How to size a 480 to 240/120V Transformer

[goodoboy]

Hello all,

I am still learning as I go and have a good opportunity to size a transformer and wires for the first time. I have size wires before, but not for the primary side wires.

Can you please provide NEC section or some sizing instructions on how to size for transformer and primary side wiring:

Primary Source Specs:

-coming from our MCC
-480V 3 phase 60HZ

Secondary Load
- See attachment
-120/240V Single phase 60 amp power panel. 4 wire.

Transformer will be mounted 1000ft from MCC 480V power source and about 20ft from 240/120 panel.

This is Nonhazardous area installation.

Questions:
1. Can you please tell me where in the NEC I can read on how to size the transformer? Or steps please?
2. What calculations do I need to perform the size the wire and fuse coming from the MCC 480V primary side.

Thank you very much

 

[kwired]

Hello all,

I am still learning as I go and have a good opportunity to size a transformer and wires for the first time. I have size wires before, but not for the primary side wires.

Can you please provide NEC section or some sizing instructions on how to size for transformer and primary side wiring:

Primary Source Specs:

-coming from our MCC
-480V 3 phase 60HZ

Secondary Load
- See attachment
-120/240V Single phase 60 amp power panel. 4 wire.

Transformer will be mounted 1000ft from MCC 480V power source and about 20ft from 240/120 panel.

This is Nonhazardous area installation.

Questions:
1. Can you please tell me where in the NEC I can read on how to size the transformer? Or steps please?
2. What calculations do I need to perform the size the wire and fuse coming from the MCC 480V primary side.

Thank you very much

The basics of this are if you need say 1.2 kVA for the secondary - you still need 1.2 kVA for the primary, it is just at different voltage and current levels.

So with my example of 1.2kVA @ 240 volts it draws 50 amps. It will draw 25 amps @ 480 volts.

The ability to start some motors could complicate selection some but the basics are VA in is equal to VA out, losses in conversion are typically low enough they can be disregarded.

 

[petersonra]

The NEC does not have any wire sizing instructions for transformers.

You are required to size all conductors so they can carry the calculated load.

You are also required to protect the conductors from short circuits and overloads.

You are also required to provide both secondary and primary protection for the transformer. It is not hard to make the protection required for the conductors and the transformer itself the same device. It is just a matter of making sure that all the requirements for all conditions are covered.

Personally, I make the transformer primary OCPD as large as is allowed. This generally requires that the wire size on the primary size be increased so that the conductors are protected by the the transformer primary OCPD. However, this is a design decision and has nothing to do with the NEC.

You can make the wires as large as you want as long as they meet or exceed the size required to meet the calculated load.

The NEC does not have any requirements for upsizing wires to account for voltage drop. That is a design decision. It is generally a good decision to make the wires large enough that the voltage drop is not much of a concern, but the NEC does not require it.

 

[goodoboy]

The basics of this are if you need say 1.2 kVA for the secondary - you still need 1.2 kVA for the primary, it is just at different voltage and current levels.

So with my example of 1.2kVA @ 240 volts it draws 50 amps. It will draw 25 amps @ 480 volts.

The ability to start some motors could complicate selection some but the basics are VA in is equal to VA out, losses in conversion are typically low enough they can be disregarded.

Thank you kwired,

For the secondary side I need 60amps x 240V = 14.4kVA.

Does this mean I need a 14.4kVA or 15kVA transformer?

For sizing the primary wire, do I use the transformer specifications or do I use the current draw on the primary side (480V 3phase?

 

[Smart $]

Thank you kwired,

For the secondary side I need 60amps x 240V = 14.4kVA.

Does this mean I need a 14.4kVA or 15kVA transformer?

For sizing the primary wire, do I use the transformer specifications or do I use the current draw on the primary side (480V 3phase?

Let's cover motors first. If this is associated with your other thread, your biggest motor is the A/C powered 240V, correct? Do you know the min' circuit ampacity (MCA) and max' OCP on nameplate?

 

[augie47]

You mentioned "steps"
The basic steps are fairly straightforward (a) calculate your load (b) select the transformer
(c) check Art 450 to assure correct transformer protection {450.3 in particular} and (d) check Art 240 to aid in sizing overcurrent protection and conductor sizing {240.21(c) in particular for transformer secondaries).
Smart$ has highlighted a key area in that motors will most often require a higher KVA than a purely restive load.
From your info you will be selecting a 480/240-120 single phase transformer and only using two phases of your 480 primary so you can pretty much ignore any 3 phase calculations.

 

[kwired]

Thank you kwired,

For the secondary side I need 60amps x 240V = 14.4kVA.

Does this mean I need a 14.4kVA or 15kVA transformer?

For sizing the primary wire, do I use the transformer specifications or do I use the current draw on the primary side (480V 3phase?

Basically yes, you need a 15 kVA transformer, but that is based on 60 amps of actual load more so then just because you have a 60 amp main - next standard size overcurrent device for continuous load at 10 KVA would still give you a 60 amp device.

As has been said before the primary conductors need to be sized per the load and protected at their ampacity, secondary conductors are essentially like feeder taps if you have multiwire secondary and secondary conductors are protected according to 240.21(C)...(forget which subsection). Transformer itself needs protected in accordance with art 450, but there are several combinations and conditions that could come into play. Your application you likely do fine with 125% of both primary and secondary rated current as a maximum setting.

Transformer basics just says if you have a 15kVA transformer that primary fully loaded to 15kVA will be 31.25 amps and secondary would be 62.5 amps. There are some losses in the transformer and primary actually would draw slightly more but this is generally disregarded.

 

[goodoboy]

The NEC does not have any wire sizing instructions for transformers.

You are required to size all conductors so they can carry the calculated load.

You are also required to protect the conductors from short circuits and overloads.

You are also required to provide both secondary and primary protection for the transformer. It is not hard to make the protection required for the conductors and the transformer itself the same device. It is just a matter of making sure that all the requirements for all conditions are covered.

Personally, I make the transformer primary OCPD as large as is allowed. This generally requires that the wire size on the primary size be increased so that the conductors are protected by the the transformer primary OCPD. However, this is a design decision and has nothing to do with the NEC.

You can make the wires as large as you want as long as they meet or exceed the size required to meet the calculated load.

The NEC does not have any requirements for upsizing wires to account for voltage drop. That is a design decision. It is generally a good decision to make the wires large enough that the voltage drop is not much of a concern, but the NEC does not require it.

Thank you,

The OPCD is fuses at the mcc for the primary wiring.
The OPCD for the panel is fuses in the panel.

Do I need OPCD on the secondary side of transformer?

Thanks

 

[kwired]

Thank you,

The OPCD is fuses at the mcc for the primary wiring.
The OPCD for the panel is fuses in the panel.

Do I need OPCD on the secondary side of transformer?

Thanks

Yes but it can be in the first panel/disconnect if you don't exceed tap rule lengths given in 240.21(C).

 

[templdl]

Thank you kwired,

For the secondary side I need 60amps x 240V = 14.4kVA.

Does this mean I need a 14.4kVA or 15kVA transformer?

For sizing the primary wire, do I use the transformer specifications or do I use the current draw on the primary side (480V 3phase?

I am in agreement with you, a 15kva transformer would be correct based upon 60a@240vv. A 15 KVA xfme is actually rated at 62.5a. You have to be careful not to get the cart before the horse as you are basing the size on the panel rating and not the calculated load. 15kva may be greater than required. But the next smallest std xfmr is 10kva which is rated at 41.7a which may be too small.
When ordering the transformer you didn't specify if it is indoor but probably is. Start out with a basic ventilated transformer 1ph xfmr, then the temp rise of 150, 115, or 80 deg c rise, you will probably settle on 150deg C rise, with a choice of copper or aluminum windings, there in nothing wrong with aluminum windings, with a 480v primary and a 120/240v secondary. Since the 480v f3der is so long make sure that you also specify (2)+2-1/2% and (4)- 2-1/2% full capacity primary taps sometime also stated as +(2)-(4) 2-1/2 taps. Because of the voltage drop that you will have you can correct the output voltage. With (2)2-1/2% bellow normal taps you have the ability to correct for up to a 5% voltage drop.
If you want to mount it on a wall include a set of wall mounting brackets. If you would rather not have a ventilated transformer because of a dirty environment that my interfere with transformer coping then opt for an caspsulated transformer. Which is wall mounted.
This should be more than plenty to mess with your mind today.

 

[goodoboy]

Let's cover motors first. If this is associated with your other thread, your biggest motor is the A/C powered 240V, correct? Do you know the min' circuit ampacity (MCA) and max' OCP on nameplate?

Yes, this is related to my other thread.

The A/C load is 2880 VA per phase, so 12amps with 240V.

Thanks

 

[goodoboy]

Thank you

When ordering the transformer you didn't specify if it is indoor but probably is. Start out with a basic ventilated transformer 1ph xfmr

The transformer will be mounted outside in a non hazardous area. It will mount in a Heavy Duty ventilated type 3R

specify (2)+2-1/2% and (4)- 2-1/2% full capacity primary taps sometime also stated as +(2)-(4) 2-1/2 taps. Because of the voltage drop that you will have you can correct the output voltage. With (2)2-1/2% bellow normal taps you have the ability to correct for up to a 5% voltage drop.

What do you mean by this?

Thank you,

The

 

[templdl]

Thank you kwired,

For the secondary side I need 60amps x 240V = 14.4kVA.

Does this mean I need a 14.4kVA or 15kVA transformer?

For sizing the primary wire, do I use the transformer specifications or do I use the current draw on the primary side (480V 3phase?

I am in agreement with you, a 15kva transformer would be correct based upon 60a@240vv. A 15 KVA xfme is actually rated at 62.5a. You have to be careful not to get the cart before the horse as you are basing the size on the panel rating and not the calculated load. 15kva may be greater than required. But the next smallest std xfmr is 10kva which is rated at 41.7a which may be too small.
When ordering the transformer you didn't specify if it is indoor but probably is. Start out with a basic ventilated transformer 1ph xfmr, then the temp rise of 150, 115, or 80 deg c rise, you will probably settle on 150deg C rise, with a choice of copper or aluminum windings, there in nothing wrong with aluminum windings, with a 480v primary and a 120/240v secondary. Since the 480v f3der is so long make sure that you also specify (2)+2-1/2% and (4)- 2-1/2% full capacity primary taps sometime also stated as +(2)-(4) 2-1/2 taps. Because of the voltage drop that you will have you can correct the output voltage. With (4)2-1/2% bellow normal taps you have the ability to correct for up to a 10% voltage drop.
If you want to mount it on a wall include a set of wall mounting brackets. If you would rather not have a ventilated transformer because of a dirty environment that my interfere with transformer cooling then opt for an incaspsulated transformer. Which is wall mounted.
This should be more than plenty to mess with your mind today.

 

[Smart $]

Yes, this is related to my other thread.

The A/C load is 2880 VA per phase, so 12amps with 240V.

Thanks

That didn't answer my question.

I asked for MCA and MOCP on nameplate. May also be in electrical spec's documentation, if you don't have physical access to the unit).

 

[Smart $]

Yes, this is related to my other thread.

The A/C load is 2880 VA per phase, so 12amps with 240V.

Thanks

That didn't answer my question.

I asked for MCA and MOCP on nameplate. May also be in electrical spec's documentation, if you don't have physical access to the unit).

PS: It's probably not that critical if it pulls ~12A when running and you size the transformer for the 60A panel rating... as 5x for largest motor FLA is usually good to go.

 

[goodoboy]

PS: It's probably not that critical if it pulls ~12A when running and you size the transformer for the 60A panel rating... as 5x for largest motor FLA is usually good to go.

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

 

[goodoboy]

Hello,

Here is my results so far for the design:

Transformer:
15 kVA, http://attachments.temcoindustrialpower.com/Data_sheet/Hammond-NMF015LE.pdf

Secondary Sizing:
60 amps, 240V (single phase), 30ft away:
Conductor sizing: #4 wire
OPCD: 60amp fuses on the 240/120V panel

Primary Sizing:
How do I size the OPCD (fuse) and wires on the primary side (480V 3 phase)? Do I use the 60amps load? The fuses will be installed in a Disconnect.

Thank you for the help

 

[goodoboy]

Hello,

Here is my results so far for the design:

Transformer:
15 kVA, http://attachments.temcoindustrialpower.com/Data_sheet/Hammond-NMF015LE.pdf

Secondary Sizing:
60 amps, 240V (single phase), 30ft away:
Conductor sizing: #4 wire
OPCD: 60amp fuses on the 240/120V panel

Primary Sizing:
How do I size the OPCD (fuse) and wires on the primary side (480V 3 phase)? Do I use the 60amps load? The fuses will be installed in a Disconnect.

Thank you for the help

Table 450.3(B) states OPCD should be 250% on primary. But I am not sure what primary current to size wire and OPCD.

 

[kwired]

Table 450.3(B) states OPCD should be 250% on primary. But I am not sure what primary current to size wire and OPCD.

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.

 

[templdl]

Hello,

Here is my results so far for the design:

Transformer:
15 kVA, http://attachments.temcoindustrialpower.com/Data_sheet/Hammond-NMF015LE.pdf

Secondary Sizing:
60 amps, 240V (single phase), 30ft away:
Conductor sizing: #4 wire
OPCD: 60amp fuses on the 240/120V panel

Primary Sizing:
How do I size the OPCD (fuse) and wires on the primary side (480V 3 phase)? Do I use the 60amps load? The fuses will be installed in a Disconnect.

Thank you for the help

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.