Maximum Fuse Size

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Dale Hayes

Senior Member
My full resistive load is 19Amps.

A control panel is available that is fused at 50Amps with 8ga 90C wire. Can I connect the 19Amp device to the 50Amp circuit and stay within applicable codes? A 19 amp load on a 50Amp fuse has a 263% fudge factor. Standard practice is to size the fuse at 125% of the full 19Amp resistive load (23.75Amps for the fuse in this case).

What NEC codes apply?
 
kwired

kwired

Electron manager
Location
NE Nebraska
Dale Hayes said:
My full resistive load is 19Amps.

A control panel is available that is fused at 50Amps with 8ga 90C wire. Can I connect the 19Amp device to the 50Amp circuit and stay within applicable codes? A 19 amp load on a 50Amp fuse has a 263% fudge factor. Standard practice is to size the fuse at 125% of the full 19Amp resistive load (23.75Amps for the fuse in this case).

What NEC codes apply?
Click to expand...

We need to know what the load is in case there is a section with specific rules for that particular load.

If it is perrmitted the conductors likely need to be 50 amp conductors with some exceptions for tap conductors at the location where the load is.
 
kwired

kwired

Electron manager
Location
NE Nebraska
If it is used for space heating then any requirements in 424 apply.

If it is used for process heating general requirements of 240 and 310 are all that likely apply.

If it is a listed unit instructions may tell you what size overcurrent protection is allowed to comply with listing.

Like I said before if there is no limit for overcurrent protection you still have to run 50 amp conductors if the overcurrent protection is 50 amps.
 
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Dale Hayes

Senior Member
To kwired,

The control panel is pre-wired with 50amp fuses and 8ga 90C wire.

By transposing numbers and using the 125% derating factor the full load should not be more than 40Amps if protected by a 50Amp fuse. (1.25 x the 40Amp Full Load = 50Amp).

I have a 19Amp load. My concern is that the fuse derating factor is not 1.25 but 2.68.

Using the 1.25 derating factor:

A 19Amp load, with 14ga 90C wire is protected by a 25Amp fuse. (19 x 1.25 = 25)

A 40Amp load with 8ga. 90C wire is protected by a 50Amp fuse. (40 x 1.25 = 50)

What about the 19Amp load, 8ga, 90C wire, protected by 50Amp fuse?
 
kwired

kwired

Electron manager
Location
NE Nebraska
Article 240 addresses overcurrent protection for conductors. For overcurrent protection of the load you need to refer either to code section that applies to the equipment to see if there is anything specific for that equipment or to equipment instructions if they state what overcurrent protection should be.

I am guessing outside of equipment instructions the applicable rules will be in either 422, 424 or 427 for your application.

I would read 422.11 and check out the other sections in case they apply to you.

Your installation is likely acceptable. It is very likely you can have multiple units on one circuit up to 50 or 60 amps max, if that is the case then you would need higher overcurrent device.
 
D

Dale Hayes

Senior Member
I cannot change out the 50Amp fuses with 30Amp. The control panel is designed for 50Amp. The control panel nameplate is stamped at 50Amp. The wiring schematic is drawn at 50Amp.

I'm back to square one: 19Amp device on a 50Amp fuse?
 
kingpb

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
Add a fused disconnect at the load side of the control panel, within 10ft and use the appropriate size fuses for the load.

But I am confused, because isn't the control panel usually designed specifically for the equipment? Also, just because the control panel is designed for 50A doesn't mean you can't fuse it lower. The fuses have to be replaceable, even if it's for like kind.

I guess I'm just not seeing the whole picture:dunce:
 
kwired

kwired

Electron manager
Location
NE Nebraska
kingpb said:
Add a fused disconnect at the load side of the control panel, within 10ft and use the appropriate size fuses for the load.
Click to expand...

Why 10ft?? if the load is 19 amps you could feed the control panel from a 25 amp fuse or breaker at the point this all starts at. And leave the 50 amp fuses in the control panel if you wish.
 
Smart $

Smart $

Esteemed Member
Location
Ohio
Dale Hayes said:
I cannot change out the 50Amp fuses with 30Amp. The control panel is designed for 50Amp. The control panel nameplate is stamped at 50Amp. The wiring schematic is drawn at 50Amp.

I'm back to square one: 19Amp device on a 50Amp fuse?
Click to expand...
Are there any additional fuses or circuit breaker for the output circuit in the control panel?

Is the control panel, heater, and interconnecting wire furnished from the manufacturer as a complete unit... or is the interconnecting wiring not supplied?

If the latter, how far, in one-way feet of wire, from the control panel is the heater?

What type of controller is used?

Is there an output rating on the control panel?
 
kingpb

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
kwired said:
Why 10ft?? if the load is 19 amps you could feed the control panel from a 25 amp fuse or breaker at the point this all starts at. And leave the 50 amp fuses in the control panel if you wish.
Click to expand...

Thinking of 10ft tap rule, however are you confirming that this is a branch circuit and 240.21(A) applies, therefore there is no length restriction?
 
kwired

kwired

Electron manager
Location
NE Nebraska
kingpb said:
Thinking of 10ft tap rule, however are you confirming that this is a branch circuit and 240.21(A) applies, therefore there is no length restriction?
Click to expand...

If you put in a fused disconnect with appropriate sized fuse for the conductor it does not matter if it is 6 inches away or 20 miles you do not have a tap.
 
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