By Tony Marino — “If the heat kit’s starving, your wiring’s lying.”
A 3.5 kW electric heat kit inside a 12,000 BTU PTAC unit is a simple machine on paper.
Give it the right voltage, the right wire size, the right breaker, and the right airflow, and it’ll pump out steady, reliable heat for years.
But here’s the uncomfortable truth most installers never admit:
99% of heat kit problems aren’t caused by the heat kit.
They’re caused by the disconnect box feeding it.
Loose lugs. Wrong gauge wire. Oxidation. Back-stabbed connections. Undersized breakers. Cheap disconnects.
And the big one nobody checks:
Hidden voltage drop under load.
If your 3.5 kW heat kit needs 208/230V and it’s only getting 194V during heating, your system is already in trouble. That’s why Tony rewires every disconnect—every job, every time.
Amana Distinctions Model 12,000 BTU PTAC Unit with 3.5 kW Electric Heat
Let’s break down exactly why.
⚡ 1. The Ugly Secret: Disconnect Boxes Are the #1 Cause of Weak Electric Heat
Nobody talks about it.
Everybody blames the unit.
But Tony’s been in the field long enough to know the pattern:
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Heat kit smells hot?
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Trips occasionally?
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Weak heat output?
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Breaker feels warm?
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Unit humming louder in heat mode?
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Slow to raise room temperature?
Disconnect box.
Every time.
Why?
Because 3.5 kW at 240V pulls roughly:
14.5 amps continuous (and more during inrush).
If your wiring or terminations aren’t perfect, you get:
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Voltage sag
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Element underheating
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Excessive amperage draw
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Hot spots at connection points
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Premature heat kit failure
Most disconnect boxes installed on PTACs come from leftover job stock—cheap, weathered, rusted, or already thermally stressed.
Tony replaces and rewires because he doesn’t like callbacks.
🔌 2. Understanding Voltage Drop — The Silent Heat Killer
Voltage drop is invisible to the naked eye.
The equipment doesn’t warn you.
The thermostat doesn’t warn you.
But electrically?
When voltage drops… amps rise.
And when amps rise… things burn.
A 3.5 kW heat kit starves when voltage dips below 212–215V under load.
That’s only a 6–8% drop, and it will absolutely kill performance.
Tony’s thresholds:
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Acceptable: ≤ 3% drop under load
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Borderline: 3–5% drop
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BAD: > 5% drop
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Danger Zone: 8–10% drop
Most bad installations land in that last category.
Why?
Because the disconnect box acts like a bottleneck—dirty contacts, loose lugs, rust, and cheap metal all raise resistance and cause voltage loss.
🪛 3. Rewiring the Disconnect: Tony’s Rulebook
There’s “installing.”
Then there’s Tony installing.
Here’s the exact process he uses on every PTAC job:
🔧 Step 1 — Remove the Factory Connections
Factory wiring inside disconnect boxes is:
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Pre-stripped inconsistently
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Over-torqued
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Under-torqued
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Using low-grade copper
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Sometimes even aluminum-copper mixed
Tony cuts all that junk out and starts fresh.
⚙️ Step 2 — Strip to Fresh Copper
Oxidation = resistance
Resistance = voltage drop
Voltage drop = weak heat
Tony strips back ½”–¾” until he sees:
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Shiny
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Clean
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Untarnished copper
If he finds corrosion deeper than an inch, he pulls new wire from the breaker to the disconnect.
🧰 Step 3 — Use Correct Wire Gauge (No Exceptions)
3.5 kW at 240V requires 14.5A continuous.
Factor in NEC 80% continuous-load rule:
14.5A ÷ .80 = 18.125A
So your wiring must handle ≥ 20A continuous.
Tony uses:
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12 AWG copper minimum
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10 AWG preferred for long runs
Why 10 AWG?
Because thicker wire = less voltage drop.
Less drop = the heat kit stays hot, efficient, and safe.
🔩 Step 4 — Torque Lugs to Manufacturer Spec
Loose lugs cause:
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Arcing
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Carbon buildup
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Heat kit cycling
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Flickering lights
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Melted disconnects
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Burnt wire insulation
Tony never “hand tightens.”
He uses a torque screwdriver or torque wrench.
This step alone prevents half the problems he sees in the field.
🧱 Step 5 — Replace Cheap Disconnects With Heavy-Duty Ones
Thin stamped-metal disconnect boxes warp from heat.
Warp = spring tension loss
Spring tension loss = loose contacts
Loose contacts = burned heat kit
Tony avoids that disaster by installing:
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Heavy-gauge steel
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Copper blade pullouts
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Weather-sealed enclosures (for exterior installs)
Cheap disconnects cost $9–12.
Tony uses disconnects that cost $25–40.
Why?
Because he’s not trying to save $16 and lose $600 on a callback.
🔥 4. What Starvation Looks Like — Symptoms of Voltage Drop in a PTAC Heat Kit
Here’s how you know your heat kit isn’t getting full voltage.
🔥 Weak or Lukewarm Discharge Air
Should be 95–115°F depending on outdoor temp.
If you’re getting 78–88°F, it’s starving.
🔄 Blower Running Faster Than Heat Output
The airflow overwhelms the element.
Discharge air feels cool.
This trick fools a LOT of people.
⚠️ Breaker Warms Up
Not tripping, just warm.
That’s a voltage drop signature.
💡 Lights Dim When Heat Turns On
That’s not “normal.”
That’s a wiring bottleneck.
🔌 Disconnect Box Feels Warm to the Touch
If you can feel heat on the metal casing?
You’ve found the choke point.
🔥 Heat Kit Smells Like ‘Hot Dust’ Every Time
Starved elements run hotter at the coil contact points.
That smell isn’t dust—it’s metal oxidizing.
⏳ Room Takes Way Too Long to Heat
A PTAC heat kit should raise room temp by:
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3–5°F in the first 10 minutes
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6–10°F within 20–30 minutes
Anything slower = voltage starvation.
🧪 5. Tony’s Load Test: The Only Test That Actually Matters
Most techs check voltage with the heat off.
That’s useless.
The number you need is the under-load voltage.
Here’s Tony’s method:
🔌 Step 1 — Turn on Electric Heat
Not heat pump mode.
Not fan.
Electric heat.
⚡ Step 2 — Measure Voltage at the Disconnect Lugs
Should be:
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208–230V depending on building
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Within ±2% from no-load voltage
⚡ Step 3 — Measure Voltage at the PTAC’s Heat Kit Terminals
If there’s more than a 2–5V difference between disconnect and heat kit, the disconnect is the bottleneck.
🔥 Step 4 — Measure Amperage
A 3.5kW kit should pull:
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~14.5A @ 240V
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~16.8A @ 208V
If amps are higher?
Voltage is dropping.
If amps are lower?
Voltage is too low to fully energize the element.
Either way = rewire time.
🔥 6. The Physics Behind Starvation: Why Low Voltage Destroys Heat Kits
Electric heat elements are resistive loads.
Here’s the formula:
P = E² / R
(Power = Voltage squared ÷ Resistance)
If voltage drops:
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Power drops exponentially
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Heat output falls dramatically
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Element temperature rises locally
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Service life shortens
A 3.5 kW heat kit at 240V produces 3,500 watts.
Drop voltage to 215V?
Now you’re at 2,760 watts.
That’s a 21% heat loss.
Drop voltage to 200V?
Now you're at 2,300 watts — nearly 35% loss.
No wonder rooms stay cold.
🧯 7. Safety Hazards of a Poorly Wired Disconnect
This isn’t just performance—it's safety.
A bad disconnect box can cause:
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Overheating conductors
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Melting insulation
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Arc faults
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Element burnout
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Breaker failure
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Wall cavity fires
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Heat kit overtemperature trips
Tony rewires because he’s seen what happens when people cut corners.
“Fire marshals don’t care who installed it. Only who wired it wrong.”
🪨 8. Tony’s Disconnect Wiring Checklist (Steal This)
If the disconnect box doesn’t pass all 10 of these, Tony rewires it.
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12 AWG copper minimum
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10 AWG for long runs
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Voltage drop < 3% under load
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Torque-spec tightened lugs
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No aluminum wire unless rated
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Clean, fresh copper ends
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OX-guard on aluminum if used
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Heavy-duty disconnect enclosure
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Proper strain relief
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Breaker sized to NEC
If one fails → Tony rewires.
🧠 9. Tony’s Rule of Thumb: “If You Didn’t Pull the Wire, You Don’t Know the Wire.”
Tony never trusts:
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Pre-installed disconnects
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“Contractor grade” anything
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Landlord wiring
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Hotel retrofits
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Old motels
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Add-on panels
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Aluminum to copper transitions
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Old BX cable in walls
If he didn’t install it,
he assumes it’s wrong until proven right.
And he’s usually right.
📚 10. External Verified Resources for Electrical Standards & Safety
Here are reliable resources that align with best practices mentioned in this guide:
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Energy.gov – Air Sealing Guidelines
https://www.energy.gov/energysaver/air-sealing-your-home -
OSHA – Construction Saw Safety (for proper wall cuts)
https://www.osha.gov -
International Building Code (Wall Framing Requirements)
https://codes.iccsafe.org/ -
UL Guidelines for Electric Heat Components
https://ul.com/ -
ASHRAE Handbook – HVAC Fundamentals (Airflow & Pressure)
https://www.ashrae.org/technical-resources/ashrae-handbook
These align with the wiring, load, and voltage-drop standards Tony uses.
🏁 Final Word From Tony
A PTAC heat kit is only as good as the power feeding it.
Most techs think electric heat fails because:
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“The unit’s old.”
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“The element is burned out.”
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“It must be the control board.”
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“Probably the thermostat.”
Tony knows better.
Electric heat fails because the disconnect box lies.
And Tony never lets bad wiring lie twice.
