Sizing transformer for 3ph 480 primary to 110/240 panel

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weall138

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Dallas,TX
Hello all, first actual post here. :?
I have been task with adding some 110v receptacles where there are none in our plant. I have a 3 ph 480 panel to feed from and need to feed a 1ph - 110/240 panel. I've came up with different answers on my sizing my transformer and , OCP for primary and secondary main ocp. Bare essentials of it- I am looking to add (10) 110v breakers to feed general receptacles. Thanks in advance for your info, ideas, and references.
 

charlie b

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Lockport, IL
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Retired Electrical Engineer
If you are looking for help in sizing your transformer, then you haven't given us enough information. Things like this need to start with a load calculation. In general, we can count each duplex receptacle at 180 VA. But you haven't told us how many receptacles will be installed. Also, if you know that there will be a specific piece of equipment plugged into a certain receptacle, then that receptacle should be counted at the VA value of that equipment.

One simple way to look at it would be to assume each of the 10 breakers will be loaded to 80% of its 20 amp rating. That gives you 80% times 10 times 20 amps times 120 volts (it's not 110 anymore!), or 19,200 VA. I should expect that there would be a desire to have capacity for future additions. So I would pick a 30 KVA transformer.

Now, how about telling us how you would approach sizing the primary and secondary feeders and overcurrent devices.

Welcome to the forum.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Hello all, first actual post here. :?
I have been task with adding some 110v receptacles where there are none in our plant. I have a 3 ph 480 panel to feed from and need to feed a 1ph - 110/240 panel. I've came up with different answers on my sizing my transformer and , OCP for primary and secondary main ocp. Bare essentials of it- I am looking to add (10) 110v breakers to feed general receptacles. Thanks in advance for your info, ideas, and references.

you will need to buy a single phase 480/240-120 xfmr.

the rating of the transformer will be determined by how much power you need. pick the smallest standard sized xfmr that meets that requirement. or go a size larger if you think you might need more juice down the road. it is difficult to tell you what size xfmr you might need based on what you have said. I think the smallest std size xfmr is 15 kVA. That would give you about 62 Amps at 240V on the secondary side, which may well be adequate for 10 receptacle circuits at 120V. it just depends on what you want to plug into the receptacles.
 

infinity

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Location
New Jersey
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Journeyman Electrician
Since you have 3Ø, 480 volts available for the primary I would use a 3Ø, 480-208Y/120 transformer.
 

infinity

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Location
New Jersey
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Journeyman Electrician
3 phase 4 wire 208/120 panels cost a lot more than 120/240 3 wire panels.

True, but the labor is about the same for a sizeable increase in capacity. For me from a design perspective I wouldn't use a 1Ø panel in a building that already had 3Ø, 480 unless maybe if they absolutely needed 240 volts.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
True, but the labor is about the same for a sizeable increase in capacity. For me from a design perspective I wouldn't use a 1Ø panel in a building that already had 3Ø, 480 unless maybe if they absolutely needed 240 volts.

he only wants ten circuits.

any "capacity increase" is really irrelevant at that number of circuits. in any case the capacity is limited by the xfmr rating and not the PB.
 

charlie b

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Location
Lockport, IL
Occupation
Retired Electrical Engineer
It would be the analogous 240.21(C) rule, which is approximately the same in concept, except the conductors are called transformer secondary conductors instead of taps.
It is not at all the same concept. 240.21(B) and (C) share the notion that a wire is not protected at its ampacity at the location at which it gets its power. There the resemblance ends. For a feeder tap, there is an OCPD that protects the initial feeder, but its rating is higher than the ampacity of the tap conductors. For a transformer secondary conductor, there is no upstream OCPD; you don't have any overcurrent protection until you reach the end of the secondary conductors. There is, of course, the primary side OCPD, but most of the time we don't get to take credit for it as serving to protect anything on the secondary side. The rules that allow these circumstances are different, because the physical situations are different.

 
802ecmCBfig2.jpg


I miss spoke, I should have said secondary transformer conductors not over ten feet.

http://ecmweb.com/code-basics/transformer-secondary-conductors

Secondary Tap Rules
10-Foot Secondary Tap Rule [240.21(C)(2)]
Secondary conductors can be run up to 10 ft without overcurrent protection, but they must be installed in accordance with the following requirements: Figure 4
(1) The ampacity of the tap conductor is:
Not less than the computed load in accordance with Article 220, and
Not less than the rating of the device supplied by the tap conductors or the overcurrent protective device at the termination of the tap conductors.
(2) The secondary conductors do not extend beyond the switchboard, panelboard, disconnecting means, or control devices they supply.
(3) The secondary conductors are enclosed in a raceway.

Author’s Comment: Lighting and appliance branch-circuit panelboards require overcurrent protection to be located on the secondary of the transformer in accordance with 408.16(A), (B) and (E).

Source:
https://www.mikeholt.com/mojonewsarchive/NEC-HTML/HTML/FeederSecondaryTapRules~20020326.htm

Thank you for the correction.
 

kwired

Electron manager
Location
NE Nebraska
you will need to buy a single phase 480/240-120 xfmr.

the rating of the transformer will be determined by how much power you need. pick the smallest standard sized xfmr that meets that requirement. or go a size larger if you think you might need more juice down the road. it is difficult to tell you what size xfmr you might need based on what you have said. I think the smallest std size xfmr is 15 kVA. That would give you about 62 Amps at 240V on the secondary side, which may well be adequate for 10 receptacle circuits at 120V. it just depends on what you want to plug into the receptacles.
You can go much smaller then 15 kVA, though some of the smallest ones you can get are really only good for control transformers for the most part. But if you had only one receptacle to feed and knew there wasn't going to be any other load you could use a 1.5 or 2 kVA transformer in many instances. Works great out in middle of nowhere where you have 480 volt equipment and just need one general purpose receptacle for maintenance purposes - and cost a lot less then a 15 kVA transformer does.

With OP's 10 circuits and no other information I'd lean toward a 7.5 or 10 kVA minimum but also would be asking for other information on what loads may be supplied.

3 phase 4 wire 208/120 panels cost a lot more than 120/240 3 wire panels.
As well as three pole breakers (primary or secondary side) as well as three phase transformers. If more then 15 or 25 kVA is needed it is worth looking at cost vs other performance issues - like do we want up to 25 kVA of single phase load or do we want it balanced across phases as much as possible?
 
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