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Kevin, Licensed Electrical Contractor
Category: Electrical
Satisfied Customers: 3667
Experience:  30 years Licensed Electrical Contractor in Illinois, Adjunct College Electrical Instructor, Former Electrical Inspector, Diploma: Digital Electronics, FCC Technician License
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Hello, I have a question about SER sizing for a sub-panel with

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Hello, I have a question about SER sizing for a sub-panel with a calculated load of 114 amps.

The main breaker panel is 200 amps, it will feed 1 stove, the heat pump and water heater.

The sub-panel feeder will be routed 60' through an accessible crawlspace. What size SER will I need to provide for 114 amp requirement?

In using Table 310.15(B)(16), I am not sure how to calculate temperature, and length.

I would like to know which size I need for copper, and for aluminum. And also what size breaker. name is XXXXX XXXXX I will be happy to assist you with your electrical question. My goal is to exceed your expectations on Just Answer!


1) Just to confirm, you have an existing 200 amp main electrical panel and you wish to install a sub-panel feeding from the 200 amp main and the calculated load for the sub-panel will be approximately 114 amps?


2) What size feeder breaker are you planning on running from the main 200 amp panel to the sub?


3) You mention that you will feed 1 stove, a heat pump and a water heater? Are these appliances all going to be fed from the new sub-panel?

Customer: replied 4 years ago.

1. Correct


2. I believe a 125 should be adequate to handle the calculated load, plus the resistance of the length of feeder, does that sound accurate?


3. Actually, no. These devices will be fed from the first panel (200 amp), that will also feed the sub-panel. The rest of the house will be fed by the new sub-panel. We are putting in an in-law suite, that will be fed off the sub, along with half of the existing house.

1) If you have an existing 200 amp main electrical panel, you should have a panel that has 42 available spaces. You wish to add an additional 114 amps onto a new sub-panel, that tells me that your 42 space panel is most likely maxed out in terms of available breaker spaces. This also suggests that if adding 114 amps onto a new sub, the main panel only has capacity of 200 amps minus 114 amps for the sub panel which = 86 amps. If you have an existing 200 amp main panel, I would suspect that the existing 200 amp panel is pulling more than 86 amps with all existing LOADS connected? If so and by adding an additional 114 amps or so, the 200 amp main breaker is gonna trip. The existing 200 amp service will be overloaded and undersized. Even by not performing the calculations, you will need a minimum of a 350 or a 400 amp service to handle all of these loads.


An electric stove will require a 50 amp. A water heater requires a minimum of a 30 amp and the heat pump probably requires a minimum of a 60 amp. If you have Central Air, that's another 40 amps. Not too mention the remainder of the existing 15 and 20 amp circuits throughout the house for receptacles, general lighting, garbage disposal, dish washer, etc.


The only true ways to determine this are to either recalculate the entire existing branch circuits in the house plus the sub-panel calculated load or to turn on every electrical appliance within the house , lighting, Central Air, Furnace, Sump Pumps, Well Pump, etc and to use a clamp-on amp meter to determine the actual amperage draw on each leg of the main breaker.



Hope this helps.........If you have any additional questions, let me know and I'll be glad to answer them for you.


Otherwise, don't forget to rate me before you log Off.

The next time you have an electrical question, you can also request for me at:


Customer: replied 4 years ago.

I may have explained incorrectly. The main disconnect box next to the meter socket is 200 amp, 12 spaces. It will ONLY have the following breakers:


1 double pole 125 for the upgraded sub that is currently 60 amp (feeds all the lights and outlets, soon to feed the in-law suite) load calc is 114 amps


1 double pole 30 for water heater


1 double pole 25 for the condensor


1 double pole 35 for the air handler


1 double pole 50 for the range


The demand load for the entire house will be 175 amps, 114 will be fed by the new sub. I just need to know the SER size to feed the sub from the 125 amp breaker at the main breaker panel.

1) Eric......Thanks for the replies


If using 240 volts over a 60 foot run and 125 amps, the calculated voltage drop equals .8 volts which is less than the National Electrical Code recommended maximum of 3%. This VD is based on using 1/0 copper type THHN. If using aluminum as the feeders, the VD results in 1.1 voltage drop using 1/0 aluminum, THHN. Therefore, either 1/0 copper or 1/0 aluminum will be adequate for a 240 volt, 60 foot run supporting a 125 amp feeder load.


2) Since the sub will be located 60 feet away from the main panel, the sub will need to be a 125 amp main breaker panel and not a main lug. The sub requires a 4 wire circuit, ie, 2 hots, 1 neutral and 1 equipment ground. The equipment grounding conductor needs to be 6 AWG Stranded copper or 4 AWG Stranded aluminum. At the sub, the neutral must be isolated from the panel metal enclosure. DO NOT install the green main bonding screw or a copper main bonding jumper at the sub. The neutral from the main 200 amp will land on the sub's neutral bus bar. The equipment ground will land on a separate equipment ground bar in the sub. The 200 amp main panel will also require an equipment grounding bar.


3) If using Romex as the branch circuits for the new sub, the bare copper grounds only land on the sub's equipment ground bar and NOT on the sub neutral bus bar. DO NOT intermix bare copper grounds and white neutrals on the sub-panel neutral bus bar as this will be a code violation.


Hope this helps.........If you have any additional questions, let me know and I'll be glad to answer them for you.


Otherwise, don't forget to rate me before you log Off.

The next time you have an electrical question, you can also request for me at:

Kevin and 4 other Electrical Specialists are ready to help you

Eric............Thank you for the excellent service rating as well as the bonus............much appreciated!


If you have any other follow-up questions, just reply back to this question and I'll be more than happy to answer them for you.


Take care and have a great weekend..............Thanks............Kevin!