Heat Strip Kit Sizing and Circuit Field Card

Why this matters

A heat strip kit is chosen from one chart in the air handler's installation instructions, and that chart answers four questions at once: how much heat each kit makes, how it has to be fed, what circuit each feed needs, and which air handlers it is allowed in. Techs who pick a kit by kW alone get caught on the other three. The 10 kW kit that makes 7.5 kW on a 208 V service. The kit that is not listed for the cabinet it was slid into. The kit that needs a 60 A circuit where the wall holds a 40 A one, discovered with the customer's house at 55 F. This card is about reading the chart so the kit, the feed and the circuit are settled on the quote, not on install day.

Safety at the air handler

An air handler with strip heat often has two or three separate feeds: the blower and control circuit, and one or two strip circuits from their own breakers. Open and lock every one of them before reaching into the cabinet, and prove each conductor dead with a tester proven on a known live source before and after, per NFPA 70E Article 120 (120.5 in the 2021 edition, renumbered in 2024), with electrical work practices under 29 CFR 1910.333(b)(2) for qualified persons. Opening the air handler disconnect alone can leave a strip feed live. Element terminals stay hot to the touch for minutes after the strips de-energize; let them cool. Restoring is its own check: covers on, breakers closed one at a time, voltage read at the kit's terminal block, then each stage run and clamped as described at the end of this card. Never jumper a limit switch to get a kit running; a limit that opens is telling you the airflow is short or the kit is wrong.

The chart, as a maker prints it

Values are illustrative, computed from the kW and an assumed blower FLA so you can see how the columns relate. Read the real chart for the cabinet in front of you.

HEAT KITS FOR AIR HANDLER SERIES AH  (ILLUSTRATIVE)
Rated at 240 V. MCA includes blower FLA 4.0 A.

Kit    kW     Stages  Feed       MCA    MOCP
       240 V
HK05    5     1       single     31.0    35
HK08    8     2       single     46.7    50
HK10   10     2       single     57.1    60
HK15   15     3       single     83.1    90
                      dual, c1   57.1    60
                      dual, c2   26.0    30
HK20   20     4       dual, c1   57.1    60
                      dual, c2   52.1    60

Approved:  AH24  HK05, HK08 only
           AH36  HK05 through HK15
           AH48, AH60  all kits
Minimum heating airflow per kit: see Table 6

Reading each column

kW at 240 V. Resistance heat output depends on voltage squared. At 208 V a kit makes (208 / 240) squared of its 240 V rating, which is 0.751, so a 10 kW kit makes about 7.5 kW and an 8 kW kit about 6.0 kW. Many apartment buildings, condos and houses fed from a commercial three-phase service measure close to 208 V between legs. Measure the supply before choosing a kit; do not assume 240.

Stages. The kit's elements come on in steps through sequencers or relays, wired to the thermostat's auxiliary and emergency heat terminals. Staging is how the control brings on only as much resistance heat as the deficit needs; the lockout that keeps strips off above the balance point is owned by Backup Heat Sizing and Lockout Configuration.

Feed: single or dual. A kit is either wired to one branch circuit (single point) or split across two (dual circuit). Under NEC 424.22 (2023 edition, as adopted by your jurisdiction), resistance heating elements in fixed space-heating equipment are subdivided in substance so that no subdivided load exceeds 48 A and each is protected at not more than 60 A; check the current edition for the exact conditions. A 15 kW kit draws 62.5 A at 240 V (15,000 / 240), so it is subdivided internally and offered either way: one large circuit feeding the kit's internal subdivisions, or two circuits each under the 60 A ceiling. Single point usually needs a factory jumper or single-point accessory the chart names.

MCA and MOCP per feed. Fixed electric space heating and its blower motor are treated as continuous loads, so the branch circuit is sized at 125 percent (the NEC Article 424 branch-circuit sizing rule: 424.4(B) in recent editions, 424.3(B) in older ones; check the edition adopted). On the chart, HK10 is 41.7 A of heater (10,000 / 240) plus 4.0 A of blower, 45.7 A, times 1.25, which gives the printed 57.1 A. On a dual feed, the blower rides on circuit 1, which is why HK15's circuit 1 (10 kW plus blower) shows 57.1 A and circuit 2 (5 kW alone, 20.8 A times 1.25) shows 26.0 A. Each circuit gets its own conductors sized to its own MCA and its own breaker at or under its own MOCP. The chart is rated at 240 V; at 208 V the kit draws less current, so a circuit sized from the 240 V chart is conservative.

Approved air handlers. The kit is part of the air handler's listing only in the cabinets the chart names. The approval reflects element clearance, limit placement, airflow and wiring space. A kit from the right maker in the wrong cabinet is an unlisted installation, and the airflow table for that cabinet may not even reach the kit's minimum.

Minimum heating airflow. Each kit has a minimum blower setting. Below it, elements overheat and cycle on their limits. The airflow side of this belongs to Airflow Per Ton Targets and What Moves Them and How to Set Blower Speed Taps for Cooling and Heating Airflow.

Matching a kit to the house

The sizing method is owned by Backup Heat Sizing and Lockout Configuration, which sizes the bank from the gap between the house's design heat loss and the heat pump's published capacity at design temperature, not from the whole-house load. That gap is the design deficit. This card takes the deficit as given and turns it into a kit at the voltage the house actually has. The heat pump's balance point, where its capacity equals the house's loss, sets where the strips start to be needed; the deficit at design sets how many kW they must deliver when they are.

Worked example: a 3-ton heat pump on a 208 V condo service

An AH36 air handler, matched to a 3-ton heat pump. From the balance point worksheet, the design deficit is 22,000 BTU/h. Measured supply at the air handler: 208 V. The existing strip circuit: 8 AWG copper in NM cable on a 40 A breaker, feeding a 5 kW kit that has been leaving the customer cold on the worst nights. The 40 A breaker is already over HK05's 35 A MOCP, a finding on its own.

Kilowatts needed. 22,000 BTU/h divided by 3,412 BTU/h per kW is 6.45 kW, delivered at the house's voltage.

What each candidate actually delivers at 208 V.

• HK05: 5 x 0.751 = 3.76 kW, or 12,830 BTU/h. That is 9,170 BTU/h short of 22,000 (22,000 minus 12,830), which explains the complaint.
• HK08: 8 x 0.751 = 6.01 kW, or 20,500 BTU/h. Short by 1,500 BTU/h (22,000 minus 20,500), about 7 percent of the deficit.
• HK10: 10 x 0.751 = 7.51 kW, or 25,630 BTU/h. Covers the deficit with 3,630 BTU/h to spare (25,630 minus 22,000).

Is the kit approved here? HK10 is inside "AH36, HK05 through HK15". Yes.

The circuit. HK10 on a single feed needs MCA 57.1 A and a breaker no larger than 60 A. The existing 8 AWG copper in NM cable is held to the 60 C column, 40 A, which is less than 57.1 A. The circuit fails on conductor ampacity before the breaker question comes up. HK08's 46.7 A MCA fails the same conductor. Any kit larger than the existing 5 kW needs a new branch circuit, which goes to the electrician, not onto the HVAC ticket. The quote says so, with the electrician's work scheduled before the kit swap.

The customer conversation. HK08 leaves the house about 7 percent short on the coldest design night, which the thermostat will show as a slow slide of a degree or two; HK10 covers it. Both need the same electrician visit, so the circuit cost does not separate them. The shop recommends HK10 and writes down why.

What would change the answer. On a 240 V service the same deficit is met by HK08 (8 kW, 27,300 BTU/h at 240 V), which would make HK10 an oversize. If the customer wants the strips to carry the whole house through a compressor failure (emergency heat sized to the full design load rather than the deficit), that is a larger kit and a larger circuit, and it is a choice the sizing SOP asks the shop to make deliberately, with the service capacity checked first. If the air handler were an AH24, HK10 would not be on its approved list at all, and the answer would be a different air handler or a smaller kit with a known shortfall.

What goes wrong with strip kits

The kit chosen at 240 V for a 208 V house. A common reason a new heat pump install with "the right size strips" runs cold on design nights.

Dual-feed kit landed on one circuit. Both kit circuits jumpered onto one breaker without the single-point accessory puts the full kit load on conductors sized for one subdivision.

Old circuit reused because the breaker "is already 60". The breaker size is evidence of what someone installed, not of the conductor behind it. Read the cable.

Strips running in mild weather. That is a lockout or droop problem, not a kit problem; see Backup Heat Sizing and Lockout Configuration.

How to verify the install

• Supply voltage at the kit terminal block, measured and written on the ticket, with each stage's delivered kW computed at that voltage.
• Each stage energized in turn and clamped: a 5 kW element at 208 V draws about 18.0 A (3,755 W / 208 V), at 240 V about 20.8 A. A stage reading near zero is an open element, a failed sequencer or a tripped limit.
• Kit model on the air handler's approved list, photographed with the cabinet's plate.
• Each feed's conductor ampacity and breaker size written next to the chart's MCA and MOCP for that feed.
• Heating airflow at or above the kit's minimum, with temperature rise measured per Temperature Rise and the Sensible Heat Airflow Formula.

References

• NFPA 70, National Electrical Code (2023 edition as adopted by your jurisdiction): Article 424 branch-circuit sizing (424.4(B) in recent editions, 424.3(B) in older), 424.22.
• NFPA 70E (current edition), Article 120; 29 CFR 1910.333(b)(2).
• Air handler installation instructions and heat kit chart (kW, feed options, MCA, MOCP, approved cabinets, minimum airflow).
• See related: Backup Heat Sizing and Lockout Configuration; Airflow Per Ton Targets and What Moves Them; Temperature Rise and the Sensible Heat Airflow Formula; How to Wire a Heat Pump With an Air Handler and Heat Strips.