100 Amps electrical panel question

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zeefrank

Member
Location
Canada
So I have a 100A electrical panel in my condo. I can use 240V appliances such as oven, heating, water heater and dryer.

My question are:

1) Is my total power 100A*240V or 100A*120V? Obviously there is a big difference.

2) If I do get 24,000W, do I get only 100A of 120V for each "column" of my electrical panel or does that stuff doesn't matter?

Thanks!
 
Jraef

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
The CAPACITY of your panel is 100A @ 240V, or 24,000 VA. That doesn’t mean you are actually using that much, your actual consumption is based on what is running and how much it uses.
It’s also true that this is divided into two busses that have a 100A capacity at 120V each.

And yes, it all matters.
 
G

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
zeefrank said:
So I have a 100A electrical panel in my condo. I can use 240V appliances such as oven, heating, water heater and dryer.

My question are:

1) Is my total power 100A*240V or 100A*120V? Obviously there is a big difference.

2) If I do get 24,000W, do I get only 100A of 120V for each "column" of my electrical panel or does that stuff doesn't matter?

Thanks!
Click to expand...
Since you put parentheses around the word column, I hope that you do realize that the physical columns of the panel alternate from one phase to the other as you go down, rather than all of one side being L1 and all of the other side being L2. This allows you to have two pole breakers that deliver a 240 circuit from L1 and L2 sitting on either actual column of the panel.
 
G

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Consulting Electrical Engineer - Photovoltaic Systems
zeefrank said:
So I have a 100A electrical panel in my condo. I can use 240V appliances such as oven, heating, water heater and dryer.

My question are:

1) Is my total power 100A*240V or 100A*120V? Obviously there is a big difference.

2) If I do get 24,000W, do I get only 100A of 120V for each "column" of my electrical panel or does that stuff doesn't matter?

Thanks!
Click to expand...

A misconception I run into occasionally is the idea that if you have a 100A 240/120 split phase service you have 50A on L1 and 50A on L2. This is not how it works. A 240V 100A load on a 240 service draws 100A on both L1 and L2. A 120V 100A load draws 100A on L1 or L2 and sends 100A back to the service on the neutral.
 
J

JFletcher

Senior Member
Location
Williamsburg, VA
Welcome to the forum. As others have mentioned, you have 24,000W, or more appropriately, volt-amps. If you are wondering if the capacity of your panel (and service) is sufficient, a load calculation would have been done at the time it was built, and 100A would have been fine. It likely still is unless you are adding loads, like on-demand water heaters, baseboard heat, or converting gas appliances to electric.

As for the "columns", there will be a some difference in amperage between A and B legs. How much depends on what 120V loads you are using and legs they are on. 240V loads usually, but not always, pull equal amperage from each leg - it depends on whether or not they have any 120V components inside.

Each "column" attaches to both bus bars, with each slot alternating. Usually, on a single phase panel, the left side has all the odd number breakers, so 1, 5, 9, 13 would be on A leg, and 3, 7, 11 and 15 (and so on) would be on B. A 2 pole breaker occupying spaces 1 and 3 sees A and B legs - it would also if it were occupying spots 3 and 5, 34 and 36, etc. The right side is the same just with even numbers.

K8MHZ said:
Why not just turn everything on and take a reading?
Click to expand...

That ignores demand factors.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Zeefrank, here is a page to calculate your loads:

https://www.ecmag.com/section/codes-standards/residential-calculations

are you asking your questions because you are looking to learn more, are you experiencing a problem, or looking to add a load?
 
U

user 100

Senior Member
Location
texas
JFletcher said:
are you asking your questions because you are looking to learn more, are you experiencing a problem, or looking to add a load?
Click to expand...

I'm going with him wanting to learn more- op profile says he is an apprentice and this kinda sounds like a test question...

And he is right where he should be.:)
 
kwired

kwired

Electron manager
Location
NE Nebraska
K8MHZ said:
If you measured more than 100 amps on a residential panel, wouldn't you opt for a 200 amp one?

One ten second measurement tops hours of calculations every time.
Click to expand...
I was just saying it more expensive to put in the 100 amp panel, take the measurement, discover you should have put in 200 amp panel, then rip out the 100 and install the 200, compared to doing proper calculations and determining you need 200 amp panel before installing anything.;)
 
K8MHZ

K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
Occupation
Electrician
kwired said:
I was just saying it more expensive to put in the 100 amp panel, take the measurement, discover you should have put in 200 amp panel, then rip out the 100 and install the 200, compared to doing proper calculations and determining you need 200 amp panel before installing anything.;)
Click to expand...

Of course. I was referring to an existing installation ("I have a condo") and whether or not it would be prudent to suggest an upgrade based upon ampacity alone.
 
J

JFletcher

Senior Member
Location
Williamsburg, VA
drcampbell said:
Unless it's a Pushmatic® panel.
They were manufactured in Detroit in the 1940s & 1950s, maybe longer. There are quite a few of them installed around here.

View attachment 19427
Click to expand...

Fair enough. There's always exceptions, and ofc 3ph panels follow a similar but different pattern (than newer 1ph panels).

K8MHZ said:
If you measured more than 100 amps on a residential panel, wouldn't you opt for a 200 amp one?

One ten second measurement tops hours of calculations every time.
Click to expand...

Depends. If I have, say, 109.4/106.8 A, but only with everything conceivable running, then no, I'd not upgrade the panel (and service); I'd personally just not run heavy loads all at once. I dont see a typical 1200ft2 condo running over 100A for any amount of time (>15 min) anyway.

If I was well over 100, like 120+A, without trying too hard, yes, upgrade time. and if the loads were badly unbalanced, like 77.1/115.9A, I'd move them around - that much imbalance would only happen if say you had multiple higher-draw 120V loads all on the same leg (but on different circuits). It's rare that higher (10+) amperage 120V loads like microwaves, hair dryers, vacuums, etc run for any real amount of time. Portable space heaters are about the only 120V load I can see running longer than 15 minutes.

Short of multiple HVAC units w/electric heating, or very high draw units like a spa or on-demand water heaters, it's often difficult to load the average dwelling to/over 100A, as we'd do with generator testing. Heat/AC, oven, range, dryer, and water heater, if all electric, can do it, but such houses usually have 150-200A services already installed. The oven, range and dryer are not going to be using their heating elements continuously anyway.
 
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