Figuring out main panel capacity

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alikp

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retired
I'm trying to figure out my house main breaker panel capacity in order to see if I can add a 50A EV charger and I am stomped.

The first problem is that the panel (which I know is a Murray from the breakers) has no marking to tell me the capacity or even model number. There's the bare minimum remnant of some sticker, but the only part left is: "240V".

So I figured I'll just look at the main breaker and assume it was properly sized for the full capacity of the panel.
But then I run into an even bigger conundrum. The main breaker is clearly labeled 175A.
But adding up all the currently installed other breakers adds up to 485-505 Amp (depending if one of them is a Triplex or quadruplex, I didn't bother testing).

All, but 70A solar have been there since I bought the house 22 years ago. And the main breaker never tripped.
And that last solar addition was supposedly engineered by their people and installed by their certified techs.

Now I know you're supposed to install breakers about 25% over expected usage, but can actually go 10% over breaker capacity safely (according to a San Diego city inspector). But this is clearly way beyond that.

Can anyone tell me what I'm missing?
And how do i really verify if I can add the 50A?
 
alikp said:
I'm trying to figure out my house main breaker panel capacity in order to see if I can add a 50A EV charger and I am stomped.

The first problem is that the panel (which I know is a Murray from the breakers) has no marking to tell me the capacity or even model number. There's the bare minimum remnant of some sticker, but the only part left is: "240V".

So I figured I'll just look at the main breaker and assume it was properly sized for the full capacity of the panel.
But then I run into an even bigger conundrum. The main breaker is clearly labeled 175A.
But adding up all the currently installed other breakers adds up to 485-505 Amp (depending if one of them is a Triplex or quadruplex, I didn't bother testing).

All, but 70A solar have been there since I bought the house 22 years ago. And the main breaker never tripped.
And that last solar addition was supposedly engineered by their people and installed by their certified techs.

Now I know you're supposed to install breakers about 25% over expected usage, but can actually go 10% over breaker capacity safely (according to a San Diego city inspector). But this is clearly way beyond that.

Can anyone tell me what I'm missing?
And how do i really verify if I can add the 50A?
Click to expand...
You need to preform a service load calculation or get some measurements/demand load data.

But anyhow service capability is not figured by adding the sum of all the branch breakers.

Just because you have a 20 amp breaker doesn’t mean that there all being used to there maximum capability.

Further more most if not all residential breakers are 80% rated for continuous loads which is 3hours or more. In other words for continuous loads you size wire and breaker 125% of the load.

Example, 17amp load that is continuous.
17x125%=21.25amp. So you would need a wire ampacity of at least 21.25amps minimum and a breaker size to correlate and protect it. #10awg/30ampere OCPD.
 
Last edited:
Dsg319 said:
Just because you have a 20 amp breaker doesn’t mean that there all being used to there maximum capability.
Click to expand...

Of course not. But shouldn't the sytem be designed for the assumption that they might all get used at once?
And even if not, we're talking more that 4 times the main breaker 175A. Is that normal?
 
alikp said:
Of course not. But shouldn't the sytem be designed for the assumption that they might all get used at once?
And even if not, we're talking more that 4 times the main breaker 175A. Is that normal?
Click to expand...
No. If that was the case everyone would have extremely large services.

Many instances a dedicated 20amp breaker serves a load that never sees anything near that.

Just like general purpose lighting and receptacle circuits that’s why we have demand factors when load calculating because never is anything used all at once at the same time.
 
So you're saying that the panel's capacity is irrelevant and I have to use some formula base don what all the actual devices in the home may use?
 
alikp said:
So you're saying that the panel's capacity is irrelevant and I have to use some formula base don what all the actual devices in the home may use?
Click to expand...
Basically yes. You can overview article 220 or check out the Mikeholt tool box app.
 
So instead of trying to figure out the wattage of every single appliance in the household, I used a suggestion from another site and simply looked at the meter's usage history on the local utility website.
Smallest shown increments are 15min and I looked at all 60+ days of last year's highest use two months, when we use heat.
The biggest peak was 3.63KWh. Prorated to a full hour that's 14.52KWh.
I.e. - a 14,520 Watt peak.

At 240V that works out to only 60.5 Amp.
Taking only 80% of the 175 Amp main breaker, (175 x 0.8=140 Amp), that leaves 79.5 Amp available. More than enough.

Granted we don't use A/C and the next resident might. But it's a heat pump, so theoretically it shouldn't draw much differently than the heating.
And even with the 50Amp EV circuit added, we'd have to have a peak of almost 1.5 times that one before hitting 80% of main breaker.

Does that sound right?
 
Your profile says retired and since we don't know what you are retired from I am closing the thread according to the forum rules.

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