Solar Combiner Boxes FAQs


What benefits does the integrated disconnect switch provide?

Opening the switch will open circuit a section of the array served by that combiner box and subsequently stop current flow. An open circuited section of the array will allow typical service for portions of the array without shutting down the entire inverter system. The disconnecting combiner box serves:

  1. System integrators, who wish to reduce PV system costs and complexity.
  2. First responders, who can locate the disconnect next to the array during an emergency event.
  3. System installers, who can check circuits, insert fuses, and more easily commission sections of the system at the combiner box.
  4. Maintenance crews when repairing the array.
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How is the solar combiner box properly fused?

The Blue Oak PV Products HomeRun™ Combiner Boxes use a MIDGET class fuse to protect the individual solar array circuits. This class of fuse is available from manufacturers such as Littelfuse, Bussmann, and Ferraz Shawmut in both 600VDC and 1000VDC rated versions. The fuses are offered in a range of current ratings up to 30A, which is also the maximum allowable fuse size for our combiner box products.

Fuses used in photovoltaic systems are sized per the National Electrical Code Article 690 (NEC 690). Typically the fuse sizes is selected based on the short circuit current rating (Isc) for the circuit, multiplied by a 1.56 safety factor. You would properly select the fuse by using the next fuse size above the result from this simple multiplication.

The main conductors leading from the combiner box are required to be fused as well. This is a requirement in the product’s safety listing to protect the combiner box and switch from backfed current. The downstream fuse is required to be 200 Amps or less and is typically included in the inverter’s DC cabinet.

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How can solar combiner boxes be mounted?

Reference the individual model installation manual for mounting details. All of our commercial/utility scale products are housed in Type 4 or Type 4X enclosures. They can be mounted in any orientation. We don’t recommend mounting any of the disconnecting models with the switch handle facing up when exposed to the elements.

The combiner boxes are typically mounted using strut on a roof or to a wall with the appropriate interface devices for the service. In ground mount applications, the combiner boxes can be mounted directly to racking systems or on their own strut system.

Our HCB4 residential combiner is Type 3R to be mounted vertically or up to 30° from vertical.

Combiner boxes must be mounted in an area to allow for access per NEC 110.26.

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Can you build a combiner with a custom number of input circuits (poles) required for my design?

We have worked diligently to offer a range of products which fit most residential, commercial and utility scale solar projects. By offering a limited number of products, we have the opportunity to streamline our operations and keep product costs to a minimum. We cannot offer modified products without field or shop approved modification and review by our listing agency. Our products are all listed to the UL 1741 standard with the specific number of poles listed in the model. What to do with that extra couple of poles? Consider them as spares. Over the next 20 years, it is likely that spare might come in handy.

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Are your combiner boxes "touch-safe"?

The HomeRunTM combiner boxes from Blue Oak PV Products utilize UL listed touch-safe fuse holders and include barriers (on most models) around all positive portions of the circuits to protect against any incidental contact. Even though this protection is installed, these products could be energized with up to 600VDC if the sun is energizing the solar array. Only trained and certified electricians should open and service combiner boxes after de-energizing the circuits.

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Do you offer any surge suppression in your combiner boxes?

We currently do not offer surge suppression. The value of surge suppression in combiner boxes is still open for debate in many areas. If the project calls for it, surge suppressors are often added in the field during installation.

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I am using 90°C rated conductors. Are your terminals rated for 90°C?

Our terminals can accept 75°C and 90°C rated conductors. All Blue Oak PV Products are listed to UL 1741. If the product is being used within its nameplate ratings, the “actual” terminal temperature is not an issue. Within the nameplate ratings, our products have been safety tested and approved for use.

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Could you recommend the correctly sized output conductor for my project?

Yes, we would be glad to accommodate your design needs to size the DC output conductors from the combiner box to the inverter. Please provide the following information:

  1. Total short circuit current (in Amps) connected to the combiner box.
  2. Total one-way conductor distance from the combiner box to the inverter.
  3. System installation location (i.e.; a rooftop in Sacramento, CA, or a ground mount in Phoenix, AZ)

E-mail the information to sales@blueoakpvproducts.com.

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