sure I have this but I wanted to make sure I had the same alternate methods down as the other bidders.

I was just intrigued by the tap box, 75 deg. / 90 deg. concept.
 
tryinghard

tryinghard

Senior Member
Location
California
electriciansnet said:
I'm sorry. I think see where your going with this and how using the tap box may work in my favor and I'm in the table which is about adjustment factors for >3 conductors but I'm not picking up on 90 deg. wire based on 75 deg. ampacity.

Even using THHN, I still dont see how I can get a 100A circuit in a 1"
Click to expand...

You can't without the 90? tap, the max is 70A with a downsize neutral and 60A without downsizing the neutral.

And with the 90? tap you still can't, the max circuit would be 3-#4?s 1-#8 (downsized neutral) & 1-#8 grnd and 70A OCP. I don't think 240.4(B) applies when using the adjustment factors.

The adjustments you do from Table 310.15(B)(2)(a) are applied to the appropriate column of conductor type used i.e. THHN is 90? rated therefore >3 CCC?s means #4?s rating adjusts by 80% of 95A which is 76 allowable ampacity.

Your RFI should include the fact that a 100A circuit is not possible with existing design and 1? conduit system.
 
Smart $

Smart $

Esteemed Member
Location
Ohio
tryinghard said:
...

The adjustments you do from Table 310.15(B)(2)(a) are applied to the appropriate column of conductor type used i.e. THHN is 90? rated therefore >3 CCC?s means #4?s rating adjusts by 80% of 95A which is 76 allowable ampacity.
Click to expand...
Why are you counting the circuit as 4 CCC. I don't see any reason to do so... so no multi-conductor derating.

This is not a service nor secondary tap conductors, so 95A wire is permitted on 100A OCPD, with the calculated load below 95A.

...and dare I mention that this is an underground run where the ambient temperature is likely less than 30?C, so adjustment for ambient would actuall increase the ampacity of the conductors.
 
Smart $

Smart $

Esteemed Member
Location
Ohio
electriciansnet said:
... but I'm not picking up on 90 deg. wire based on 75 deg. ampacity.
Click to expand...
From tap boxes to disconnect and panelboard you will have to use the 75?C rating of the wire regardless of being rated higher than 75?C. Thus for a 100A (or rather 95A...) circuit, you have to use #3 Cu with 75?C-rated insulation or better.

PS: The tap boxes should be as far away as possible from the disconnect and the panelboard.
 
Last edited:
Smart $

Smart $

Esteemed Member
Location
Ohio
electriciansnet said:
This is intreuging. Can you give an example? I'm going to let the architect deal with their kiosk problem but I'm just curious about how the tap box would help. Thanks
Click to expand...
It's pretty much straightforward....

Between tap boxes you can use 90?C rated wire at it full ampacity potential because all terminations at both ends are 90?C rated. In the disconnect and panelboard the termination ratings are 75?C... so you are limited to the 75? ampacity of the conductor even when you are using 90?C rated wire.

In other words, #4 THWN-2 can be used in your kiosk situation between tap boxes at a pre-adjusted ampacity of 95.
 
beanland

beanland

Senior Member
Location
Vancouver, WA
Totally Not-Cost-Effective Option

Totally Not-Cost-Effective Option

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!
 
Smart $

Smart $

Esteemed Member
Location
Ohio
tryinghard said:
Smart $ said:
Why are you counting the circuit as 4 CCC. I don't see any reason to do so... so no multi-conductor derating.
Click to expand...
Becaues the current design is secondary to the Kiosk.
Click to expand...
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.
 
Status
Not open for further replies.
Top