# sizing main fuses



## keithg (Jan 17, 2011)

I have a piece of automated equipment to refurbish. There are no drawings or information to work with. The main fuses were never sized. I know how to size the motors, but am not sure how to size the other components, control trnsformers, power supplies, etc. Any suggestions would be greatly appreciated.
Thanks.


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## John Valdes (May 17, 2007)

Size the motor fuses to NEC article 430.52.

Can you list whats left? And include watts. I would not want to give you bad information.


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## MDShunk (Jan 7, 2007)

Maybe there's more to your question, but my answer to you so far is to use a calculator. Calculate it much like you'd calculate demand load on a service. 

NFPA 79 is what you need to get yourself a copy of.


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## MDShunk (Jan 7, 2007)

Just be aware that if this machine is sufficiently old (older than 10 years), you're likely going to have to gut that whole panel and re-do it to remain compliant. So much has changed. 2-circuit guard and e-stops, AWM prohibited, fingersafe requirements, control cabinet door interlocks, Grace ports, etc. Get a good copy of NFPA 79 and study it.


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## keithg (Jan 17, 2011)

thanks for the help.

I have sized the motors in accordance with the NEC. The only components left would be a 480v-24v power supply and a 480v-120v control transformer. Most of the control voltage is 24vdc, with the expeption of about 4 contactors that have 120v coils.

The machine is being refit to reflect the current requirements of NFPA79. The machine is actually only a year or two old. It was built by a company that went bankrupt before finishing it and basically left it "as is". They did not follow any rules of the NFPA 79 so there are quite a bit of things to change.


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## MDShunk (Jan 7, 2007)

keithg said:


> thanks for the help.
> 
> I have sized the motors in accordance with the NEC. The only components left would be a 480v-24v power supply and a 480v-120v control transformer. Most of the control voltage is 24vdc, with the expeption of about 4 contactors that have 120v coils.


Yeah, the power supply will have a watt rating, and the transformer will have a kva rating.


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## keithg (Jan 17, 2011)

So is it just a matter of adding the values of the power supply and transformer to the total motor load?


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## MDShunk (Jan 7, 2007)

keithg said:


> So is it just a matter of adding the values of the power supply and transformer to the total motor load?


Eh, yeah, but transformers have some crazy inrush if they're too terrible big. From what you describe you have running on 120, I'd be surprised if your control transformer was over 100va. 

Anything else run on the 480 other than the motor, your DC power supply, and your control transformer? If not, you've pretty much got it licked.

Ideally, you'll have enough headroom to let the motor reach FLA with just tripping the overloads and not pop the main fuses.


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## keithg (Jan 17, 2011)

Thanks, 
I appreciate all of the help. This looks like a really good site, I am glad to have found it.


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## JRaef (Mar 23, 2009)

Manufacturers of Control Power Transformers almost all have easy to use charts on what fuses to use on the primary and secondary. it doesn't excuse you from doing the math yourself, but it does make it easier to find confirmation of your math :whistling2:

http://www.solahd.com/products/transformers/industrial/FuseRecommend.htm

The same is generally true for DC power supplies, but it's a little easier if its a Switch Mode Power Supply because they don't really have much inrush.


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