Okay, let's break down why this is a bad idea and how quickly that breaker is likely to trip.

The Problem



Total Load

Two 1500W heaters draw a combined 3000W.

Amperage Calculation

3000W / 120V = 25A. This means they are trying to draw 25 amps from a circuit designed for only 15 amps.

Overload

This is a significant overload (25A trying to flow through a 15A circuit).

How Quickly Will the Breaker Trip?

This is the tricky part, as it's not an exact science. Here's why: Breaker Trip Curves

Circuit breakers don't trip instantaneously at exactly 15.0001 amps. They have a "trip curve," meaning the amount of time it takes to trip depends on how much over the rated current they are.

Slight Overload

A small overload (say, 16 or 17 amps) might take a few minutes to trip the breaker.

Significant Overload

A significant overload (like 25 amps on a 15 amp circuit) will trip much faster.

Breaker Type

Different types of breakers exist (e.g., slow-blow, standard). Slow-blow breakers can handle brief surges better, but they are not typically used for standard household circuits.

Breaker Age and Condition

Older or worn-out breakers might trip faster or slower than their rating. They might even fail to trip at all, which is extremely dangerous.

Inrush Current

Heaters often have a brief "inrush current" when they first turn on, which is higher than their normal operating current. While brief, this can contribute to tripping the breaker, especially in an already overloaded situation.

Ambient Temperature

A breaker's ability to carry its rated current is affected by temperature. A hot breaker will trip sooner than a cold one.

Likely Scenario

Given the significant overload (25A on a 15A circuit), the breaker will likely trip relatively quickly. Here's a possible timeline, but it's not guaranteed:

Very Soon

In many cases, it will trip within seconds of both heaters being turned on.

At most, a minute or two

Under ideal conditions (cold breaker, healthy breaker, no inrush current issue) the breaker might take up to a minute or two to trip.

Worst Case

If the breaker is old and weak, and the house wiring is old, or if the breaker does not trip at all, this could cause overheating of the circuit's wires and potentially start a fire, creating a dangerous situation.

Important Warnings for Your Friend



Fire Hazard

Overloading a circuit is a serious fire hazard. The wires in the wall are not designed to carry that much current, and they will overheat. This can melt insulation, cause shorts, and ignite nearby materials.

Damaged Wiring

Even if a fire doesn't start immediately, repeated overloading can damage the wiring in the wall, making it brittle and more prone to future problems.

Code Violation

What your friend is proposing is almost certainly a violation of electrical codes.

Safety Issue

Electrical shock hazards are greatly increased by improper or unsafe wiring.

Recommendations


Absolutely Do Not Connect Both Heaters to the Same Circuit

This is not safe.2.

Use Separate Circuits

The only safe way to run these heaters is to plug them into separate 15A (or preferably 20A) circuits. Each circuit must be capable of handling the 12.5A load of each individual heater.
3.

Consult an Electrician

The best course of action is to have a qualified electrician assess the situation. They can: Verify the existing wiring is up to code. Determine if additional circuits are needed for the greenhouse. Install new circuits if necessary, with proper wiring, breakers, and outlets.

In Conclusion:

This is a dangerous situation. Your friend needs to understand the risks and take the proper steps to ensure the electrical system is safe. He should not proceed until a qualified electrician has assessed the situation. He should listen to your advice on this matter, as it is a matter of safety and could prevent significant damage or worse. Flag for review