Enter if you dare - Come and destroy my calculations

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cyberspaceghost

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Engineer 0.5
Hello All,

I am here posting for the first time. I hope I've come to the right place.

I am new to the field and I had a few questions about a project I decided to take on.

We are installing dock levelers at a facility in North America.

We are wanting to replace three 120V single phase dock levelers with 3 480V 3 phase dock levelers. I am wanting to size the protection equipment and wire to accomodate all 3 docks.

Here is the layout:

Circuit Breaker in MCC room -> ?AWG Wire -> Distribution Box -> Local Equipment Fused Disconnects -> Equipment

Equipment includes:

1) 3 Phase 480VAC Motor, 1.5FLA, 1 HP, @3450RPM, 60Hz; fused with 6A fuses
2) 500 VA Single Phase Transformer 480VAC to 120VAC, fused with 2.5A fuses
3) Transformer feeds a circuit board that powers a 120VAC motor with 5.2FLA, 0.25HP, 60Hz AND some lights and sensors varying in voltage from 12VDC and 120VAC

So, Here are the questions:

1) I've added the two electrical loads: 1.5FLA + (500VA/4) = 2.8 --> Round up to 3A @ 480VAC. If I'm using a circuit breaker to feed 3 of these docks (that all might be working at once at some point) I'd get a load of 9A. Thus, if I do a 250% calculation I'd get 22.5A MAX LOAD. Would I then just use a 25A breaker? and 10AWG wire to power a power distribution block and 12AWG wire to feed the fused disconnects?
2) What size would I need the fuses in the local fused disconnects to be?

Hope you made it this far.

Thank you for your time.
 
Not sure why you are doing the 250% to your 9 amp load when a 15 amp circuit would be adequate,
If you wanted a safety factor you could use a 20 amp breaker and #12 throughout.
Keep in mind branch circuits and feeders to a dock need GFP protection in accordance with 555.3
 
augie47 said:
Not sure why you are doing the 250% to your 9 amp load when a 15 amp circuit would be adequate,
If you wanted a safety factor you could use a 20 amp breaker and #12 throughout.
Keep in mind branch circuits and feeders to a dock need GFP protection in accordance with 555.3
Click to expand...
I think I needed to hear it come from someone else to believe it.

I saw this post on sizing a circuit breaker.

Can I use a 20A breaker with #10 to feed the junction box and #12 to feed the individual dock levelers? Is that allowed?

I'll read 555.3 to ensure the circuit has GFP protection. Thank you for that.
 
augie47 said:
Not sure why you are doing the 250% to your 9 amp load when a 15 amp circuit would be adequate,
If you wanted a safety factor you could use a 20 amp breaker and #12 throughout.
Keep in mind branch circuits and feeders to a dock need GFP protection in accordance with 555.3
Click to expand...
I think dock levelers are the ramps on loading docks.
 
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