Existing Load Calculator — NEC 220.87
A paper load calculation assumes a house might run everything at once. Real houses never do. NEC 220.87 lets you establish the existing load from what the building actually drew — the utility's highest recorded demand over twelve months, times 125% — then add your new load on top. It is the single strongest argument against an unnecessary service upgrade.
Test the existing service
EV supply equipment is continuous by rule (625.41). Ranges, dryers, and ovens are not. Ask the utility for twelve months of interval or peak-demand data — most provide it free through the account portal.
Total calculated load
Fits the existing service
Existing load
55.0 A
Headroom
5.0 A
See the breakdown
The 100 A service covers this with 5.0 A to spare, so no upgrade is required on the basis of load.
The method, explained in plain English
Measured beats calculated
A load calculation is deliberately conservative. Metered demand is what the building really did, which on an older house is routinely half what the paper number says.
Two different 125% rules
One is 220.87's safety factor on measured demand. The other is 210.20(A) on a continuous new load. They share a number and nothing else.
Spaces aren't capacity
A full panel is a physical problem — tandems or a subpanel fix it. This calculation tests capacity, which is a different question entirely.
Ask before you rely on it
220.87 is model-code text, but what data your AHJ accepts — and whether a 30-day logger will do — is a local question. Confirm before quoting.
Worked examples
The same house — 100 A service, 44 A recorded peak — with three different additions.
32 A EV charger — fits
The defaults above. A 7.7 kW charger, ample overnight.
32 A × 1.25 = 40.0 A new (continuous, 625.41)
total = 95.0 A on a 100 A service → 5.0 A spare
Result: no upgrade needed. A 32 A charger delivers roughly 25–30 miles of range per hour, so a car parked overnight is comfortably covered.
48 A EV charger — does not fit
The same house, a bigger charger.
over a 100 A service by 15.0 A
Result: the charger, not the house, is the problem. Dropping to 32 A or adding load management under Article 750 both solve it for a fraction of an upgrade.
30 A electric dryer — non-continuous
A dryer cycles, so no 125% adder applies to it.
15.0 A remaining on the 100 A service
Result: comfortably inside. Note how much the continuous flag matters — the same 30 A treated as continuous would add 37.5 A instead.
What fits a 100 A service
All rows assume a 44 A recorded 12-month peak, giving a 55.0 A established existing load under 220.87. Change the inputs above for your own figures.
| New load | Nameplate | Applied | Total | On 100 A |
|---|---|---|---|---|
| Nothing added | — | — | 55.0 A | Fits |
| Electric dryer | 30 A | 30.0 A | 85.0 A | Fits |
| EV charger, 32 A | 32 A | 40.0 A ×1.25 | 95.0 A | Fits |
| EV charger, 40 A | 40 A | 50.0 A ×1.25 | 105.0 A | Over |
| EV charger, 48 A | 48 A | 60.0 A ×1.25 | 115.0 A | Over |
| Heat pump, 30 A | 30 A | 37.5 A ×1.25 | 92.5 A | Fits |
Sources & standards: NEC (NFPA 70) 2023 — 220.87 determining existing loads from measured demand, 220.82 optional dwelling calculation, 220.83 additional loads in an existing dwelling, 210.20(A) continuous load, 625.41 EV supply equipment as a continuous load, 230.79(C) minimum service rating, Article 750 energy management systems. What demand data an AHJ will accept varies locally; local amendments override the model code and the AHJ has final say.
Frequently asked questions
Common questions about NEC 220.87 and measured existing load.
What is NEC 220.87?
It is the method for determining an existing load from measured data rather than a paper calculation. The existing load may be taken as the maximum demand recorded over the previous 12 months × 125%. Where a full year of data is not available, 30 days of recorded demand data may be used instead. Because real buildings draw far less than a load calculation predicts, it frequently shows an existing service has capacity the calculation says it does not.
How do I get my peak demand from the utility?
Ask for interval or peak-demand data for the last 12 months — most utilities provide it free, often through the online account portal, and many will email a CSV. If the meter is not a demand meter, some jurisdictions accept a recording ammeter or a data logger left on the service for 30 days, which is what the 30-day provision is for. Confirm what your AHJ will accept before relying on it.
Can I add an EV charger to a 100 amp service?
Very often yes, and this is the calculation that proves it. A house with a 44 A recorded peak establishes a 55 A existing load; a 32 A charger — continuous under 625.41, so 40 A — brings the total to 95 A, inside a 100 A service. A 48 A charger would add 60 A instead for 115 A and would not fit, which is why specifying the smaller charger is often the whole difference between a $300 circuit and a $4,000 upgrade.
Why is the new load multiplied by 125% but the existing load also 125%?
They are two different rules that happen to share a number. The 125% on the existing load is the safety factor 220.87 itself applies to measured demand. The 125% on the new load is 210.20(A), which requires the overcurrent device and conductors to be sized at 125% of a continuous load — one operating for three hours or more. A non-continuous new load such as a range or dryer is added at 100%.
Does 220.87 replace a load calculation?
For establishing the existing load, yes — that is exactly its purpose. It does not size the new circuit, which still follows the normal rules, and it does not apply to a building with no operating history. For a new dwelling or an addition with no metered data, use the 220.82 optional method instead.
What if the load exceeds the service?
Three routes before an upgrade. Specify a smaller version of the new load — a 32 A charger instead of 48 A is usually ample overnight. Add a load-management system under Article 750, which lets a large load back off when the house is busy and is a fraction of an upgrade's cost. Or, if neither works, upgrade the service — the calculator names the next standard rating that would cover the total.
Is a full panel the same as a full service?
No, and conflating them is the most common reason people are sold an upgrade they do not need. Running out of breaker spaces is a physical problem solved by tandem breakers or a subpanel. Running out of capacity is what this calculator tests. A panel with no free slots may still have plenty of unused capacity.
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