3 phase 480 sub to feed 120 volt load

sailonjazz

Member
Location
Charletown, RI
:?Hello to all. I've been gleaning very good information from this forum for a while but feel i need more advise than i can find in the posts. I have been licensed since 1986 and have done a lot of industrial and commercial work but find myself a little out of my depth of field. I have been tasked to engineer a remodel of an existing space in a very large building. The building has a 1200a 480v 3 ph service consisting of a 1977 ITE series 5 switchgear and panel board with 3 open spaces next to a JL3-F400 breaker rated at 30 KAIC. My 120 volt loads total 65836 va, 44364 va is 240v lighting and the rest is some light duty fans. I also have 2 480v 3 ph HVAC roof units drawing 21.5 amps and 24 amps each. The new area is 175' of pipe run away from the electric room.
My original plan, working backwards, is to have a 400 amp main 120/240 1 ph 42 cir panel-board able to feed a 200 amp sub panel. Fed with (3) 600 kcmil and (1) #3 gnd, or (6) 4/0 and (2) #6 in parallel, from a 75 kva 208/480v - 120/240v transformer. The transformer would be bonded with a 1/0 jumper to building steel and fed with (2) 3/0 and (1) #6 gnd from a 200 amp 2-pole breaker. This breaker would be installed in a 400 amp 480 volt 3 ph MLO panel also containing (2) 30 amp 3 ph breakers for the two HVAC roof units. I intend to feed this panel with (3) 350 kcmil and (1) #4 gnd from an ITE JL3-F400 with a 250 amp setting.
So finally to my questions. I am concerned about balancing my 3 ph panel.
1) Could i use a 3 ph 120/240 v panel along with a 3 ph transformer. This would bring my sizing down.
2) Should the 480 v 3 ph sub panel have a main breaker.
3) Are my calculations messed up
 

augie47

Moderator
Staff member
Location
Tennessee
If your 44364 "240 volt" load is truly 240 only and not compatible with a 208v system you are somewhat locked into a 240/120 secondary, however, unless things have changed, you will find a 3 phase 240/120 transformer will have a severely reduced neutral capacity so you are looking at single phase or more than one transformer
(If balancing is an issue you could select a 480/240 delta transformer for your 240 loads and a smaller 208/120 for yout 120v loads.)

If your "240 load" is compatible with a 208 secondary transformer, obviously you can install a 3 phase 208/120 transformer and have a more balanced load.

The only problem I see with your numbers is the #3 secondary "ground". Your transformer secondary bonding jumpers size would be based on your secondary phase conductors in this case requiring a 1/0
 

sailonjazz

Member
Location
Charletown, RI
Please clarify. Is the 44364 VA included in the 65836 VA? ...or are the completely separate.
Sorry for the confusion
I have recalculated using a better calculator including 125% rule for the motor loads
37980va - 240v lighting
11070va - 120v Fluorescent lighting,small pumps & heaters
6030va - 120v small motors & fans
9705va - 240v exhaust fan & two 240v heat pumps
64785va - Total
 

GoldDigger

Moderator
Staff member
Sorry for the confusion
I have recalculated using a better calculator including 125% rule for the motor loads
37980va - 240v lighting
11070va - 120v Fluorescent lighting,small pumps & heaters
6030va - 120v small motors & fans
9705va - 240v exhaust fan & two 240v heat pumps
64785va - Total
Putting all of these loads on one phase-to-phase primary on the three-phase supply will create an unbalanced load on the supply. You will need to look at that contribution in addition to the balanced loads to see whether this will cause a problem.
 

sailonjazz

Member
Location
Charletown, RI
If your 44364 "240 volt" load is truly 240 only and not compatible with a 208v system you are somewhat locked into a 240/120 secondary, however, unless things have changed, you will find a 3 phase 240/120 transformer will have a severely reduced neutral capacity so you are looking at single phase or more than one transformer
(If balancing is an issue you could select a 480/240 delta transformer for your 240 loads and a smaller 208/120 for yout 120v loads.)

If your "240 load" is compatible with a 208 secondary transformer, obviously you can install a 3 phase 208/120 transformer and have a more balanced load.

The only problem I see with your numbers is the #3 secondary "ground". Your transformer secondary bonding jumpers size would be based on your secondary phase conductors in this case requiring a 1/0
Thanks everyone for the replies.
the 400a 3 ph 480v panel is fed with 175' of 350kcmil & #4 ground, OCPD is an ITE JL3-F400 with 300a trip rated at 50KAIC.
The transformer is fed from a 200a 3 ph breaker in the 480v 3 ph panel with three 3/0 & #6 ground bonded with 1/0
the 400a 120/240 1 ph panel is fed with three 600kcmil & #3 ground
 

sailonjazz

Member
Location
Charletown, RI
Putting all of these loads on one phase-to-phase primary on the three-phase supply will create an unbalanced load on the supply. You will need to look at that contribution in addition to the balanced loads to see whether this will cause a problem.
Thats my concern, exculding transformer inrush, i have
201a on A phase
201a on B phase
45a on C phase
Anyone have a fix
if i used a 3 ph transformer and panel how would it handle my neutral loads?
 

Smart $

Esteemed Member
Location
Ohio
Thats my concern, exculding transformer inrush, i have
201a on A phase
201a on B phase
45a on C phase
Anyone have a fix
if i used a 3 ph transformer and panel how would it handle my neutral loads?
It seems you are jumping all over the issues. We need to hone in on what you have (existing) and what you need (without any preconceived notions).

You said you have a 1200A, 480v 3? main. It may not matter, but is that 480V grounded (3- or 4-wire) or ungrounded (3-wire only); if grounded, corner grounded delta (3-wire), delta center-tap grounded (4-wire, 480/240V), or wye (4-wire, 480/277)?

Also, you said three available spaces. On this end, we have to assume some spaces are used. If so, what are the connected loads?

This all leads back to the quoted post, because you indicate current on all three phases...

Next, you need to separate your loads better. 240V lighting is okay, 120V lighting is okay. But you have to separate motor loads and HVAC, best for now to list individually, using namplate ratings in HP (FLC if HP not shown) or MCA/MOCP (minimum circuit ampacity/maximum overcurrent protetction).

FWIW, 3? would be preferred for your end panel(s) supply... IMO. Just a question whether to make it 240/120V 3? 4W or 208/120V 3? 4W.
 

sailonjazz

Member
Location
Charletown, RI
Thanks to all

Thanks to all

Thank you everyone who responded
I just met with the building owner and we our seeking a more desirable responce to his problem.
A NEW SERVICE
Woo Hoo
Thanks again
this forum is a wealth of information:)
 
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