20A 4500w water heater Manufactured house

After 12 yrs inspecting I saw what looks to be my first 20A 240v breaker and #12 wire going to a 40 gal 4500w dual element hot water heater. Is this allowed because it is a modular home? It looked like an original wiring job because the hot water tank is in the bedrom closet in the interior of the home. The home is a 2009 built on a crawlspace. I ran the dishwasher and showers and washing machine back to back and the wire wasn’t warm. This defys logic. Any input? Thanks, bob

It will work (4500/240=18.75) but the breaker should be 25 or 30 if they follow the 125% rule.

What did the appliance data plate state?

And a modular home is not a manufactured home.

Agree with Dominic, :smile:

Storage type water heaters of 120 gallons or less are considered a continuous load and require a branch circuit that is rated at 125% of the nameplate rating of the heater. As Dominic stated (4500/240=18.75 amps * 125%) is 23.4 amps so the #12 conductors are too small. If the same heater were used on a 208 volt system then lowering the voltage would also lower the current by about 25% and the 20 amp circuit would work.

If you think that it is original then it was factory approved, by HUD, and therefore is safe and code compliant.

I took this today on a new set. This is an off grid install and the electric tank was replaced with a gas unit.

25 AMP breaker - 10-2 AWG

Edit - 12awg = 25amps consult the tables in article 300.

For a HWH #12 AWG is limited to 20 amps in article 240.

No disagreement.

Maybe HUD is using an engineered method or:

So is it good or not? how do you know HUD applies, it’s not clear if it’s a manufactured or modular home, OP never clarified. :face_with_raised_eyebrow:

I looked at the OP title.
(1) Chapter 4781-6 of the Administrative Code is designated as the “Model Manufactured Home Installation Standards.” The model manufactured home installation standards as adopted by the department of housing and urban development pursuant to 42 U.S.C. 3535, 5403, 5404, and 5424 , and as set forth in 24 C.F.R. 3285 , as the adoption date of this rule is incorporated fully as if set out at length herein but as altered with additions, substitutions, and deletions indicated in this rule.


80% of the branch circuit rating or 125% of the unit nameplate are the same thing so that can’t be it. :smiley:

But I’m guessing HUD used 25 amps as their branch circuit rating that’s why they meet the limits of 240.

Michael can you elaborate I’m not sure what you’re saying.

HUD units are inspected prior to shipment and to HUD standards. I am guessing that a 25 AMP (75c) rating from the table then they used 80% to size the ocpd . In other words HUD approved the calculation using article 550.

This is a guess on my part. Also, the circuit length is often 10’ or less. HUD may think that a non continuous load on a 12 awg is safe under these conditions?


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The ‘unit’ as it is delivered is exempt from inspection by the AHJ. Now additional work is subject to the NEC.

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I know nothing about HUD but I will add that the NEC has no provisions for using a 4500 watt HWH on a 20 amp circuit with #12 conductors.

But the NEC only applies to modifications to the “unit”. If it comes with an approved installation as described in the OP then it is safe and code compliant.

I agree that it would not be allowed in a stick built home.

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Wow! thanks guys for the info. This is a real interesting case. I’ve done dozens of mobile homes but only a few modular homesnever seen this size wiring for a HWH. Also this is a modular home. Sorry for the confusion.
Also because w=i x v wouldn’t amps go up for 208v as apposed to 240v if the watts are the same? according to the formula volts and amps are inversely related.


That is a common misconception. For a purely resistive load like a HWH the current and voltage are directly proportional, one goes up the other goes up and vice versa. The wattage is the output which varies with the voltage. The constant in the equation is the resistance. So you cannot just divide the watts by the different voltages. You need to find the resistance and use that with the different voltages to find the current.

Since the HWH is rated for 4500 watts at 240 volts the formula would be as follows:

I=P/E=4500/240=18.75 amps

R=E/I=240/18.75=12.8 ohms

For applied voltage of 208 volts:

I=E/R=208/12.8=16.25 amps

If you cut the voltage in half you would get 1/2 the current (18,75/2=9.375}:

I=E/R=120/12.8=9.375 amps


If you drop the voltage without also reducing the resistance now your water takes longer to warm up. Another example is running 220v European appliance on 120v, the darn hairdryer blows cold air :slight_smile:

interesting. I need to brush up on my electrical laws. thanks.