300 KVA Transformer Calc

[Vlietracer64]

I have a location that wants a 800 Amp 120/208v of current 480v gear.

They have drawn up and requested a 480V 350Amp breaker to feed 300KVA transformer feeding a main lug 800-Amp panel.
They didnt account for 125%
Calculated at 350A 480V primary i get a KVA of 290.976, that gives me a 807.69 Amp on secondary, can this still feed a 800-amp main lug panel or do i need 800-amp main breaker to protect the buss,
feeders are listed as parallel 500 MCM 90*C for 430 amps

Thanks in advance

 

[infinity]

The secondary conductors require protection as does the panel. If you go with an 800 amp Main or other 800 amp OCPD then the conductors need to be rated for at least 800 amps which makes parallel 500 kcmil (380*2=760 amps) too small. You cannot use the 90° C ampacity. Parallel 600 kcmil conductors would work.

 

[Vlietracer64]

The secondary conductors require protection as does the panel. If you go with an 800 amp Main or other 800 amp OCPD then the conductors need to be rated for at least 800 amps which makes parallel 500 kcmil (380*2=760 amps) too small. You cannot use the 90° C ampacity. Parallel 600 kcmil conductors would work.

Trying to grasp the 10 and 25' tap ruels, correct me if im wrong;

The instance above they have 300KVA 480 Delta Primary to 120/208 Y secondary looking to power a 800-Amp 120/208 Distribution Panel

Primary is 480V off 350Amp Breaker although it could be a 450Amp at the 125% but spec says there just looking to feed the 800-Amp secondary

Secondary 120/208V 800-Amps, calculated to 807.69amps with 350-Amp 480 input = KVA of 290.98



1. 10-Foot conductor max = No OCPD on secondary as long as the calculated load and conductors, panel board is rated for calculated load. (not using this just want to better understand it, seem this would be more useful if transformer was connected directly to a single piece of equipment)
Do you have an example of this, im thinking you could take project above and run parallel 600mcm in conduit from transformer to a main lug 800-amp panel board following 408.36 exceptions to 240.21(C)(2)


2. 25-Foot conductor max = I can use the 800-Amp main breaker in panel board as secondary protection for the transformer
Example: Using project above Parallel 600MCM in conduit up to 25' conductor between transformer and 800-Amp main breaker would be compliant.

3. Over 25-Foot conductor = OCPD within 10' of transformer to protect secondary and then
Example: Using project above 800-Amp fused disconnect at transformer secondary and then parallel 600mcm ran 80' to 800-amp main lug panel board. OCPD at secondary of transformer protect wire and panel board for exceeding 800-Amps

 

[augie47]

Once you have all that straight in your head, you will find 250.92

 

[electrofelon]

1. 10-Foot conductor max = No OCPD on secondary as long as the calculated load and conductors, panel board is rated for calculated load. (not using this just want to better understand it, seem this would be more useful if transformer was connected directly to a single piece of equipment)
Do you have an example of this, im thinking you could take project above and run parallel 600mcm in conduit from transformer to a main lug 800-amp panel board following 408.36 exceptions to 240.21(C)(2)

Regarding Red: I dont think that is the intent of 240.21(C)(2)(1)(b), to allow the secondary conductors to land on a MLO panelboard. The wording isnt the best.

Regarding Blue: I dont see any exceptions in 408.36 that would let you off the hook for protection.

 

[Vlietracer64]

Regarding Red: I dont think that is the intent of 240.21(C)(2)(1)(b), to allow the secondary conductors to land on a MLO panelboard. The wording isnt the best.

Regarding Blue: I dont see any exceptions in 408.36 that would let you off the hook for protection.

My only thought and possible purpose for 10' rule would be transformer mounted to a single piece of equipment then you could have 10' or less on secondaries
*That one is confusing to me

408.36(B) state over current protection shall be located on secondary side of transformer,

Then right below it
Exception: Shall be considered as protected by the OCPD on primary side when in accordance with 240.21(C)(1) which would be a 2 wire in 2 wire out single phase or delta to delta 3-phase transformer?

 

[Vlietracer64]

Am i understanding 240.21(C)(6) correctly,

Under 25' i can run 800-Amp wire to 800-amp main breaker in conduit and that provides my secondary OCPD

Over 25' and ill set 800- fused disconnect at transformer to protect secondary and then can feed a 800-amp main lug panel.

 

[Vlietracer64]

Regarding Red: I dont think that is the intent of 240.21(C)(2)(1)(b), to allow the secondary conductors to land on a MLO panelboard. The wording isnt the best.

Regarding Blue: I dont see any exceptions in 408.36 that would let you off the hook for protection.

OK i see 240.21(C)(2)(1)(b) states not less than Equipment containing over current device.

I can see that as a main panel breaker or main lug panel and counting distribution breakers.

 

[infinity]

Am i understanding 240.21(C)(6) correctly,

Under 25' i can run 800-Amp wire to 800-amp main breaker in conduit and that provides my secondary OCPD

Over 25' and ill set 800- fused disconnect at transformer to protect secondary and then can feed a 800-amp main lug panel.

Yes, the first part for 25' or less the second part if you exceed 25'. In just about every scenario a panelboard requires protection.

 

[Vlietracer64]

Yes, the first part for 25' or less the second part if you exceed 25'. In just about every scenario a panelboard requires protection.

Wouldn't the 800-Amp fused protection on the secondary at transformer protect the 800-amp main lug panel board and be compliant then for runs over 25'

 

[infinity]

Wouldn't the 800-Amp fused protection on the secondary at transformer protect the 800-amp main lug panel board and be compliant then for runs over 25'

Yes, the OCPD can be ahead of the MLO panel which is what would happen when the length exceeds 25'.

 

[Vlietracer64]

Yes, the OCPD can be ahead of the MLO panel which is what would happen when the length exceeds 25'.

Thank you,

Once other questions trying to wrap my head around.

They want 800-amp supply 120/208

Its specked for 300KVA

300KVA 208v = 832.74 amps
at 800 amps 208 = 288 KVA

288kva 480V = 346 Amps,

They have specked for 350 amp breaker but if i got 125% then next size on primary i can go 450Amp primary side

Is there a reason to or not to add the 125% on primary side when secondary is going to me limited to 800-Amp and require a 346A on primary.

Were re-feeding the existing 208V system off the new 480v service, current service peak 76KW

 

[Vlietracer64]

Thank you,

Once other questions trying to wrap my head around.

They want 800-amp supply 120/208

Its specked for 300KVA

300KVA 208v = 832.74 amps
at 800 amps 208 = 288 KVA

288kva 480V = 346 Amps,

They have specked for 350 amp breaker but if i got 125% then next size on primary i can go 450Amp primary side

Is there a reason to or not to add the 125% on primary side when secondary is going to me limited to 800-Amp and require a 346A on primary.

Were re-feeding the existing 208V system off the new 480v service, current service peak 76KW

Do you have to size the primary OCPD at 125% of rating

 

[electrofelon]

Thank you,

Once other questions trying to wrap my head around.

They want 800-amp supply 120/208

Its specked for 300KVA

300KVA 208v = 832.74 amps
at 800 amps 208 = 288 KVA

288kva 480V = 346 Amps,

They have specked for 350 amp breaker but if i got 125% then next size on primary i can go 450Amp primary side

Is there a reason to or not to add the 125% on primary side when secondary is going to me limited to 800-Amp and require a 346A on primary.

Were re-feeding the existing 208V system off the new 480v service, current service peak 76KW

Do you have to size the primary OCPD at 125% of rating

The reason you generally want to size the primary OCPD to at least 125% is because of the inrush current of the transformer. The higher setting ,of course the less likely it is to trip upon energizing. I have a few times in a pinch I had a primary OCPD about 3/4 of the transformer full load current and you had to try several times to get it to hold. Whether this is a problem depends on what its for and if qualified people are there. 450.3(B) allows going up to 250% for the primary protection, but keep in mind you then need conductors and equipment to match, and especially if that gets you into a larger frame size it can get expensive. Note 125% the FLC of a 300 KVA 480 primary is 451 so you could actually go up to 500 and still be in the 125% category.

 

[Vlietracer64]

The reason you generally want to size the primary OCPD to at least 125% is because of the inrush current of the transformer. The higher setting ,of course the less likely it is to trip upon energizing. I have a few times in a pinch I had a primary OCPD about 3/4 of the transformer full load current and you had to try several times to get it to hold. Whether this is a problem depends on what its for and if qualified people are there. 450.3(B) allows going up to 250% for the primary protection, but keep in mind you then need conductors and equipment to match, and especially if that gets you into a larger frame size it can get expensive. Note 125% the FLC of a 300 KVA 480 primary is 451 so you could actually go up to 500 and still be in the 125% category.

Thank you

I was able to get the 350 that was originally asked for to feed transformer upgrade to a 450, above that and changes to the gear design and placement would have to be changed guessing frame of breaker.

 

[Vlietracer64]

The reason you generally want to size the primary OCPD to at least 125% is because of the inrush current of the transformer. The higher setting ,of course the less likely it is to trip upon energizing. I have a few times in a pinch I had a primary OCPD about 3/4 of the transformer full load current and you had to try several times to get it to hold. Whether this is a problem depends on what its for and if qualified people are there. 450.3(B) allows going up to 250% for the primary protection, but keep in mind you then need conductors and equipment to match, and especially if that gets you into a larger frame size it can get expensive. Note 125% the FLC of a 300 KVA 480 primary is 451 so you could actually go up to 500 and still be in the 125% category.

So if you were using same 300kva transformer for a smaller load (temp power or due to availability) powering a 400-amp panel you would still want to over size primary to cover for inrush current.
You would still size primary OCPD to 125% or transformer rated current of 450/500-amps even thoe with 400-Amps on secondary KVA = 144, That put primary at 173 amps,

Since transformer internals didn't change you still have large iron core, same number of winding so you still have a constant inrush just to turn transformer on so i can see how you would be limited to a min Amperage just to power up transformer.

Thanks in advance just trying to get a better grasp, little bit more to this than i originally thought but enjoying it.