Monday, March 20, 2006

Estimating Electrical Needs

After reading Chicago 2-Flats blog entry the other day about the mile-high estimates they were getting from contractors I started to picture them in a Medieval battle. I pictured Jocelyn and Steve armed with battle axes and broadswords desperately trying to protect the checkbook, as contractors came at them from all sides and tried to bash in their house with trebuchets and battering rams.

It almost does feel like you’re at war when you are dealing with those guys. Their not all crooks, and in fact, I’m sure the majority of them are honest and hard working individuals. Still, even the honest ones, when faced with 2 ways to solve a homeowners problem, will probably suggest the more expensive way just as often as the less expensive way. I don’t really fault them for this, but as a homeowner it is our responsibility to make sure we are not going to be paying for work that goes way above and beyond what needs to be done. The best weapon to prevent being taken advantage of is knowledge.

One of the things Jocelyn mentioned that she might need to do is to upgrade from a 100 amp electrical service to a 200 amp service. She may end up needing to do this but there is no reason she should just accept what an electrician says. With that in mind, I thought I would share something I learned in a residential wiring class I took a few years back.

There is a worksheet electricians use to determine what type of service a house may need. At the time I did this for my house I had 60 amp service that was last upgraded in 1951. I was thinking I was going to have to upgrade to a 200 amp service. I have a 3000 sq ft 2 story house with a walk-up attic that I might want to convert to living space someday. It turned out my estimated usage was much lower because my heating, cooking, water heater, and clothes dryer are all natural gas. I also don’t have A/C.

So here is the worksheet I used. (Va = Volt Ampere)

Lighting
3 Va for each sq. ft. of house.
For me this is 3000X3=9000

Small Appliance circuits (code requires 2, 20=amp circuits for each kitchen)
2 X 1500 Va
Me: 3000

Laundry Circuit
1500 Va
Me: 1500

Appliances

Electric Range = 8000 Va
Me: 0

Electric Dryer: 5000 Va
Me: 0

Electric Heater: 5000 Va
Me: 0

Microwave: 2000 Va
Me: 2000

Dish Washer/Disposal: 1500 Va (separate circuit in the kitchen)
Me: 1500

Motors
If you have well pumps, basement pumps, or other motors you would need to figure their Va.
Me: 0


Total For Me: 15,000 Va

(This part figures that you are not going to be maxing out all circuits at the same time)
Less 10,000 Va @ 100%
Total For Me 5,000 Va

40% of Balance
For Me: 2,000

Plus 10,000 Va required at 100%
For Me: 12,000 Va

Divide by 240 to get the total amps needed

12,000 / 240 = 50 Amps

As you can see I only require 50 Amps, so when I upgraded the 55 year old panel I went with a 100 amp service. Even if I do add a Jacuzzi or A/C in the future there is plenty of room. This is taken from the 2002 NEC. If you Google Service Entrance Calculation you can find tutorials and other information that gives you more detail. I may have left off some things that don’t apply to me. But it’s a good start.

We wired a new house in the second semester of the course. The service entrance calculation for that house was 92.5 amps. We installed a 100 amp service panel and it passed code and everyone was happy.

To figure the Va for things not listed you multiply the amps times the current in a circuit. If something requires 3 amps on a 120 volt circuit it requires 360 Va. If it is 3 amps on a 240 volt circuit it is 720 Va.

9 comments:

John said...

Cool, thanks for the calculation.

We installed a 200 Amp box in the Queen. The Queen is 100% electric, so once you figure in the two water heaters, the heat pump/ac, appliances, etc we really needed it even though we only have 1850 sq. feet. No two houses are equal, get what you need.

Marg. said...

I could hug you.

We've got a 1400 sq ft (current living area) one story house that we are probably going to be finishing the cellar on (so that adds another 1200 sq ft of living space to budget for). We are also just about all gas, and we're rewiring the whole shebang due to sheer scariness.

We did buy a 200amp rated box, and will probably upgrade to a higher amp service (100 is where it is now) when we put in a central a/c unit (say, next year?), but you've just saved us a HUGE amount of headache.

Greg said...

John, I sounds like you definitely needed 200 amp service. And Marg., I’m glad it helped. Even with the A/C you may be able to keep your 100 amp service.

nc said...

You should also take in account for all of the technology we all have these days (tvs, dvd, steros, computers). The other factor - if you are goint to remodeling your kitchen some time in the future. It is best to plan a head then have to pay someone to come in and update the service and or panel. It shouldn't cost too much more for a the large panel. Better to pay once. I am an estimator for construction and found this to be the best tactic with our clients.
Good luck

Greg said...

You're the expert, not me. This is based on the 2002 National Electric Code (NEC).

The worksheet accounts for 3, 20-amp circuits on the kitchen. Also, the NEC requires that kitchen lighting (15 amps) NOT be on one of those circuits. That is essentially 4 circuits and 75 amps just for the kitchen. That is a lot. What the heck would you need more for?

As for TVs, DVDs, Stereos, etc. I don’t think they figure all those in to the equation because they draw relatively little power compared to an A/C unit or oven. For instance, my 12 year old VCR draws 28 watts at 120 volts. To convert this to amps you divide Watts by Volts. So 28/120= .23 amps. At that rate I could run 60 VCRs on the 15 amp circuit just in my front parlor before I blew a fuse. The 4 bedrooms are run on 4, 20 amp circuits. I could run 320 VCRs in the bedrooms! You would go nuts trying to estimate that much detail. This worksheet is used for a general idea of the overall needs of the house, not an exact number.

If you use this worksheet, and you wire enough circuits to meet the current code, your house will most likely be able to carry the load for the next 75 to 100 years and that is pretty much the life expectancy of the system. Who knows what the future will bring?

Greg said...

Oops! That should read:

The 4 bedrooms are run on 2, 20 amp circuits.

So that would be 240 VCRs, not 360. And I'll add, that to meet code the bedroom circuits are AFCI (arc fault) circuits. Even more protection!

Trissa said...

And you can't forget power for the power tools!

Jocelyn said...

I love that image Greg. It made me laugh and I definitely needed some laughs after the attempted pillaging of our nest egg last week. The whole process is so traumatic for us and we know alot- imagine what someone who knows nothing feels like.

The codes may vary here, but I appreciate this tip and we should (and will) work on calculating our needed amps too. We found out that our smoke detectors have to be hard wired and every bedroom outlet has to have an arc circuit breaker in the main panel to prevent sparking from frayed extension cords (a major cause of fire in bedrooms i.e. space heaters etc..). What is so funny about this is that the new codes also make you have an outlet every 6 feet, which makes the possibility of even using an extension cord remote- but better safe than sorry I guess.

Again, I really like the swords and armour image- it does feel that rigorous at times!

slateberry said...

Greg wrote, "Who knows what the future will bring"

Very a propos. I think the future is bringing more energy efficient appliances and bulbs. I was trying to figure out the temperature my clothes dryer reaches at various settings and stumbled on some appliance old-timers talking about how the high heat setting on a dryer ain't what it used to be. In this case, I'm OK with that. It is interesting to think (or hope) that household electrical needs have historically peaked and with energy star, cfls, and leds, we are on the way back down. Wouldn't it be funny if in 100 years we're back down to 1-2 circuits per house, and people scratching their heads when they see one of those old-timey service panels with 20 breakers? OK maybe not.