Subpanel – Planning

Before embarking on any projects, it is critical to first sit down and logically figure out what is needed and what steps are necessary to achieve project goals.

1. Subpanel placement
2. Electrical Load Requirement
3. Subpanel Size
4. Feeder Cable size
5. Subpanel Type

Subpanel placement

One of the most critical component was determine where I was going to install my subpanel.   That meant figuring out things like the distance from my main to subpanel, overcoming obstacles (literally), etc. Please note that the distance is directly proportional to the feeder cable size. That means that due to wire resistance, the longer the distance, “fatter” the wire has to be to overcome this distance (I will get into it more in detail later).

Ideally, the subpanel should be located where you can easily run the feeder cable from the main panel.  Draw a simple stick diagram to describe the location of the main panel, how the wiring will be pulled, and where the subpanel will be located.  Figure 1.2 shows how I decided to run my cable and install my subpanel in the basement.

TIPS: Purchase some cheap, long rope to gauge the actual distance to your new subpanel location. I “estimated” my distance and ended up purchasing $200 worth of feeder cable that ended up being short (ouch!) Add about 15% linear feet to the total distance to ensure that you have enough cable length to connect to the subpanel.

Electrical Load Requirement

I got this nifty Residential Load Calculator some time ago (sorry, I forgot where so if you know the author, please let me know so that I can give proper credit). If you are very new, it may be slightly complicated but give it a try (ResidentialLoadCalculations).  Or if you prefer, you can follow my simple instructions below:

First, before anything else, please remember this formula:  Wattage = Voltage x Amperage.  This will help you to understand when calculating a circuit load requirement

Second, all circuits should never exceed the 80% of the circuit capacity.  This creates a safety margin to minimize circuit overloading (less trip to the circuit breaker to reset).  That means for 120 volts, a :

• 15 amp (1,800 watts) circuit – Plan on max utilization of 12 amps  or 1,440 watts
• 20 amp (2,400 watts) circuit – Plan on max utilization of 16 amps or 1,920 watts
• 30 amp (3,600 watts) circuit – Plan on max utilization of 24 amps or 2,880 watts

Third, look at the tags on all appliances and lighting fixtures and try to evenly distribute across different circuits.  Amperage heavy appliances like refrigerator should be on its dedicated circuit.

TIP: Check with your township’s electric inspector. Some towns follow various iterations of NEC codes.

For my project, I planned on renovating my kitchen in the near future so I wanted to future-proof my electrical load requirements as much as possible.  That meant I had to place new dedicated circuits to support a dishwasher, refrigerator, small appliances, gas oven as well as well as dedicated circuits for the counter-top outlets and lighting fixtures as per NEC 2005 code.In the end I chose to go with Square D 100 Amp Subpanel

Subpanel Size

Although 60 amp subpanel was sufficient based on my load requirement, I decided to spend extra $60 to upgrade to a 100 amp sub panel. This upgrade gave me confidence that I can handle any future power growth, whether it be a kitchen upgrade, or finishing my basement (on a side note, most basement remodels will require a dehumidifier to run during the spring and summer months; humidifier has a large amperage requirement)

The panel I chose was Square D QO 100 amp Value Pack (why?).  It came with main panel with 32 circuit spaces, pre-installed 100 amp breaker, and five, 20 amp circuit breakers which was nice. There were other good choices like Cutler Hammer and Siemens brands out there, but in my opinion, Square D was THE BEST due to its design, engineering and quality. In addition to the sub panel, I had to purchase a separate Square D 100 amp circuit breaker to be installed on the main panel.

TIP: If you plan on using the Square D solution as I have, please avoid using the 70 amp sub panel solution. Square D makes 70 amp circuit breakers, but Square D does not manufacture sub panels with 70 amp main circuit breakers pre-installed. In addition, the cost of a 70 amp circuit breaker was higher than the 100 amp circuit breaker!

Feeder Cable Size

My next step was to determine the wire size. In the US, a standard acronym is American Wire Gauge (AWG). For your project, you should look at the NEC 2005 Section 316.15 code (below) dwelling column and find a corresponding amperage requirement. Then look at the AWG column to determine the wire size requirement.

TIPS: I cannot stress enough that you SHOULD CONTACT your township’s electrical inspector to ensure that you can use either the aluminum or copper feeder cables! Certain towns do not allow the use of aluminum feeder cables.

Aluminum Cable

Aluminum Conductor	Stranding		Allowable Capacity (1)
Size (AWG)	Phase Conductor & Neutral	Equipment Ground Conductor	60 oC	75 oC	90 oC	Dwelling
SER Three Conductor with Bare Ground (a.k.a. “Four Conductor”)
8-8-8-8	1	1	30	40	45	- -
6-6-6-6	7	7	40	50	60	- -
4-4-4-6	7	7	55	65	75	- -
3-3-3-5	7	7	65	75	85	- -
2-2-2-4	7	7	75	90	100	100
1-1-1-3	18	7	85	100	115	110
1/0-1/0-1/0-2	18	1	100	120	135	125
2/0-2/0-2/0-1	18	1	115	135	150	150
3/0-3/0-3/0-1/0	18	1	130	155	175	175
4/0-4/0-4/0-2/0	18	1	150	180	205	200
Allowable ampacity based on National Electrical Code (NEC) 2005, Section 310.15: 60 oC – When terminated to equipment for circuits rated 100 amp or less or marked for 14 through 1 AWG conductors 75 oC – When terminated to equipment for circuits rated over 100 amp or marked for conductors larger than 1 AWG. 90 oC – Wet or dry locations. For ampacity derating purposes Dwelling – For units, conductors shall be permitted at listed ampacities as 120/240-volt, 3-wire, single-phase services and feeders

Copper Cable

Conductor			Ground Wire		Allowable Ampacity
Size (AWG)	# of Conductors in Cable	# of Strands in Conductors	Size (AWG)	# of Strands
Two Conductors
14	2	1	14	1	15
12	2	1	12	1	20
10	2	1	10	1	30
8	2	1	10	1	40
6	2	1	10	1	55
Three Conductors
14	3	1	14	1	15
12	3	1	12	1	20
10	3	1	10	1	30
8	3	7	10	1	40
6	3	7	10	1	55
4	3	7	8	1	70
2	3	7	8	1	95

For me, I decided to use Aluminum 2-2-2-4 (also known as 4 conductor) cable

(Figure 1.6 is the cable I purchased). Copper was my first choice, but since I made my mistake of buying a wrong sized copper cable, I went with aluminum. Please note that copper cable is more expensive and it’s quite a bit heavier than aluminum. In my township, I was not required to use conduit system, but again, you need to check with your township.

Subpanel Type

There are two types of sub panels : a panel with main-lugs only (Figure 2.2) and panel with a pre-installed main breaker (Figure 2.1). The difference between the two types is that a main lug panel DOES NOT come with a main breaker installed.

Although the NEC 2005 does not require a main breaker to be installed at the sub panel, I decided to add it anyway (extra $70 on top of the panel cost) because I wanted to have the flexibility to turn off the power from my sub panel. You should check with your township’s electrical inspector before deciding for or against this option.

Related posts:

1. Subpanel – Getting Started