PV DC Fuse Sizing. Why min fuse size rather than max fuse size requirement?

[parallel_fuses]

2017 NEC 690.9(B)(1) "Not less than 125 percent of the maximum currents calculated in 690.8(A)."

Why must the fuses used be NOT LESS than 156% of Isc rather than NOT MORE than 156% of Isc?

In every other situation I can think of, the purpose of OCPD is to protect the conductors and/or other equipment. I understand that the current produced by a string can exceed the STC Isc when in the field. But what other reasons are there for setting a minimum fuse size?

Is there a maximum fuse size requirement that also applies? I do not expect using a 100A fuse on 10A strings using 10AWG CU would be to code, but where in the text is the maximum fuse size capped for DC source circuits?

 

[wwhitney]

2017 NEC 690.9(B)(1) "Not less than 125 percent of the maximum currents calculated in 690.8(A)."

Why must the fuses used be NOT LESS than 156% of Isc rather than NOT MORE than 156% of Isc?

So the fuse doesn't blow during normal operations?

Is there a maximum fuse size requirement that also applies?

Certainly for a given conductor size, as you must comply with 240.4.

Cheers, Wayne

 

[ggunn]

2017 NEC 690.9(B)(1) "Not less than 125 percent of the maximum currents calculated in 690.8(A)."

Why must the fuses used be NOT LESS than 156% of Isc rather than NOT MORE than 156% of Isc?

As wwhitney says, so that the fuse doesn't blow during normal operation. Fusing on DC lines isn't there to protect the conductors from the modules they are connected to; the conductors are sized so that the maximum current that a string can produce will not endanger them. The fuses are there to protect the conductors from other strings that are combined with them to keep the other strings from backfeeding a fault with more current than the conductors are rated for. That's why if there are only two strings being combined, fuses are not necessary.

 

[jaggedben]

PV circuits are generally expected to operate at or near max current continuously for several hours on most days of the year. So the code requires OCPDs and conductors to be sized higher than the expected max current to ensure that the conductor sizes are truly adequate and avoid nuisance tripping from the normal operating current.

The 156% is also for the rare case when brighter sunlight conditions could cause the current to exceed the normal nameplate current.

 

[ggunn]

PV circuits are generally expected to operate at or near max current continuously for several hours on most days of the year. So the code requires OCPDs and conductors to be sized higher than the expected max current to ensure that the conductor sizes are truly adequate and avoid nuisance tripping from the normal operating current.

The 156% is also for the rare case when brighter sunlight conditions could cause the current to exceed the normal nameplate current.

That 156% is confusing to many newcomers to solar; when I was training people I kept the two 125% numbers separate. I defined DC Imax as 125% of Isc and used it instead of Isc; thereafter DC and AC conductors are handled identically.

 

[Zee]

It wasn't the question..... but many fuses I sized properly (156% Isc rule above) have blown on my systems. I have gone back and replaced many of these 15A fuses with 20A fuses, or 10A ,12A fuses with 15A fuses.
The rooftop heat and sunlight on a rooftop J-box is no joke.
The hottest boxes are ones like Soladeck which attaches to the roof itself.

 

[ggunn]

It wasn't the question..... but many fuses I sized properly (156% Isc rule above) have blown on my systems. I have gone back and replaced many of these 15A fuses with 20A fuses, or 10A ,12A fuses with 15A fuses.
The rooftop heat and sunlight on a rooftop J-box is no joke.
The hottest boxes are ones like Soladeck which attaches to the roof itself.

As long as you stay under the maximum fuse size shown on the module data sheet, you are fine.