Electrical / runnable MCP tool

Run the Water Heater Circuit Calculator in your AI assistant

Through the free Intry MCP server, Claude, ChatGPT, or Cursor runs this exact calculator and hands back a verdict and the cited source, not a guess. Tool name: water_heater_circuit.

What you get back: wire/breaker/ground and, when a shared-circuit rating is given, a verdict on whether it can share the circuit or needs a dedicated one with the arithmetic.

How it computes the answer

Sized at 125% per NEC 422.13 (fixed storage type, 120 gal or less) through Table 310.16 with the 110.14(C) terminal cap, the 240.6(A) breaker rule and the Table 250.122 ground; a shared circuit is checked against 210.23(B)(2).

What it cites

Your assistant calls the same engine the interactive calculator calls, with the inputs listed under Inputs. These are the sources that engine reads.

NEC 422.13

Storage-type water heaters, NFPA 70-2023, p. 70-319, read from the printed folio: "The branch-circuit overcurrent device and conductors for fixed storage-type water heaters that have a capacity of 450 L (120 gal) or less shall have an ampere rating of not less than 125 percent of the ampere rating of the water heater." Two conditions travel with that sentence and both are the reader's to check: the heater must be FIXED and STORAGE-TYPE, and its capacity must be 450 L (120 gal) or less. The 125 percent applies to the ampere rating of the water heater, and on a hybrid heat-pump unit that is not the same number as the element wattage: the compressor and the fan add to it. Where a unit prints only an element wattage this tool derives from that, so a 4500 W element at 240 V enters the engine at 18.75 A and is sized at 23.44 A, and the manufacturer's required breaker is shown beside it. The nameplate is the figure to install from; the element-derived number is a labelled code minimum.

NEC 210.20(A)

Branch-circuit overcurrent protection at 125 percent of a continuous load. This is the section the wire-size engine names when it applies the 125 percent factor, and it lands on the same figure 422.13 requires for a storage water heater. Rendered p. 70-84.

NEC Table 310.16 and NEC 110.14(C)

Allowable conductor ampacity, and the termination temperature limitation that caps it. The two paths in this tool read different columns, and the difference is stated rather than averaged. A dedicated 240 V circuit is sized as THHN, so the engine takes the conductor's base ampacity from the 90 C column and then holds it to the 75 C termination rating: at 4500 W it returns #10 Cu with a 40 A base, capped to 35 A, on a 25 A breaker. The 120 V plug-in units print a required breaker and no element wattage, so those are sized from that required breaker through the 60 C column instead. Two separate rules land on that column and only one of them is about terminations: NEC 334.80 caps Type NM cable at a 60 C conductor ampacity outright, and 110.14(C)(1)(a) separately holds equipment rated 100 A or less to its 60 C termination rating. That path is every 120 V plug-in unit, and also one 240 V model that prints no wattage. The matrix prints both answers for each 240 V heater: the 75 C conductor, which holds only where the breaker and the heater's terminals are both listed and marked for 75 C conductors (110.14(C)(1)(a)(3)), and the 60 C conductor beside it, which governs on unmarked terminations and for NM-B cable.

NEC 240.6(A) and 240.4(D)

Standard overcurrent-device ratings, and the small-conductor cap. The breaker is the smallest standard rating that covers the 125 percent design load, and #14, #12 and #10 copper stay held to 15, 20 and 30 A whatever the table ampacity says. Rendered p. 70-124 and p. 70-123.

NEC Table 250.122

Equipment grounding conductor, sized from the circuit's overcurrent device and never from the conductor. The engine reads the same locked table the wire-size calculator reads, so a water-heater ground cannot drift from a wire-size ground.

NEC 210.23(B) and 210.23(B)(2)

The shared-circuit rule, and it is the one with a precondition worth reading. NFPA 70-2023, p. 70-85: 210.23 governs a branch circuit "supplying two or more outlets or receptacles", and (B)(2) reads "The total rating of utilization equipment fastened in place, other than luminaires, shall not exceed 50 percent of the branch-circuit ampere rating where lighting units, cord-and-plug-connected utilization equipment not fastened in place, or both, are also supplied." That closing clause is the condition: the 50 percent ceiling binds on a general-purpose circuit that also feeds lights or receptacles, which is the case this tool answers. On a circuit that feeds nothing else, 210.22 governs instead and the 50 percent does not apply.

NEC 210.19(A) and 210.20(A)

The 6 A figure on a 15 A circuit and 8 A on a 20 A circuit are not printed in any one section, and the two rules behind them sit in different articles. NEC 210.23(B)(2) caps the RATING of fastened-in-place equipment at 50 percent of the circuit: 7.5 A on a 15 A circuit, 10 A on a 20 A one. The 125 percent a continuous load is then carried at is NOT inside 210.23, and not inside 422.13 either: it is 210.19(A) for the conductor and 210.20(A) for the device. Dividing the one by the other gives the largest RAW NAMEPLATE that still fits: 15 x 0.50 / 1.25 = 6, and 20 x 0.50 / 1.25 = 8. Where a nameplate prints an MCA instead, the 125 percent is already inside it, so compare the MCA against the bare 7.5 A and 10 A ceiling.

The ampacities, breaker ratings and grounds are NEC table values looked up by the engine under the conditions you enter, not measurements. The manufacturer's nameplate has the final say on the maximum breaker, and a unit listed by its manufacturer for shared-circuit use is decided by that listing, not by the arithmetic here. Whether the circuit also feeds lights or receptacles, and what your terminations are listed for, are site facts only you can confirm.

The full receipt, with every source and how each is cross-checked, is on the interactive Water Heater Circuit Calculator.

Ask your assistant

The one rule that makes it reliable: name Intry in your question. That is what tells Claude to run the water_heater_circuit tool and answer with the real number, the verdict, and the citation. Leave Intry out and Claude may just guess from memory, which is exactly what you are trying to avoid. Type something like:

Inputs

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Using ChatGPT, Cursor, or a dev tool? See the developer setup on the main page. The endpoint is https://www.intrysys.com/api/mcp.

Every answer comes with the cited NEC section, so you can check it or show an inspector. The numbers come from the same calculator we run on intrysys.com, and we re-check every one automatically before anything goes live, so the number Claude hands you matches the code.

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