Required conductor ampacity when fuses are oversized?

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PaulHartley2

Member
Location
Pennsburg, PA
Occupation
Solar Engineer
In a situation where one wants to "oversize" fuses to avoid overheating/tripping (i.e a fused AC Combiner on a hot rooftop in Texas--collecting Inverter output circuits),
do the conductors have to be "oversized" proportionally? I see that NEC ('17) 240.4 (B) allows for the "next higher standard" overcurrent device rating to be used, but is that the greatest ampacity difference permitted (one standard fuse size)?

The current in these circuits is limited by the inverter outputs on the PV side of the fuse, and is connected to the Utility on the other side of the fuses.

In other words, in this type of circuit, can the fuses be rated significantly higher than the conductor ampacity....higer than just the "1 standard device rating size"?

thank you.
Paul Hartley
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
If you are worried about heating of the conductor than you can run larger conductors to compensate for the heat. I am not a pv guy but I don't think you can oversize the overcurrent protective device for a smaller conductor.
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
If you look at a TCC curve from one breaker size to the next, you can see some of the justification for going up to the next size

I’m not quite following the logic here. The wire is overheating yet you want to increase the breaker size?
 

PaulHartley2

Member
Location
Pennsburg, PA
Occupation
Solar Engineer
Thank you. This is a 480V circuit. The concern is that the excessive heat on the rooftop combiner box may cause a typically-sized fuse to overheat/melt under normal operation, hence the desire to over size the fuses (not the conductors). The question is do the conductors have to be sized to match the overcurrent device rating?
 

Carultch

Senior Member
Location
Massachusetts
In a situation where one wants to "oversize" fuses to avoid overheating/tripping (i.e a fused AC Combiner on a hot rooftop in Texas--collecting Inverter output circuits),
do the conductors have to be "oversized" proportionally? I see that NEC ('17) 240.4 (B) allows for the "next higher standard" overcurrent device rating to be used, but is that the greatest ampacity difference permitted (one standard fuse size)?

The current in these circuits is limited by the inverter outputs on the PV side of the fuse, and is connected to the Utility on the other side of the fuses.

In other words, in this type of circuit, can the fuses be rated significantly higher than the conductor ampacity....higer than just the "1 standard device rating size"?

thank you.
Paul Hartley

If you do put in larger than NEC-required fuses or ocpd's in general, the corresponding wire needs to be large enough to "be protected" by them. Either 240.4(B) applies and you get to "round up" to the next standard size (such as 351A of conductor protected by 400A fuses). If 240.4(B) doesn't apply, such as over 800A or non-standard OCPD rating, then the wire would need to have at least as many amps as the OCPD.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Thank you. This is a 480V circuit. The concern is that the excessive heat on the rooftop combiner box may cause a typically-sized fuse to overheat/melt under normal operation, hence the desire to over size the fuses (not the conductors). The question is do the conductors have to be sized to match the overcurrent device rating?
One thing to keep in mind is that the OCPD on the AC side is to protect the conductors from fault current coming from the service, not from the inverter(s), so yes, when you upsize the OCPD you need to verify that the conductors are protected, i.e., their ampacity derated for temperature and conduit fill must be greater than the rating of the next size down OCPD.
 
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