Sizing OCPD for 3 Phase

[Zyb]

Hi there,

Do I need to divide by 1.732 in sizing the fuse in disconnect or not?

IQ7 Max. Continuous Current = 1.0 A

Total No. of Inverter = 42

1.0 x 42 x 1.25 = 52.5A = 60A Fuse

1.0 x 42 x 1.25 / 1.732 = 30.3 A = 30A Fuse

30A or 60A Fuse?

Thank You!



3 Phase.png

[SunEagle]

If you are talking about the fuses in the 3 phase Fuse Disconnect then it should be what the buss is rated in the load center.

[Zyb]

If you are talking about the fuses in the 3 phase Fuse Disconnect then it should be what the buss is rated in the load center.

Thank you for your response SunEagle. Is that a code that fuse will be the same with the bus in load center? Let say the PV system will be back feed to a 400A Service Panel, what will be the size of the 3pole breaker? Thanks

[PVAndy]

Thank you for your response SunEagle. Is that a code that fuse will be the same with the bus in load center? Let say the PV system will be back feed to a 400A Service Panel, what will be the size of the 3pole breaker? Thanks

SunEagle was talking about the fuse in the Fused AC disconnect feeding the panel. The breaker in the panel is sized at the next standard size up from 1.25 times the max operating current. For 3 phase as you have indicated divide by the square root of 3 (1.732)

Andy

[solarix]

OK, first understand that IQ7s are a single phase animal and are wired line to line on a 3ph service. So assuming your 3ph panel is 208VAc line to line so that the leg voltages are 120Vac, then you wire the microinverters line to line with 14 on each set of phases. Actually, the IQ7 spec sheet says that 13 microinverters is the max per branch circuit and max output is then 1.15A at 208Vac. So 13 x 1.15A x 1.25 = 19A. So that is three sets of dual 20A breakers - one on each pair of phases to balance the load. Now when the current from these three sets combine together in the 3ph buss, the ampacity becomes just 19A /1.73 = 11A. With three circuits, your utility will probably require a single disconnect switching all three at the same time, so they should be combined together (using a small 3ph load center) before the 3ph disconnect - which would then be rated for at least 11 amps or 30A. The backed breaker in the main panel would then be a triple 20amp breaker.

[SunEagle]

Thank you for your response SunEagle. Is that a code that fuse will be the same with the bus in load center? Let say the PV system will be back feed to a 400A Service Panel, what will be the size of the 3pole breaker? Thanks

The main breaker or fuse is sized based on the panel amp rating and not the amps of the loads. Since you have a fused disconnect that is where the over current protection sits unless the panel has a main breaker. If it is a 400amp service panel then the fuses can be 1.25% times 400amps as mentioned by PVAndy

[PVAndy]

Also take note that with many utilities when using single phase inverters on a 3 phase service, phase loss and phase unbalance circuity is required. When we've needed to do it we've used a phase loss detector and a 3 pole contactor. https://assets.omron.eu/downloads/da...tasheet_en.pdf