How to size a 480 to 240/120V Transformer

[Smart $]

Thank you,

Can you help me out please? I do not see the MCA and MOCP.

This is the A/C here: http://www.ajmadison.com/cgi-bin/ajmadison/WY09D33.html.

Thanks

Okay... that's a wall-mount, plug-in unit. This type unit just gives a circuit rating and plug type. There may be an MCA and MOCP on the nameplate, but all I can glean from documentation is 20A circuit and NEMA 6-20R required.
http://friedrich.gear.host/documents/wallmaster/WallMaster.pdf

You should be good as the "quickview" spec's on this page say 4.5A cooling, 4.3A heating.

BTW, this model is rated for 208V operation according to the "quickview" spec's... so you may want to rethink your current plan.
http://residential.friedrich.com/Products/ShowBrandDetails.aspx?AcBrandID=3

 

[templdl]

How about this package, an Easton mini power center, SqD has a similar product called a mini power zone.
http://m.cesco.com/b2c/product/Eaton-Cutler-Hammer-P48G11S15P-Mini-Power-Centers/609361

A transformer/loadcenter all in one.

 

[ActionDave]

BTW, this model is rated for 208V operation according to the "quickview" spec's... so you may want to rethink your current plan.
http://residential.friedrich.com/Products/ShowBrandDetails.aspx?AcBrandID=3

I'd have made money on that bet.

 

[goodoboy]

Otherwise than II am not familiar with the transformer manufacturer you have selected one with the correct aspects excerpt that it didn't specify the taps as I had suggested. It is extremely important that you specify taps to assure that you have the ability to correct for the anticipated voltage drop, +(2)2-1/2%FCAN and (4)2-1/2% FCBN taps.
Also, is a NEMA 3r necessary? If located indoors it may add cost.

Thank you templdl,

What does taps mean? The transformer will be located outside by the panel.

Thanks

 

[goodoboy]

Okay... that's a wall-mount, plug-in unit. This type unit just gives a circuit rating and plug type. There may be an MCA and MOCP on the nameplate, but all I can glean from documentation is 20A circuit and NEMA 6-20R required.
http://friedrich.gear.host/documents/wallmaster/WallMaster.pdf

You should be good as the "quickview" spec's on this page say 4.5A cooling, 4.3A heating.

BTW, this model is rated for 208V operation according to the "quickview" spec's... so you may want to rethink your current plan.
http://residential.friedrich.com/Products/ShowBrandDetails.aspx?AcBrandID=3

Thank you,

Yes, I noticed yesterday the unit is rated for 208V, but the panel is already built and installed for 120/240V.

I will stick to the transformer for now cause I enjoy the design. I am off work now for Thanksgiving holiday, so I am still thinking and trying to spec out the transformer design (transformer, fuses, primary and secondary wiring?

 

[goodoboy]

Table 450.3(B) states the max OPCD can be 250% not that it must be 250%.

If you want to have 60 amps of secondary capacity then you will need at least 30 amps of primary ability since primary voltage is 2 times secondary voltage.

If you have lightly loaded situation nothing says you couldn't go with lesser conductors/overcurrent protection then the unit rating as long as the overcurrent device holds during energization or during a supplied motor starting event.


Also notice there is an informational note after 450.3 referring you to sections in art 240 for overcurrent protection of the conductors.

Thank you kwired,

So do I size my wires and OPCD on the primary side for 30amps?

I'm a bit confused now. if using 30amps primary current rating for this design, this mean OPCD can be up to 250% (or 75amps)? How do you make decision on OPCD with phrase "450.3(B) states the max OPCD can be 250%" Shouldn't the OPCD be for 30amps to protect the wires that will be sized for 30amps. If I get a 75amps OPCD, then my wires will not be protected because they will be sized for 30amps max current draw.:?:?

 

[kwired]

Thank you kwired,

So do I size my wires and OPCD on the primary side for 30amps?

I'm a bit confused now. if using 30amps primary current rating for this design, this mean OPCD can be up to 250% (or 75amps)? How do you make decision on OPCD with phrase "450.3(B) states the max OPCD can be 250%" Shouldn't the OPCD be for 30amps to protect the wires that will be sized for 30amps. If I get a 75amps OPCD, then my wires will not be protected because they will be sized for 30amps max current draw.:?:?

You are catching on, if you put a 75 amp (80 next standard size)OPCD chances are good you will need a 75 (71 ampacity can be protected by 80 amp device) amp conductor. This is general rules, things like motors can throw a wrench into the gears. Read through art 240 carefully or wherever it may send you to for certain equipment if you are thinking of putting more then 30 amp protection on a 30 amp conductor.

Most general purpose uses, you are probably going to be fine with 30 amps though, if you are on the edge of your device holding while starting a particular motor maybe will need 35 or 40 amps at the most.

 

[goodoboy]

You are catching on, if you put a 75 amp (80 next standard size)OPCD chances are good you will need a 75 (71 ampacity can be protected by 80 amp device) amp conductor. This is general rules, things like motors can throw a wrench into the gears. Read through art 240 carefully or wherever it may send you to for certain equipment if you are thinking of putting more then 30 amp protection on a 30 amp conductor.

Most general purpose uses, you are probably going to be fine with 30 amps though, if you are on the edge of your device holding while starting a particular motor maybe will need 35 or 40 amps at the most.

Thank you kwired,

How can "Table 450.3(B) states the max OPCD can be 250% not that it must be 250%." help me for my decision on what load amp to size the primary conductors and OPCD for the design.

I think your first recommendation of 30 amps on the primary size should be good enough to size for but the 450.3(B) is confusing me.

 

[goodoboy]

Most general purpose uses, you are probably going to be fine with 30 amps though, if you are on the edge of your device holding while starting a particular motor maybe will need 35 or 40 amps at the most.

Thanks,

What confusing me is I sized the transformer based on 60amps load on the primary side? So I keep thinking to use 60amp to size the OPCD and wires on the primary size. But early you mention that because the primary size is 480V 3 phase, the current to power up the transformer will cut in half. :?

 

[kwired]

Thanks,

What confusing me is I sized the transformer based on 60amps load on the primary side? So I keep thinking to use 60amp to size the OPCD and wires on the primary size. But early you mention that because the primary size is 480V 3 phase, the current to power up the transformer will cut in half. :?

60 amps x 240 volts = 14400 VA

30 amps x 480 volts = 14400 VA

If you are using a 15KVA transformer full 15KVA is 15000 / 480 = 31.25 A primary 15000 / 240 = 62.5 A secondary. 30 amp primary device likely holds when energizing but actually is putting slight limitation on the output cabability in relation to full load rating.


I have feed a 25 KVA unit before (480 volt primary) with a 30 amp primary OCPD and had no issues, because we were not loading it all that much - was a temporary situation, the 25 kVA transformer happened to be available as well as a long 10 AWG cable to feed it with - so we protected it with 30 A and had no problems for the duration that setup was needed.

 

[goodoboy]

60 amps x 240 volts = 14400 VA

30 amps x 480 volts = 14400 VA

If you are using a 15KVA transformer full 15KVA is 15000 / 480 = 31.25 A primary 15000 / 240 = 62.5 A secondary. 30 amp primary device likely holds when energizing but actually is putting slight limitation on the output cabability in relation to full load rating.


I have feed a 25 KVA unit before (480 volt primary) with a 30 amp primary OCPD and had no issues, because we were not loading it all that much - was a temporary situation, the 25 kVA transformer happened to be available as well as a long 10 AWG cable to feed it with - so we protected it with 30 A and had no problems for the duration that setup was needed.

Thank you kwired,

Happy Thanksgiving.

http://prntscr.com/97ogw6

I am still not understanding the primary current to use to size the OPCD and wires on the primary. According the screen shot I should be using another equation. I am just confused.

 

[Smart $]

Thank you kwired,

Happy Thanksgiving.

http://prntscr.com/97ogw6

I am still not understanding the primary current to use to size the OPCD and wires on the primary. According the screen shot I should be using another equation. I am just confused.

I didn't read all that kwired told you so uncertain what you do and don't understand...

There are two schema's for primary OCPD: 1) primary only transformer protection, and 2) primary and secondary transformer protection. Let's say for now that you are going to use the latter. Referring to Table 450.3(B) your primary OCPD is allowed to be rated up to 250% the primary rated current [typically; see Note 3]. Primary rated current at 15kVA 480V is 31.25A and 250% of that is 78.1A.... but you can't go up to next size, so max' std. OCP rating is 70A.

Keep in mind that you do not have to use that size. You can go smaller... but I would not go any smaller than the secondary OCP rating times the primary-to-secondary voltage ratio (and up to next std. size not over 70A). If you plan on using 60A secondary protection, that means I wouldn't go smaller than 35A on the primary.

Note for a 15kVA transformer, the secondary rated current is 62.5A and you can go up to 125% (78.125A) and go up from there to the next std. size (80A). So if you go this high, that would raise the smallest I would use for the primary to 45A.

Sometimes the deciding factor for primary OCPD is the primary conductor, which has to be protected per 240.4. If you go with a 70A OCPD for the primary, you essentially have to run 70A conductor.

 

[goodoboy]

I didn't read all that kwired told you so uncertain what you do and don't understand...

There are two schema's for primary OCPD: 1) primary only transformer protection, and 2) primary and secondary transformer protection. Let's say for now that you are going to use the latter. Referring to Table 450.3(B) your primary OCPD is allowed to be rated up to 250% the primary rated current [typically; see Note 3]. Primary rated current at 15kVA 480V is 31.25A and 250% of that is 78.1A.... but you can't go up to next size, so max' std. OCP rating is 70A.

Keep in mind that you do not have to use that size. You can go smaller... but I would not go any smaller than the secondary OCP rating times the primary-to-secondary voltage ratio (and up to next std. size not over 70A). If you plan on using 60A secondary protection, that means I wouldn't go smaller than 35A on the primary.

Note for a 15kVA transformer, the secondary rated current is 62.5A and you can go up to 125% (78.125A) and go up from there to the next std. size (80A). So if you go this high, that would raise the smallest I would use for the primary to 45A.

Sometimes the deciding factor for primary OCPD is the primary conductor, which has to be protected per 240.4. If you go with a 70A OCPD for the primary, you essentially have to run 70A conductor.

Thank you very much for response.

Since my primary current rating is 31.25, i will use 40 amp OPCD (31.25 x 1.25). I will size my primary conductors for 40amps with the 1000 ft voltage drop.

 

[goodoboy]

Thank you very much for response.

Since my primary current rating is 31.25, i will use 40 amp OPCD (31.25 x 1.25). I will size my primary conductors for 40amps with the 1000 ft voltage drop.

Hello,

To calculate primary current should I be using the equation. k VA x 1000/ V x 1.732

15000/ 480V x 1.732 = 18amps. So my OPCD is 20 amps.

Is this correct? I know previously we been saying 30 amps on primary side.

 

[kwired]

Hello,

To calculate primary current should I be using the equation. k VA x 1000/ V x 1.732

15000/ 480V x 1.732 = 18amps. So my OPCD is 20 amps.

Is this correct? I know previously we been saying 30 amps on primary side.

Only if it is a three phase transformer. Single phase is just 15000/480 = 31.25 amps.

I believe we were talking single phase from the very start and most replies were based on that.

 

[goodoboy]

Only if it is a three phase transformer. Single phase is just 15000/480 = 31.25 amps.

I believe we were talking single phase from the very start and most replies were based on that.

Thank you kwired,

The transformer is 480V 3phase to 240V 1 phase .

The primary source voltage is 480V 3phase 60hz.

http://www.temcoindustrialpower.com/products/Transformers_-_General/HT2306.html

The spec sheet state 1 phase. But I am concered if the transformer is rated for 480V 3 phase and not 4801V 1 phase.

Does this depends on how I wire up the primary side?

 

[ActionDave]

Does this depends on how I wire up the primary side?

No. It depends on what kind of transformer you have. You have a single phase transformer. The iron and windings are pretty dumb, they don't know if the two hots come from a single phase system or a three phase.

 

[goodoboy]

No. It depends on what kind of transformer you have. You have a single phase transformer. The iron and windings are pretty dumb, they don't know if the two hots come from a single phase system or a three phase.

Thank you,

Maybe I am confusing myself. But do I need a 3 phase transformer considering my input is 480V 3 phase, and output is 240 1 phase ?

 

[ActionDave]

Thank you,

Maybe I am confusing myself. But do I need a 3 phase transformer considering my input is 480V 3 phase, and output is 240 1 phase ?

The transformer you linked to is a single phase transformer. A single phase transformer can only be fed single phase, even if the panel it is fed from is three phase.

 

[goodoboy]

The transformer you linked to is a single phase transformer. A single phase transformer can only be fed single phase, even if the panel it is fed from is three phase.

Thank you ActionDave.

I need 3 phase transformer.

I spec out the wrong transformer because the transformer will be fed with 480V 3phase on primary side, and output will be 120/240V single phase.

So this means my current rating on the primary side will be 15000/ 480V x 1.732 = 18amps. So my OPCD is 20 amps.

Thanks for clarification.