When to decide on disconnect vs fused disconnect/breaker for panel

g.mccormick

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I have a heater control panel that I am making. The heaters are 2X 25KW 480V 3phase.

The only thing in the panel will be disconnect/breaker, fusing for each scr/heater, small control transformer fused for fan.

Heaters will each consume 30A@480V. Fusing at each SCR will be 45A (125% of 30A).


Do you think I should just use an unfused disconnect in the panel and assume the customer will size feeder breaker correctly? Or would you all suggest that I use a fused disconnect (or probably breaker) sized correctly for panel?

Sizing would be (45A*1.25) + 30 + 1A (assume for control transformer) = 87.25A
Next size breaker = 90A

What is everyones opinions?
 
I don't think you required by code to provide fusible disconnect. However you have to provide note on the drawing, which states total load current and size of overcurrent protection required.

PaulB
 
45A is not 125% of 30A, its 150%... just thought I'd note that...

Also Code is a Bare minimum, nothing stopping you from going above and beyond it.
Id recommend you used a Breaker as well, even if its not required.
 
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45A is not 125% of 30A, its 150%... just thought I'd note that...

Also Code is a Bare minimum, nothing stopping you from going above and beyond it.
Id recommend you used a Breaker as well, even if its not required.

You are correct. Brain thought one thing, finger thought another.
 
Hi there,

I would calculate your load like this....

30 amp (heater 1) + 30 amp (heater 2) = 60 amp TOTAL HEATING LOAD

60 amp * 1.25 = 75 amp rated TOTAL CONTINUOUS HEATING LOAD

1 amp (motor load)(so you say) * 1.25 = 1.25 amp rated TOTAL CONTINUOUS MOTOR LOAD.
(You are doing this motor through a transformer so this may be getting picky but it is a very minimal load and will not affect the calculation if you consider it "Continuous Load" or not.)

TOTAL LOAD = 76.25 amps

Next Size Breaker = 80 amps

See NFPA-70 Article 424 "Fixed Electrical Space Heating" and Article 430 Motors.

Also it has been a LONG time since I was heavy into the code but I think I recall something about if your heat strips are in separate banks then you may need to add supplemental protectors for each bank. You say you have 2 x 25 Kw heating banks so this may apply to you.
(Don't quote me on that one though its been too long and the cobwebs have taken over the NEC section of my brain)

BCS
 
You are going to need to put an SCCR on the panel, otherwise it will be assumed to be only 5kA, which the installer will curse you for because the responsibility will fall upon them to prove that the available fault current is 5kA or less. What you really should do is investigate the ratings of the power components and see what fuses are necessary ahead of them to achieve a more reasonable SCCR for a 480V 3 phase system. I would suggest striving to achieve 35kA minimum, higher would be better. Most likely the only change you will need will be making the main a fused disconnect (non-fused disconnects are never rated more that 10kA SCCR), but it will depend on the series ratings of all of the power devices. The component manufacturers will (should) have that information available, otherwise, change mfrs.
 
The panel only has two SCR controllers, fuses, breaker. No transformer and fan now.

I'm changing to 40A J fuses ahead of SCRs. The breaker will be sized at 80A
30+30 = 60 * 1.25 = 75, Next standard size = 80A

The SCRs have a 200kA SCCR rating. The CB has a 30kA SCCR rating. The contactor has 5kA SCCR rating.
 
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I would suspect that the contactor would have a combination rating when used with appropriate current limiting fuses or breakers. If not, that 5kA is pretty painful.

Keith
 
That is truly unfortunate. At that current level there isn't much you are going to do to keep peak let-through under 5kA. If SCCR really matters to you I would suggest you look at a different contactor that has a high fault combination rating.

Keith
 
On small panels I fuse/breaker each item and put a note on the schematic:

"Customer supplied fused disconnect required" with the power requirements listed.
 
No need for over current relais, just a simple fuse rated the maximum current of the heaters, as they can not overload.
so fuse should be 60 amps, now you can calculate the cable length and its size., put the fuses on the mainboard, if the cables are smaller you may have to check max cable length and size of cable.
the customer must have fused to prevent shortage in cables as aabeck says.
 

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