Industrial Equipment 3

[KSue]

A system was designed with 1500W (3 heaters 500W each, and each fused at 10a) , and is 120VAc on a 15A circuit breaker, 14awg.

They had intermittent tripping of the internal 15A breaker. More information came to light. With the box, temperatures reach to 51deg C (has a small internal boiler.)

My first thought was the breaker is too small. De-rating the breaker at about 55C, brings it to about 12A, which is 120X12A=1440W. If it was ambient temperature, this would still be marginal.

I suggested a 20A breaker on 12awg. De-rated brings it to 16A, which is 120x15A = 1920W.

The question came back, can we use the 20A breaker with the 14awg, if the conductor is only about 30" long.

They ran a short circuit test on 20A/14awg, and it didn't blow the internal breaker, but did blow the breaker for the receptacle they were using.

The end of the (short) wire welded to the screw.

Tell me what happened here?

 

[MacGyverS2000]

You can use a 100A breaker with 14AWG wire... but it won't pass code. A 20A breaker is only legal on a 12AWG wire, at least from an NEC standpoint for homes... industrial may be different in that regard, so I throw that option out there, but I doubt it.

Are you sure these are 120V heaters and not 240V? They could be either at that power level, but I want to be sure before continuing.

If I understand you correctly, you used a 20A breaker (at the heater) attached to a 14AWG line that was connected to mains via a 15A breaker. When turned on, the 15A breaker in the main panel popped. This tells me you're likely pulling more than 15A... if you have to derate because it's on continuous for several hours, then it's pulling more than 12A, for sure.

You need to verify your heating elements are still good, for one...

Dan - Owner

http://www.Hi-TecDesigns.com

 

[KSue]

Originally this was a 15A breaker with three 120V, 500W heaters. The line wire is 14AWG. The problem was when the interior of the system heated up, there were intermittent trips in the 15A breaker. I calculated there is 1500W of power (theoretically) being drawn off by the heaters.

[Maybe for a little while there isn't a problem because the conductors are max 30" long from source to load :: negligible resistance in conductor.] But it's inconsistent. I does trip.

We have a version of 240V heaters, but these are 120V for this system.

The product manager changed the breaker to a 20A, (we get then 1920W) which covers the 1500W load and then some. He left the 14AWG in the system, didn't change to 12AWG. I was wondering if the 14AWG can carry enough current to trip the 20A breaker.

 

[DRWeig]

Hi KSue,

If you have a short circuit, even a much smaller wire will carry enough current to trip the 20A breaker.

It's the potential for sustained overload that may make your situation dangerous. At elevated ambient temperatures, if your heaters faulted in such a way as to draw, say, 19A for a prolonged period of time, the conductor insulation may fail but the 20A breaker would never trip.

As MacGyverS2000 mentioned, industrial settings sometimes are not governed by local codes. However, you really need to check the electrical code / standards / corporate rules / specifications that apply to your device.

For example, if your device is supposed to meet the national electrical code (NEC), it is not permitted to use 14 AWG conductors with protection above 15A. The same applies if there is a UL standard to meet. Often, corporate policies dictate the use of one or the other of these in cases where a local electrical inspector does not have jurisdiction.

To put it more simply, I would never put a 20A breaker on 14 AWG wire. The cost of 12 AWG over 14 AWG for a 30-inch circuit is mighty small...

Good on ya,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[itsmoked]

When does it blow. Immediately? Or after a while?

That is a 12.5A load so a 15A breaker should be holding unless it's being cooked.

Put a clamp-on ammeter on a power lead and SEE what's happening.

Maybe you have a fault.
Maybe your heaters are not really 500W.
Maybe the heaters are considerably higher power, before they're warmed up.

You won't know until you actually check the current.


You say "Product Manager". If this is a product you will be selling you have an obligation to check each one for functionality before you ship it. Part of that test should be checking the current. That will tell you if all the heaters are electrically connected correctly and that the heaters are actually the correct ones.





Keith Cress
kcress -

http://www.flaminsystems.com

 

[MiketheEngineer]

Two thoughts -

Use a "slo-blo"breaker
14 ga wire will/might burn out before the fuse/circuit breaker blows. NOT good!!

 

[KSue]

itsmoked:

I put in a request for that test. I'll see what happens.

BTW, The short circuit triggered the short to the receptacle instantly.

The tripping I'm trying to figure out is intermittent, doesn't happen all the time, but it happens when it shouldn'tgaaa... always not good.

I'll be back.

 

[LionelHutz]

You describe this as a box. Inside the box is a breaker feeding 3 x 10A fuses which each feed a heater. Outside the box is a cord plugged into an outlet which provides 120V power to the components inside the box.

If this description is true, then it doesn't really matter what breaker you use internally. You don't even need the internal breaker at all. The #14 wire feeding this box and any #14 guage wire used inside the box is protected by the 15A breaker protecting the receptacle you plug this box into.

 

[RobWard]

I've not quite got my head around this, but can you cool the electrics?
Are you saying that you are deliberately heating the electrics (i.e. they are in the same box as the heaters) or are they in a separate control panel? If the latter, could you not add a small cooling fan and vent?

"I love deadlines. I love the whooshing noise they make as they go past." Douglas Adams

 

[KSue]

"you describe this as a box. Inside the box is a breaker feeding 3 x 10A fuses which each feed a heater. Outside the box is a cord plugged into an outlet which provides 120V power to the components inside the box.

If this description is true, then it doesn't really matter what breaker you use internally. You don't even need the internal breaker at all. The #14 wire feeding this box and any #14 guage wire used inside the box is protected by the 15A breaker protecting the receptacle you plug this box into."

The breaker keeps tripping. Not right away, but after about an hour. Changed it to a 20A breaker, and it quit tripping. Didn't upgrade the wire from 14awg to 12awg tho.

 

[KSue]

"I've not quite got my head around this, but can you cool the electrics?
Are you saying that you are deliberately heating the electrics (i.e. they are in the same box as the heaters) or are they in a separate control panel? If the latter, could you not add a small cooling fan and vent? "

The breaker switch is on the box which hoses an internal boiler. (not a separate panel for switch, so no fans allowed). We'll try a magnetic switch (higher heat tolerance). We are also looking to keep field modification costs down.

 

[LionelHutz]

That response was useless. Did you even understand what I posted?

 

[KSue]

New PostLionelHutz (Electrical)

Let me say something less "useless"... The internal breaker is a UL requirement, therefore it must be in/on the box. On that note, it must work also.

BTW, There is a circuit board internal also, and it handles the heat fine. The small boiler puts out heat 41° C to 55° C. The 15A switch/breaker (655-W33-T2N1Q-1) is attached to the front of the unit, and obviously can't handle the heat. The 20A switch/breaker can handle it.

I intended to get opinions on the 14awg conductor used in a 20A circuit breaker. I have been given several excellent responses.

 

[KSue]

CORRECTIONS

The breaker switch is on the box which HOUSES an internal boiler. (not a separate panel for switch, so no fans allowed). We'll try a magnetic switch (higher heat tolerance).

We are also looking to keep field modification costs down.

 

[KSue]

itsmoked (Electrical)

Used a clamp-type ammeter to measure steady-state current on two units:

Boiler #1: 11.6A
Boiler #2: 12.0A

This was with the 15A breakers/switches (655-W33-T2N1Q-1)

 

[LionelHutz]

Well, if you are following a UL standard then look in that standard and see what it says for wire size. Opinions from a forum don't mean anything when you meet the UL standard you are listing to.