Xfrmr Secondary Conductors

[mstrlucky74]

If I'm thinking about this correctly I see something wrong with the xfrmr schedule. One line feeder tags show the following for the secondary conductor sizes: T2/1 to be 2 sets 4#500 w/1#2 and T3/1 to be 1 set w/4#500 w/1#2. the transformer schedule calls for each to get 2 sets 4#250 w/2/0G which seems correct.

1. The loads both xfrmrs are feeding are 600A so 600A x125% is 750A. 2 Sets #250 won't work. Need 2 sets 4#500's(which gives me 380A x2=760A) as is called for with T2/1, correct?
2. Both transformers are feeding panels that have MCB's but only T2 is showing a separate disconnect switch.

 

[augie47]

IMO, Code-wise, T1 would require a #4 supply side bonding jumper and T2-T3 a #2 based on 250.30 & 250.102(C).

 

[Ingenieur]

Xfmrs
330 kva
480 vac
i rated 400
125% 500

primary V ?

 

[mstrlucky74]

Xfmrs
330 kva
480 vac
i rated 400
125% 500

primary V ?

xfrmr prim 120/208 sec 277/480

 

[Ingenieur]

xfrmr prim 120/208 sec 277/480

Assuming a supervised location
primary 916 A
sec 397 A

ocpd allowed
prim
prim w/sec 250% 2290 A
prim only 125% 1145 A use 1200

sec
prim w/sec 125% 496 A use 500 A
prim only none

Everything looks ok to me
sec cond 2 sets of 250 kcmil 510 A

 

[mstrlucky74]

Assuming a supervised location
primary 916 A
sec 397 A

ocpd allowed
prim
prim w/sec 250% 2290 A
prim only 125% 1145 A use 1200

sec
prim w/sec 125% 496 A use 500 A
prim only none

Everything looks ok to me
sec cond 2 sets of 250 kcmil 510 A

confused....I don't have to size the secondary conductors to the load it serving...the 600A panel???

 

[Ingenieur]

confused....I don't have to size the conductors to the load it serving...the 600A panel???

Yes and no, we do not know what the pnl load is
just because the bus is 600 doesn't mean connected load is
if the xfmr fuse blows because the pnl load > 500 A that is poor engineering but not a code issue
imo, from the limited info, it looks like competent engineering sans the xfmr sched vs feeder sched typos

 

[augie47]

confused....I don't have to size the conductors to the load it serving...the 600A panel???

r

I this arrangement the panels provide your secondary conductor protection and you would have to size them to you panel main.
Also 240.4(B) does not apply (no "next size up")

 

[Ingenieur]

r

I this arrangement the panels provide your secondary conductor protection and you would have to size them to you panel main.
Also 240.4(B) does not apply (no "next size up")

the 600 A pnl feeder is fused at 500
it has no main as far as I can tell

 

[augie47]

the 600 A pnl feeder is fused at 500
it has no main as far as I can tell

my error,, I saw that a while ago but forgot... I should have made the statement that he needs to size his secondary conductors based on the overcurrent device they supply. Do you agree ?

 

[Ingenieur]

my error,, I saw that a while ago but forgot... I should have made the statement that he needs to size his secondary conductors based on the overcurrent device they supply. Do you agree ?

in the case of the xfmr fed panel with no sec protection yes

 

[mstrlucky74]

the 600 A pnl feeder is fused at 500
it has no main as far as I can tell

Yes, I checked the schedule and it's a MLO. So I went back to review what you said but I'm not clear.

Assuming a supervised location
primary 916 A
sec 397 A

ocpd allowed
prim
prim w/sec 250% 2290 A
prim only 125% 1145 A use 1200

sec
prim w/sec 125% 496 A use 500 A
prim only none

Everything looks ok to me
sec cond 2 sets of 250 kcmil 510 A

Shouldn't the T3 secondary conductors be be 600A x125% for 750A? Thanks

 

[Ingenieur]

Yes, I checked the schedule and it's a MLO. So I went back to review what you said but I'm not clear.

Assuming a supervised location
primary 916 A
sec 397 A

ocpd allowed
prim
prim w/sec 250% 2290 A
prim only 125% 1145 A use 1200

sec
prim w/sec 125% 496 A use 500 A
prim only none

Everything looks ok to me
sec cond 2 sets of 250 kcmil 510 A

Shouldn't the T3 secondary conductors be be 600A x125% for 750A? Thanks

imo no

the xfmr is rated at 400 A
the conductors 2 sets of 250's are rated 510
the circuit is fused at 500 or 1.25 x 400
below the bus rating of 600
imo that is code compliant
the primary could be 250% x 916, but is only 125%

As others have noted if the 'secondary protection' was a 600 A cb in the 600 A pnl the conductors must be >600A

 

[mstrlucky74]

imo no

the xfmr is rated at 400 A
the conductors 2 sets of 250's are rated 510
the circuit is fused at 500 or 1.25 x 400
below the bus rating of 600
imo that is code compliant
the primary could be 250% x 916, but is only 125%

As others have noted if the 'secondary protection' was a 600 A cb in the 600 A pnl the conductors must be >600A

Follow exactly what your saying. Just got a little confused as I'm thinking you have to size to the LOAD....600a.

 

[Ingenieur]

Follow exactly what your saying. Just got a little confused as I'm thinking you have to size to the LOAD....600a.

Just because the bus/lugs are 600 doesn't mean the load is
if it is the xfmr is 50% overloaded

The 500 A fuse protects the conductors and bus
each 'load' off the 600 bus is protected by its own cb

 

[mstrlucky74]

So to sum it up...for the most part you size xfrmr secondary conductors based on the xfrmr KVA(converted to amps) *125%?

 

[Ingenieur]

So to sum it up...for the most part you size xfrmr secondary conductors based on the xfrmr KVA(converted to amps) *125%?

Basically yes assuming a continuous load
but the ocpd must comply with tables 450