Disconnects Vs Fused Disconnects

Merry Christmas

WA_Sparky

Electrical Engineer
Location
Vancouver, WA, Clark
Occupation
Electrical Engineer
Just a general question for you EC's out there.
If you have a 30A/2 piece of equipment, do you see more 30A/2 Disconnects or 30A/2 Fused disconnects in the field? I know code required just a disconnect, but its our company standard to provide most all with fused disconnects... Just seeing if this is an industry standard to protect equipment or an over engineered standard.

Thanks,
 

ASIsparky

Member
Location
Philadelphia, PA
Occupation
Electrician
We generally don't see fused disconnects unless multiple disconnects are sharing the same feeder (rarely ever). In this case, the breaker protects the feeder and the fuses protect the equipment.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Just a general question for you EC's out there.
If you have a 30A/2 piece of equipment, do you see more 30A/2 Disconnects or 30A/2 Fused disconnects in the field? I know code required just a disconnect, but its our company standard to provide most all with fused disconnects... Just seeing if this is an industry standard to protect equipment or an over engineered standard.

Thanks,
Perhaps your company gets the fused discos for cheap, or finds efficiencies in only stocking one component. Otherwise there's no good reason for that.
 

EC Dan

Member
Location
Florida
Occupation
E&C Manager
A fused disconnect could be used to reduce fault current and raise the SCCR of a listed panel downstream of the disconnect.
 

mayanees

Senior Member
Location
Westminster, MD
Occupation
Electrical Engineer and Master Electrician
My understanding is that the disconnect would have to be located in a fault current environment under 10,000 amps in order to be nonfused.
Given that your application is 30-amp, 2-pole, it's likely that this is under 10kA, but you'd need to do a fault current calculation in order to be sure. An infinite primary calc method of the upstream transformer fla/transformer Z should suffice.
 

Rock86

Senior Member
Location
new york
Occupation
Electrical Engineer / Electrician
Does the equipment require fused disconnect? Some equipment allows you to use get away with a non-fused, others may specify fused.
 

Buck Parrish

Senior Member
Location
NC & IN
I have heard from an instructor. For motors particularly HVAC units. The fuse protects against an over load where the breaker is for the ground fault. But now with breakers having a hcar listing. The motor has an integral overload protection. The fuse is no longer required.

Some may have to do with the manufacture of the motor and the UL listing
Just my two cents...
I still see them a lot in rural areas.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Just a general question for you EC's out there.
If you have a 30A/2 piece of equipment, do you see more 30A/2 Disconnects or 30A/2 Fused disconnects in the field? I know code required just a disconnect, but its our company standard to provide most all with fused disconnects... Just seeing if this is an industry standard to protect equipment or an over engineered standard.

Thanks,
My guess is someone just decided that it would be a good idea to have a single part number rather than multiple part numbers. Most of these kind of disconnects can be purchased for under $20 with the fuses so the cost of the part just doesn't matter.

That way you can use the same part even if by some random chance you happen to have a higher short circuit current available than normal.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I have heard from an instructor. For motors particularly HVAC units. The fuse protects against an over load where the breaker is for the ground fault. But now with breakers having a hcar listing. The motor has an integral overload protection. The fuse is no longer required.

Some may have to do with the manufacture of the motor and the UL listing
Just my two cents...
I still see them a lot in rural areas.
That's just nonsense. You cannot use a fuse as an overload device on a motor, it just won't open reliably enough.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I actually pretty much never do for my solar stuff. I'm not sure why the solar industry loves them so much (except in the case of a utility required visible blade break).
Most of the AHJs I deal with require such a switch, and when the PV system is supply side interconnected, I use a fused disco. I design commercial PV systems, so a lot of what I do is supply side connected.

One AHJ that I deal with a lot even requires a fused disco on some backfed breaker interconnected systems, which makes no sense.
 

Buck Parrish

Senior Member
Location
NC & IN
That's just nonsense. You cannot use a fuse as an overload device on a motor, it just won'
I read the following somewhere. I think it was printed in N Illinois, probably nonsense ;) Be sure to use a smiley face when you call some one's post nonsense :) I was going to say It's FRIDAY, But it's not. It's better than Friday
Here's the quote...

"Overload protection is a protection against a running overcurrent that would cause overheating of the protected equipment. Hence, an overload is also type of overcurrent. Overload protection typically operates on an inverse time curve where the tripping time becomes less as the current increases. Overload relays as well as “slow blow” fuses are commonly used to provide overload protection."
 

wbdvt

Senior Member
Location
Rutland, VT, USA
Occupation
Electrical Engineer, PE
A fused disconnect could be used to reduce fault current and raise the SCCR of a listed panel downstream of the disconnect.
This is not totally true. The SCCR is a tested value and you cannot exceed it. Therefore, the fuse would have to limit the fault current below the SCCR value and this can be done as a tested configuration.

I suspect that a fused disconnect may have been decided on as a standard to insure that the disconnect is not in a location that would exceed its SCCR rating. Non-fused disconnects are rated at 10kA whereas a fused disconnect, with the proper fuses, can be up to 200kA.

Many studies that I do on existing electrical systems, I find non fused disconnects that are installed in a location where the fault current exceeds 10kA. These need to be replaced with fused disconnects so this more costly after the fact than installing fused as part of original install.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
This is not totally true. The SCCR is a tested value and you cannot exceed it. Therefore, the fuse would have to limit the fault current below the SCCR value and this can be done as a tested configuration.

I suspect that a fused disconnect may have been decided on as a standard to insure that the disconnect is not in a location that would exceed its SCCR rating. Non-fused disconnects are rated at 10kA whereas a fused disconnect, with the proper fuses, can be up to 200kA.

Many studies that I do on existing electrical systems, I find non fused disconnects that are installed in a location where the fault current exceeds 10kA. These need to be replaced with fused disconnects so this more costly after the fact than installing fused as part of original install.
Yeah but for a 30A disco? 30A breakers above 10k exist but are very rare.
 
Yeah but for a 30A disco? 30A breakers above 10k exist but are very rare.
I'm not sure I would say "very rare". Yes probably not stocked normally at supply houses, but readily available from an online breaker supplier:


 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I'm not sure I would say "very rare". Yes probably not stocked normally at supply houses, but readily available from an online breaker supplier:


But my point is that most 30A equipment discos will have a 10k breaker upstream so there is no problem using an unfused disco. And by 'most' I mean 99.9%, at least in resi.
 
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