Transformer Secondary conductor sizing

[omega83]

Hi everyone,

I was wondering if someone could help me with sizing transformer secondary conductors? Im a little confuse of how to size the conductor.

Example: I have a transformer that has a secondary nameplate rating of 75 kva 120/240v single phase. I use formula KVA=E x I / 1000. Come up with 312.5 FLA's. Breaker size is 312.5 x 1.25 = 390.625 amps Breaker would be size at 400amp breaker next higher size.

What Im confuse on is do I size wire to FLA or do I size wire with breaker size. The transformer can only produce 312.5 FLA's, Do I have to match the breaker size? I was told to from one person to size with FLA because the transformer can not produce over FLA's and was also told by another to go by the breaker size. Can I get some feed back on this for taking my test.

Thanks

 

[jumper]

Here are a couple of links on the subject.

http://ecmweb.com/nec/code-basics/electric_transformer_secondary_conductors/

http://ecmweb.com/mag/electric_transformer_installation_made/

 

[don_resqcapt19]

In general the secondary conductors of a transformer have to be protected at or below there rated ampacity. The "round up" rule does not apply. The primary conductors have to be protected but the "round up" rule does apply. This can result in conductors that are very much oversized for the actual load if the transformer primary protection is 250% of the rated primary current as permitted in Article 450 for transformers with both primary and secondary protection.

 

[augie47]

There are a couple of points in your original post which may be misleading you.......

Hi everyone,
..........................................................................................

What Im confuse on is do I size wire to FLA or do I size wire with breaker size. The transformer can only produce 312.5 FLA's,

The kva rating is for 312.5 FLA. If there is an overload or short, the transformer secondary current can be much larger. You are protecting against that,

Do I have to match the breaker size? I was told to from one person to size with FLA because the transformer can not produce over FLA's ......

Incorrect

and was also told by another to go by the breaker size. Can I get some feed back on this for taking my test.

Thanks

If your transformer is protected by the primary OCP, then your secondary OCP's main purpose is to protect the secondary conductors. As noted in above posts, 240.21(C) precludes using 240.4(B) for protecting transformer secondary conductors (next size up). In this case I usually see 400 amp C/B and 600 kcmil wire.

 

[omega83]

Sorry guys, Still kinda of confuse from what you guys posted. Breaker is size according to table 450.3(a) or (b). In 240.21(c)2, It says The ampacity of the secondary conductor is: Not less than the rating of the device supplied by the secondary conductor or not less than the rating of the OCP device at the termination of the secondary.

How do I size my wire? Do I size it with breaker, Do I just use what the FLA rating of the transformer, or do I have to do FLA x 1.25% ?

 

[raider1]

You size the secondary conductors for the load served and then you size the secondary OCPD that they terminate on to protect the secondary conductors.

Chris

 

[omega83]

You size the secondary conductors for the load served and then you size the secondary OCPD that they terminate on to protect the secondary conductors.

Chris

So what you are saying is the FLA is 312.5 amps I would size wire with #400kcmil. Then the breaker would be size at 312.5 x 1.25= 400amp( next size larger)

 

[topgone]

You size the secondary conductors for the load served and then you size the secondary OCPD that they terminate on to protect the secondary conductors.

Chris

What Chris said. Always keep in mind that you are not allowed to use a breaker size higher than the conductor ampacity to protect them conductors.

 

[omega83]

What Chris said. Always keep in mind that you are not allowed to use a breaker size higher than the conductor ampacity to protect them conductors.

Table 450.3(a) and (b) are use for maximum rating and settings. 312.5 x 1.25= 400 is the maximum settings with next larger size. I understand this part. Im taking my test and usually they would want the minimum, not what is practical. Lets consider this not a continuos load. I would size secondary conductor to 312.5 amps right because 240.21(c) states on the last sentence we can t round up for secondary conductors.

 

[don_resqcapt19]

The secondary conductors must be protected by a OCPD that has a rating equal to or less than the ampacity of the conductor. If you are using a 400 amp breaker then the conductor must have an ampacity of at least 400 amps.

 

[omega83]

The secondary conductors must be protected by a OCPD that has a rating equal to or less than the ampacity of the conductor. If you are using a 400 amp breaker then the conductor must have an ampacity of at least 400 amps.

Yeah this is the reason why Im confuse. OKAY, will is this correct then

312.5 x 1.25(t450.3(b)) = 390.625amps
Breaker size will be 400 amps.

240.21( c) does not let conductors to be up size like breaker so we size it to 390.625 amps right? So we would size wire to 390.625 Right?

NOW If we would to use a different multiplier 167% from same table,

312.5 x 1.67= 521.875 amps
breaker next higher size would be 600 amps standard.

240.21( c) does not let conductors to be up size up so we size 521.875 amps to a 1000 kcmils. If I size this according to 240.21c then its not equal or lesser then the OCPD which would be 600amps and 1000 kcmils is only good for 545 amps(75 c)

What do I do?

 

[infinity]

Yeah this is the reason why Im confuse. OKAY, will is this correct then

312.5 x 1.25(t450.3(b)) = 390.625amps
Breaker size will be 400 amps.

240.21( c) does not let conductors to be up size like breaker so we size it to 390.625 amps right? So we would size wire to 390.625 Right?

NOW If we would to use a different multiplier 167% from same table,

312.5 x 1.67= 521.875 amps
breaker next higher size would be 600 amps standard.

240.21( c) does not let conductors to be up size up so we size 521.875 amps to a 1000 kcmils. If I size this according to 240.21c then its not equal or lesser then the OCPD which would be 600amps and 1000 kcmils is only good for 545 amps(75 c)

What do I do?

Isn't the 167% for transformers with a secondary current of less than 9 amps?

 

[omega83]

Isn't the 167% for transformers with a secondary current of less than 9 amps?

yes, Im just throwing that in if thats how you would size the wire

 

[jumper]

yes, Im just throwing that in if thats how you would size the wire

This what you use to size the wire:

215.2 Minimum Rating and Size.
(A) Feeders Not More Than 600 Volts.
(1) General. Feeder conductors shall have an ampacity not
less than required to supply the load as calculated in Parts
III, IV, and V of Article 220. The minimum feeder-circuit
conductor size, before the application of any adjustment or
correction factors, shall have an allowable ampacity not
less than the noncontinuous load plus 125 percent of the
continuous load.

 

[omega83]

This what you use to size the wire:

215.2 Minimum Rating and Size.
(A) Feeders Not More Than 600 Volts.
(1) General. Feeder conductors shall have an ampacity not
less than required to supply the load as calculated in Parts
III, IV, and V of Article 220. The minimum feeder-circuit
conductor size, before the application of any adjustment or
correction factors, shall have an allowable ampacity not
less than the noncontinuous load plus 125 percent of the
continuous load.

Well if thats the case, then we would use 312.5 amps to size wire? WOW Im sorry if Im not seeing it guys. can someone size this transformer for me so I can see. Secondary nameplate reads 75 kva 120/240v single phase. Size the OCPD and Wire for this secondary size?

 

[jumper]

Well if thats the case, then we would use 312.5 amps to size wire? WOW Im sorry if Im not seeing it guys. can someone size this transformer for me so I can see. Secondary nameplate reads 75 kva 120/240v single phase. Size the OCPD and Wire for this secondary size?

The transformer is not the load, what is connected to it is. Did you even look at the links I posted?

 

[augie47]

T he problem is you don't normally "size the wire for the transformer", you (a) size the wire for the load, (b) select the OCP device to protect the wire and (c) select a transformer to carry the load.

If your transformer is properly protected by the primary OCP device, you secondary conductors could be almost any size (some minimum limitation depending on the tap rule) from #2 to 1000 kcmil.

IF we assume you are using the "secondary OCP" Table 450.3(b) to protect the transformer, then your maximum OCP device would be a 400 amp. Based on 240.21(C) the conductor would have to have an ampacity of at least 400 amps (600 kcmil, parallel 3/0s, etc)

IF you are not using the secondary OCP to protect the transformer then you could have any size OCP you want as long as your wire is sized not less than that OCP size. You could have (6) 100 amp breakers and feed each with a #3, or even (3) 400 amp breakers each with a 600 kcmil, etc.

 

[jumper]

Augie,

For the absolute minimum, couldn't I use #14 and 1 amp fuses for primary and secondary?

 

[augie47]

Augie,

For the absolute minimum, couldn't I use #14 and 1 amp fuses for primary and secondary?

works for me :slaphead:
(my head hurts)

 

[don_resqcapt19]

Yeah this is the reason why Im confuse. OKAY, will is this correct then

312.5 x 1.25(t450.3(b)) = 390.625amps
Breaker size will be 400 amps.

240.21( c) does not let conductors to be up size like breaker so we size it to 390.625 amps right? So we would size wire to 390.625 Right?

240.21(C) does not say you can't use a bigger conductor. It says you can't use a OCPD with a rating that exceeds the ampacity of the wire. If you are using a 400 amp breaker, than the minimum wire size would be 600 kcmil copper.

NOW If we would to use a different multiplier 167% from same table,

312.5 x 1.67= 521.875 amps
breaker next higher size would be 600 amps standard.

240.21( c) does not let conductors to be up size up so we size 521.875 amps to a 1000 kcmils. If I size this according to 240.21c then its not equal or lesser then the OCPD which would be 600amps and 1000 kcmils is only good for 545 amps(75 c)

What do I do?

If you have a 600 amp OCPD, the minimum wire size would be 1500 kcmil for a single conductor. That would not be practical so you would probably parallel two 350 kcmil per phase.