1

What if I have this case:

  1. 15A circuit breaker
  2. 220V supply from the power company
  3. #14 AWG wire THHN - 6 or 7 strands
  4. Running 52 50-Watt light bulbs 20 hours per day

Max power supported in this case is 15x220=3,300 Watts
80% of the max power is 2,640 Watts

I am not planning to use 52 light bulbs in my house, I just want to understand the risk of using 52 50-Watt light bulbs in #14 AWG wire in a 15A circuit. If I will do that, even if the 52 50-Watt light bulbs will only use 2,600 Watts,

  1. Will there be any risks such as the wires melting or overheating?
  2. What are the possible hazards?
  3. Will it cause a fire if I keep the 52 light bulbs on for 20 hours per day?
  • It would help to know where you are located. I assume not North America since you are talking about lighting on 220V. – Speedy Petey Dec 14 '16 at 12:42
2

In the US under the NEC (I believe Canada is similar) you can run 80% load on a circuit 24/7. You can run 100% load intermittently, or by accident, or even occasionally if it's not expected, even for three hours or more.

The key is the actual definition of "Continuous Load", not an arm-chair definition, or what an old timer always told you.

Continuous Load.-

A load where the maximum current is expected to continue for 3 hours or more.

  • Thanks. I am from the Philippines by the way. I usually hear that each 15A circuit should only have up to 10-15 light bulbs. Even if I will only use up to 80% load, does it mean that I should not use more than 15 light bulbs? – user1764381 Dec 19 '16 at 12:25
2

What you're asking is: If you ran the wire up to NEC-authorized limits, would it start a fire?

You're fine.

That is the whole point of specifying safe limits. Now, if you really want to push the limits further and your application is commercial/industrial, Code permits you to go into dozens of pages of gory-details rules. For instance, there are three different ampacity tables depending on wire temperature of 60C, 75C or 90C, and the rating of the terminations, commonly you can't run wire at 90C ampacity because your lugs are only good for 75C. It's intended for larger industrial usage with thick wires -- but if you apply those rules and tables to household wiring, you'll see where they've built in an extra de-rate of 30-50%.

With household wiring, NFPA said "We need to simplify this so an average Joe can wire their house, and he and the next guy can be sure he did it right, without arguing about pages of picky rules in the NEC." So they said "14AWG = 15A, 12AWG=20A, 10AWG=30A, period, we're done". So you don't need to spend hours calculating all gory details, if you have a 20A circuit, you can just use 12AWG and know that you're golden.

Industry spends time/money Code-lawyering because they're dealing with thick, expensive cable and the cost of the cable justifies the thermal engineering effort. Not in a house, where it's thin wire and the cost to go up one wire size is fairly trivial.

VA is a factor

You know that AC power is a sine-wave. It's assumed most loads work like a resistor: at any instant, their current draw is proportional to the voltage. What happens the load draws unevenly, with current not proportional? Like this disaster - this is an incandescent lamp dimmer.

enter image description here (source)

Say that's a 100 watt bulb being dimmed. You notice that half the time, the load draws 100% of 100W. So the wires must handle 100W even though the load is only 50W on average.

Yes, it's as ugly as it looks.

How do we even calculate that? They created a special unit called "VA". literally, Volts-Amps. Yes, it sounds very similar to watts. VA describes the power (i.e. watts) which the wires must deliver (even if the load doesn't use it all). The above would be 100 VA, and 50 watts.

You need to plan your loads for VA. For instance your circuit with 220V x 15A = a capacity of 3300VA. (told you, it's just like watts). Look at the VA of each of your devices and calculate accordingly.

There's a USA$20 device called a "Kill-A-Watt" which will measure a device's load and read out both watts and VA.

How do we describe the relationship between watts and VA? Power factor. It is a ratio: Watts / VA. A perfect load has a power factor of 1.00. The example above has a power factor of 0.50, which is terrible by the way. One of the problems with the rush to CFL and LED bulbs has been cheap power supplies with very poor power factor, and laws in the USA have encouraged manufacturers to clean that up, which can be done by spending a few extra pennies on the power supply section.

  • Thanks. I often hear or read that for 15A circuit, you can have up to 10-15 light bulbs or 8-10 light bulbs only according to some people. But what if I only use 15-Watt light bulbs and will always sure that I will not use more than 80% of the circuit capacity even if I will replace the bulbs in the future, will that still be the case? If so, is the reason to avoid legal issues as implied by you above (if I understood correctly)? – user1764381 Dec 19 '16 at 13:10
  • That sounds like a "general guideline" to explain it to simple people. You are far past that. You're ready to learn about VA. Some loads draw power unevenly. The wires must handle their peak amperage even though they draw unevenly. VA (volts-amps) is a unit similar to watts, but accounts for that peak load. A cheap CFL might be 11 watts but 25 VA. A 220V x 15A circuit can only handle 3300 VA. – Harper - Reinstate Monica Dec 19 '16 at 15:22
  • Thanks. I bought a bulb which doesn't state the VA, just the Watts. It says "13 Watts, 780 lumens, 60 lumens per watt" but there is no VA. The brand is firefly and the appearance exactly looks like this one: fireflyelectric.magcon.net/index.php?route=product/…. – user1764381 Dec 20 '16 at 11:41
  • Since there is no VA, and only Watts is given, what would be the best way to assume that VA value? Should I just assume that the given watts is actually half of the VA? – user1764381 Dec 20 '16 at 11:43
  • US/EU has a US$30 gadget called a "Kill-a-watt" that measures watts and VA for any plug-in load. There must be similar devices in your market. You can also contact the company and ask about VA or power factor. They are not stating VA/PF because they think it won't matter for just a few small bulbs on a typical 3300W circuit. If you have many such bulbs on a circuit, VA/PF becomes critical - since they will all draw in the exact same way, accumulating. – Harper - Reinstate Monica Dec 20 '16 at 14:23
1

Even with a continuous use of all 52 lights you are below 80% as shown by your calculation. I find homes regularly that have closer to 100% and get called because the breaker is tripping. 80% is a good safety factor and even if above this level the wiring is not going to melt down as long as the wire size is matched to the circuit breakers. (If memory serves in an industrial control panel 14 awg wire is rated for 40 amps) So there are safety factors built into residential wiring and you should be fine.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.