How to wire a 4 lamp ballast for 3 lamps

As a DIY’er, I often struggle with wiring lighting. I always want to make my light as bright as possible, but this often results in a lot of heat and even dangerous wiring.

I’m writing this how-to to help other DIY’ers wire their lights more safely and efficiently.

I will be using an HPS lamp because that is the most common MH/HPS ballast type. The principles are the same for an LED lamp; however, verification should be done with the manufacturer’s documentation. First, some background information on what you’re working with:

How to wire a 4 lamp ballast for 3 lamps, Zazzy Home

First, some background information on what you’re working with:

A single lamp ballast has just one power lead (a hot), one neutral(white), one ground(green or bare wire), and a single control lead.

A 4 lamp ballast has two power leads (a hot for each side of the 120V circuit), two neutrals, two grounds, and four control leads.

In the United States, the electricity travels from the breaker box to a junction box in your main electrical panel. This is where you have your lights wired into your house’s power system. The lamp ballast is installed directly above this wiring junction in a dedicated light fixture known as a junction box.

In Europe, power enters through a wall socket and then runs straight to the lighting junction.

I will be referring to three different types of devices

The first device is the lamp ballast

It is a device that controls the voltage going to your lamps, regulates the current(amps) output to your lamps, and keeps the polarity correct.

The second device is the lamp itself. 

Its purpose is to produce light for your fixture. The most common types are magnetic metal halide (MH), high-pressure sodium (HPS), and double-ended metal halide (DE).

There are also linear fluorescent lamps, but I will not be discussing them in this article except for this note: Fluorescent tubes require specialized ballasts or electronic drivers to power them safely. It would help if you always used these fixtures as their manufacturer intended them. Attempting to wire one of these into an HID ballast can result in a fire.

How to wire a 4 lamp ballast for 3 lamps, Zazzy Home

The third device is the reflector(s) 

This is what directs and focuses the light from your lamps into your garden. The more reflective, the better for growing. Some good examples of reflectors would be mylar, aluminized (mylar with a thin layer of aluminum), and polished aluminum. Others such as white paint will suffice but leave much to be desired.

A poor quality sheet metal shop may even sell you ‘white finished’ or whatever their cheapest-looking finish is called; these do NOT work for growing plants!

You can use regular household wiring devices such as switches and outlets, but certain precautions should be taken:

– Use a GFCI

– If your HID is in its own separate four-lamp junction box, then you do not need the GFCI. However, if your main electrical panel or single light junction box contains a regular lighting fixture, you must use the GFCI.

– The neutral wire from the ballast should always be connected to either your incoming house power neutral wire or to the white wire from the secondary incoming lines(the ones that go from one switch to another)

– Always place an outlet or two on each of your switches for convenience and future expansion. Also, make sure it is a grounded duplex outlet. Then you would have one for turning on/off with a push-button switch and the other for plugging in your reflectors or fans.

A few tips to keep in mind:

– If your ballast is 120V only, you would wire it up the same as above but replace the two lamp wires with one power(hot) and one neutral(white). The device will share these two lines equally among its four lamps. This kind of ballast sometimes has a switch on it to run two or all four lamps simultaneously. You may also use this type of ballast to wire up three regular household lights instead of four HID’s. If you don’t need to grow anything right now, just remember that it is not intended for that purpose.

– Some higher wattage (300W+)ballasts come with an internal grounding screw. This can be used to secure the ballast to your metal junction box, but it should not be depended on as the sole means of grounding. Instead, ALWAYS run a ground wire directly to the house’s main electrical panel or single lighting junction box regardless of how good you think your ballast is grounded!

– ALWAYS keep moisture away from anything electrical

– If you are adding more lamps than what it was originally designed for (using two four lamp ballasts instead of one eight lamp), then be sure that there is enough ventilation and that all other wiring is equally sized; otherwise, you risk a fire.

This article does not cover dimming HID lights. However, it may cause damage to your ballast if done improperly. In some cases, it can lead to electrocution or fire as well.

In the end, it is up to you if you want to wire something yourself. I tell people that they should spend a few hours wiring their ballasts if they want them in a permanent location instead of spending hundreds of dollars on hiring an electrician who may or may not do a good job and will probably be more expensive than the equipment itself. At least now you know what you are getting yourself into before going out and doing things your way!


Meet Jeff. For the last 10 years, he's been repairing and fixing problem homes - from leaky roofs to faulty wiring. He started blogging about his experiences as a way to help others who might be struggling with home repairs, and he's become something of an expert in the field. Jeff is always up for a challenge, and he loves sharing his tips and advice with others. When it comes to home repairs, Jeff knows what he's talking about. So if you're looking for some help and guidance, be sure to check out his latest guide!

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