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#### guitarboyled

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Discussion Starter · ·
Perhaps I’m missing something obvious here but why are we allowed fusing the primary 250% of the primary current when the transformers secondary protection does not exceed 125%.

What are the advantages of doing this?

Let’s take a 75 kVA

On the primary:
75,000 / (480 x 1.732) = 90.2A
90.2A x 250% = 225A

Fuse: 225A
Wire: 0000 AWG

On the secondary :

75,000 / (208 x 1.732) = 208A
208A x 125% = 260A

Fuse: 250A
Wire: 0000 AWG

Aren’t you simply spending money for nothing on larger protections and conductors?

#### Locknutz

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guitarboyled said:
Perhaps I&#146;m missing something obvious here but why are we allowed fusing the primary 250% of the primary current when the transformers secondary protection does not exceed 125%. What are the advantages of doing this? Let&#146;s take a 75 kVA On the primary: 75,000 / (480 x 1.732) = 90.2A 90.2A x 250% = 225A Fuse: 225A Wire: 0000 AWG On the secondary : 75,000 / (208 x 1.732) = 208A 208A x 125% = 260A Fuse: 250A Wire: 0000 AWG Aren&#146;t you simply spending money for nothing on larger protections and conductors?
Inrush current

#### guitarboyled

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Discussion Starter · ·
Inrush current
But if the secondary is only at 125% won't it limit any inrush to half of the primary value anyways?

Why not simply use time delayed fuses?

#### Locknutz

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guitarboyled said:
But if the secondary is only at 125% won't it limit any inrush to half of the primary value anyways?
The wiring upstream of the secondary ocpd is higher voltage (typically) thus less current and would be a lower ocpd on the primary. The 250% would allow for the inrush and not trip the primary ocpd

#### Big John

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Inrush is a function of the transformer design. It's the current flow that occurs when a magnetic field is being initially established in the core: Until that happens there's no counter-EMF so the winding acts like a short circuit, which is why the 250% setting is necessary. This occurs regardless of the load on the secondary.

#### piperunner

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Perhaps I’m missing something obvious here but why are we allowed fusing the primary 250% of the primary current when the transformers secondary protection does not exceed 125%.

What are the advantages of doing this?

Let’s take a 75 kVA

On the primary:
75,000 / (480 x 1.732) = 90.2A
90.2A x 250% = 225A

Fuse: 225A
Wire: 0000 AWG

On the secondary :

75,000 / (208 x 1.732) = 208A
208A x 125% = 260A

Fuse: 250A
Wire: 0000 AWG

Aren’t you simply spending money for nothing on larger protections and conductors?
Well we installed 75 KVA transformers on every job and the engineer of record has a one line drawing and every 75 KVA that's on the oneline

Shows this primary 125 amp a 75 kva transformer with no load on it draws
5 amps plus or minus the secondary has a 225a feeder and breaker .

The wire size on primary is 125g =2/0 alu no. 4 awg grd
The wire size on secondary is 225 ng = 300 mcm no. 2 awg grd

You don't have to install it to the max and its safe and your not doing anything wrong trust me .
Yes theres in rush current breakers are designed for that in rush at least the ones we use .

#### ponyboy

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Every 75 I've ever done gets a 125 amp ocpd and #1 copper on the primary and a 200 amp ocpd and 3/0 copper on the secondary. Never ever have had any issue

#### MTW

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It's perfectly acceptable to use a non flexible wiring method to connect a transformer. :jester:

#### ponyboy

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MTW said:
It's perfectly acceptable to use a non flexible wiring method to connect a transformer. :jester:
For you

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#### ponyboy

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Awg-Dawg said:
Nice.:laughing: Why are the XFMRs up on pedestals?
It's the way they want them. Makes it easy to clean under them and some of the machinery is known for blowing hoses from time to time.

One time many years ago an oil line burst over Xmas break and the pumps didn't stop until the tank was dry. Ugly

Awg-Dawg

#### piperunner

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It's perfectly acceptable to use a non flexible wiring method to connect a transformer. :jester:
Well it maybe if your working for jack rabbit electric. Weres your side kick today did he finally quit or did his computer need repairs since hes 24/7 on the forum and other forums . I guess your company never did a VA hospital job or a government project of size were not talking box store if you get one your going to use flex . :thumbsup:

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#### Locknutz

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piperunner said:
Well it maybe if your working for jack rabbit electric. Weres your side kick today did he finally quit or did his computer need repairs since hes 24/7 on the forum and other forums . I guess your company never did a VA hospital job or a government project of size were not talking box store if you get one your going to use flex . :thumbsup:
We use flex....but we try and make it look like we knew what we were doing.

#### MTW

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Well it maybe if your working for jack rabbit electric. Weres your side kick today did he finally quit or did his computer need repairs since hes 24/7 on the forum and other forums . I guess your company never did a VA hospital job or a government project of size were not talking box store if you get one your going to use flex . :thumbsup:
You're not even literate. :no:

#### Big John

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Of all the pictures to post when trying to defend the use of flex, I would never, ever, have picked that.

#### ponyboy

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Big John said:
Of all the pictures to post when trying to defend the use of flex, I would never, ever, have picked that.
Yeah it looks all queered up but in his defense I think I remember him saying there was a spec for 180° loops in all transformer terminations. Can't make chicken salad out of glass houses, or something like that

#### Big John

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Yeah it looks all queered up but in his defense I think I remember him saying there was a spec for 180° loops in all transformer terminations. Can't make chicken salad out of glass houses, or something like that
Agreed. I'm just being a jerk: I've had my share of glass chicken.

#### ponyboy

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Big John said:
Agreed. I'm just being a jerk: I've had my share of glass chicken.
Washing machine pig. Bam

Big John

#### piperunner

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We use flex....but we try and make it look like we knew what we were doing.
Well do I have to explain this in detail why we do the big loop or would you just trust me on why . :thumbsup:

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