ac disconnect derating for inverter output

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rmartin

Member
Location
branchburg nj
I have a 400 amp 600 volt ac fused disconnect connected to an inverter whose output is 316 max output . The system is designed to produce the maximum output during solar noon . The blades on the disconnect are over heating. I am trying to resolve the issue . This is the calculations I was given by the designer.

316 x 1.25 = 395 amps
Disconnect is rated for 400 amps and fused with 400 amp fuses Nema 3R
Wiring is parallel 250 MCMs .
What I have concluded is that the disconnect is only rated for 320 amps for continuous use and not suitable for use.
What i am being told is that the disconnect can handle the 316 amps from the inverter and that the calculations cancel each other out.
The disconnects are sitting outside in the sun on the side of the building .
What derates would apply ?
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Disconnect switches don't get derated. What makes you think it's overheating? Just being out in the sun it might get pretty warm. That does not mean there's anything really wrong with it.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I have a 400 amp 600 volt ac fused disconnect connected to an inverter whose output is 316 max output . The system is designed to produce the maximum output during solar noon . The blades on the disconnect are over heating. I am trying to resolve the issue . This is the calculations I was given by the designer.

316 x 1.25 = 395 amps
Disconnect is rated for 400 amps and fused with 400 amp fuses Nema 3R
Wiring is parallel 250 MCMs .
What I have concluded is that the disconnect is only rated for 320 amps for continuous use and not suitable for use.
What i am being told is that the disconnect can handle the 316 amps from the inverter and that the calculations cancel each other out.
The disconnects are sitting outside in the sun on the side of the building .
What derates would apply ?

The switch/fuses is/are rated for 320A continuous and your inverter output is 316A. That is sufficient, though just barely; you do not multiply both the inverter output by 1.25 and the switch/fuse rating by 0.8.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
The switch/fuses is/are rated for 320A continuous and your inverter output is 316A. That is sufficient, though just barely; you do not multiply both the inverter output by 1.25 and the switch/fuse rating by 0.8.

switches and fuses don't get derated for continuous versus noncontinuous duty unless this is a special rule just for PV.
 

just the cowboy

Inactive, Email Never Verified
Location
newburgh,ny
1 blade or all

1 blade or all

One blade or all blades?

Did you check voltage drop across the blades to check for loose blades.

Are there hot spots or across whole blade, and are the conductors just as hot?
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
A common cause of overheating of fuse holders or switch blades is a poor connection between wires and lugs. That heats the connected parts.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
switches and fuses don't get derated for continuous versus noncontinuous duty unless this is a special rule just for PV.
Maybe not directly, but to calculate a fuse size you multiply the inverter current by 1.25 and round up to the next standard size, which is essentially doing the same thing, and then you cannot put (for example) 110A fuses in a 100A switch. The result is that both the fuses and the switch must be rated at at least 125% of the inverter current. The same is not true of unfused switches; they just need to be rated at at least 100% of the inverter max current.
 
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rmartin

Member
Location
branchburg nj
The inside of the fused disconnect says it must be de rated for continuous use by 80% non fused disconnect are continuous use no de rating is needed
so right off the bat that disconnect is only good for 320 amps continuous
doing the continuous solar calculation 316 x 1.25 = 395. Dictates the wire size and disconnect amperage to be 400amps but the disconnect for continuous use according to the manufacturer is only good for 320
therefore a 600 amp disconnect with 400amp fuses should be used
 

rmartin

Member
Location
branchburg nj
Disconnect switches don't get derated. What makes you think it's overheating? Just being out in the sun it might get pretty warm. That does not mean there's anything really wrong with it.
The blades and fuse blades are turning black and the wiring is reading about 180 degrees
we have already replaced the disconnect once before for the same issue and it heated up within 6 months
 

pv_n00b

Senior Member
Location
CA, USA
The disco is rated correctly. If it's getting too hot you are looking at a couple of causes:

  • Conductors are under sized and getting hot, that is getting transferred to the disco components through the terminations.
  • The terminations are not done correctly and are high resistance
  • The current is higher than you think
  • Bad fuses with high resistance, a long shot.

Not really much else that can be causing it that would go over two discos.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
The inside of the fused disconnect says it must be de rated for continuous use by 80% non fused disconnect are continuous use no de rating is needed
so right off the bat that disconnect is only good for 320 amps continuous
doing the continuous solar calculation 316 x 1.25 = 395. Dictates the wire size and disconnect amperage to be 400amps but the disconnect for continuous use according to the manufacturer is only good for 320
therefore a 600 amp disconnect with 400amp fuses should be used

To repeat what others have said, both the 80% derate and the 125% increase are for continuous use. See 705.60(B). I agree that you do not have to perform two calculations for continuous use. You only have to do one and your switch's nominal rating is fine.

My only suggestion to add to pv_n00b's good advice is to check whether the ambient temperature is higher than the switch is approved for.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
All the blades are turning black and heating up to the point where you cannot hold the wires
That the wires are too hot to hold doesn't necessarily mean that there is something wrong. Wiring should be sized so that the conductor temperature stays below 75 degrees C, which is 167 degrees F. That's pretty hot to the touch and I don't think I could hold onto wires that hot.
 

PWDickerson

Senior Member
Location
Clinton, WA
Occupation
Solar Contractor
You indicated that the conductors are 250 kcmil but did not specify CU or AL. AL 250 conductors have a 75 degree ampacity of 205 amps, or 410 amps for two parallel conductors, so I am guessing that is what was installed. How hot do ambient temperature get at the switch location? Did you factor in any temperature derate? Did all three poles overheat the same amount? Did anyone take any current measurements? Can you post a photo? I agree with others that the equipment is adequately rated. Are you certain the terminations were properly torqued?
 
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