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#207010 - 09/13/12 06:22 PM Control Transformer Fuse sizing refresher
sparkyinak Offline
Member

Registered: 07/08/07
Posts: 1295
Loc: Alaska
Hey boys and girls, I am working on installing some control trnasformers and it has been years since I did the calculations. The largest is a 200va with 277 pri/24sec voltage. According to 725.21(D)

Primary Side of Transformer. Class 1 circuit conductors
supplied by the secondary of a single-phase transformer
having only a 2-wire (single-voltage) secondary
shall be permitted to be protected by overcurrent protection
provided on the primary side of the transformer, provided
this protection is in accordance with 450.3 and does not
exceed the value determined by multiplying the secondary
conductor ampacity by the secondary-to-primary transformer
voltage ratio.


If I am reading it right, all I need is a 1 amp fuse for a 200va transformer because:

200/277=0.7 amp load
0.7x1.25=0.9 or 1 amp (next strd size up)per NEC Table 450.3(B)
277/24=11.5 primary/seconday winding ratio
11.5x0.9=10.4 minimum wire ampacity (14 gauge sec. wire spec'd)
Max calculated sec. load 159vA

Am I doing this correct or am I am infected by PVC glue fumes...again?
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#207017 - 09/14/12 08:16 AM Re: Control Transformer Fuse sizing refresher [Re: sparkyinak]
JBD Offline
Member

Registered: 07/12/01
Posts: 599
Loc: WI, USA
Small control power transformers often have extremely high inrush currents; notice how the NEC allows up to 300% for these small transformers.

I usually size my primary fuse at 175%.

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#207020 - 09/14/12 04:56 PM Re: Control Transformer Fuse sizing refresher [Re: JBD]
sparkyinak Offline
Member

Registered: 07/08/07
Posts: 1295
Loc: Alaska
I can go 250% if I use secondary OCPD per 450. I am mainly interested in the rule in 725 per the OP. I'm gonna be back in the office sooner then expected and I can then research it.
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#207075 - 09/19/12 07:40 PM Re: Control Transformer Fuse sizing refresher [Re: sparkyinak]
sparkyinak Offline
Member

Registered: 07/08/07
Posts: 1295
Loc: Alaska
A couple of interesting facts regarding my OP. The first, I was using an older reference (NEC 2002). Now that I am back in the office, the 2011 allows sizing up to 300% on the primary OCPD pending on its amp draw. For a 200VA control transformer per 725.43 (formally 725.21)

200va / 277v = 0.7 amp primary load
0.7a x 300% = 2.1 or 2 amp fuse maximum

However since I am using #14 on the secondary:
15 amp x 24/277 (xfer ratio) = 1.3 or 1-3/10 amp fuse is the maximum size fuse I can use or:

2 amp primary fuse and 15 amp secondary fuse

Anyone thinks different?

The other interesting fact is they reduced the ampacity for #14 wire from 20 amps to 15. Reasoning according to the ROP report for the 2011 NEC is to put it inline with the CEC. Not for safety, not for electrical issues. So much for the NEC is being about safety...
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#207141 - 09/27/12 12:41 AM Re: Control Transformer Fuse sizing refresher [Re: sparkyinak]
aspencade Offline
Member

Registered: 10/19/04
Posts: 13
Loc: BC Canada
Originally Posted By: sparkyinak


The other interesting fact is they reduced the ampacity for #14 wire from 20 amps to 15. Reasoning according to the ROP report for the 2011 NEC is to put it inline with the CEC. Not for safety, not for electrical issues. So much for the NEC is being about safety...



Now that is irony. The Canadian Electrical Code recently increased the values for #14 to 20A subject to overcurrent not set more then 15A. Basically that translates to #14 to have a better current carrying capacity when it comes to de-rating for things like temperature or when there are more then three current carrying conductors in contact.

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#207142 - 09/27/12 01:20 AM Re: Control Transformer Fuse sizing refresher [Re: sparkyinak]
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
That change only affected 60c conductors. Usually when you are looking at the derating table, you are in the 75 or 90 column.
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#207143 - 09/27/12 09:33 AM Re: Control Transformer Fuse sizing refresher [Re: gfretwell]
aspencade Offline
Member

Registered: 10/19/04
Posts: 13
Loc: BC Canada
Greg are you referring to the original post or my comment about irony?

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#207144 - 09/27/12 10:14 AM Re: Control Transformer Fuse sizing refresher [Re: sparkyinak]
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
Just referring to the change in 310.16 in case nobody noticed it (they also changed the name of the table for no particularly good reason).
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#207145 - 09/27/12 12:28 PM Re: Control Transformer Fuse sizing refresher [Re: gfretwell]
sparkyinak Offline
Member

Registered: 07/08/07
Posts: 1295
Loc: Alaska
My gripe is it was change and not for safety. Unless one can clearify the temperture rating on the terminals of the wires, the rating of a #14 THHN will be only 15 amps per 110.14(C)(1)
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