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Buzzing 400 amp main


alabama1984
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30kW should not even come close to 400A on a 240vac system, if that what this is. It will be closer to 125A.

First thing is that you need to do an amp draw to see just how many amps you are drawing. The other systems in this setup may be adding to the total draw and the 400A breaker can be at or near its maximum, therefore arcing and causing the buzz sound.

There can be a bad connection at the breaker. This type is probably a bolt in style.

The breaker can be bad. Either way, someone qualified needs to evaluate the situation to determine what is wrong.

There is something wrong.

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Originally posted by alabama1984

With six stages of electric heat turned on (totalling 30 kw) the separate 400 amp main produces a loud buzzing and vibration. Is this normal?

No. It's not normal. I frequently see 30kw electric heating systems and I've never seen one cause buzzing of breakers -- and these are on 200 amp systems.

Are you sure that it was the breaker that was buzzing?

What's in the box to the left of the switch?

Does this system use current transformers for the meter?

Is this a house?

- Jim Katen, Oregon

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The two breaker boxes are each rated at 200 amps while the main is fitted with 300 amp bolted fuses. The switch on the right contains a spring-action knife switch and the 300 amp fuses. The centre enclosure has within it the transformer for the meter (which is located outdoors). To clarify, the breakers aren’t buzzing, it is the fuse box on the right which is making the noise when under heavy load. And yes, this is a house.

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Originally posted by alabama1984

The two breaker boxes are each rated at 200 amps while the main is fitted with 300 amp bolted fuses. The switch on the right contains a spring-action knife switch and the 300 amp fuses. The centre enclosure has within it the transformer for the meter (which is located outdoors). To clarify, the breakers aren’t buzzing, it is the fuse box on the right which is making the noise when under heavy load. And yes, this is a house.

I can't explain it. Are you sure that it's the fused switch box that's buzzing and not the transformer box?

It really doesn't make sense unless . . . is this by any chance a three-phase system?

Oh yes, and is this straight resistance heat or is it something fancy?

- Jim Katen, Oregon

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Originally posted by alabama1984

Yes, I am very sure – the buzzing is louder on fuse box than on the transformer enclosure. It is a fairly modern (1987) two-phase system.

Just a small aside. It's a single phase system, not two phase. The only remaining two-phase systems that I'm aware of are for some very old railroads & elevators.

The HVAC setup is two triple-split heat pumps each with three stages of 5kw auxiliary resistance heat.

I'm stumped.

Does it only behave like this when all six are firing at once or can you make it buzz with only some of the six firing?

- Jim Katen, Oregon

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Originally posted by Erik

Hi, First time to post here. sounds like loose bolt on the service feeds at the fuses.

Also in the picture's... are you guys allowed to bundle your wires out of the breaker box? Looks like it could create quite a heat load.

Erik

Maybe at that one group of wires with the silver colored clamp in the bottom photo but I don't think it's an issue at the top of the panelboards.

Here's Douglas Hansen on the subject:

From Electrical Inspection of Existing Dwellings - 2001 Edition

Cables - NM Cables: comments about bundling

A common poor practice is to bundle several cables together and enter through one large bushing or clamp into an enclosure. The clamp is not designed for this large number of cables, and those in the center might not be properly secured.

Cables cannot be bundled together without creating a problem of heat entrapment. When a bundle is longer than 24-inches and contains enough conductors to surround the ones in the middle, their heat cannot readily dissipate.

The high ambient heat in attics can cause NM Cable to overheat and damage its insulation. Prior to 1984, conductors in NM cable were rated 60°C. Cables manufactured after 1984 contain 90° insulation. Both cables must be protected using the 60° column in the ampacity tables, but a large difference comes into play when the effect of high ambient temperature is considered. For example, if an older 60° #12 wire is in an attic with a temperature of 125°F, the maximum allowable current int that wire is 11 amps. In the same situation, a newer 90° rated wire would still have an ampacity of 20 amps. Wire can be more quickly degraded if there is both excess bundling and high ambient temperature.

ONE TEAM - ONE FIGHT!!!

Mike

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Originally posted by Erik

Hi, First time to post here. sounds like loose bolt on the service feeds at the fuses.

Good thought. I'll bet you're right.

Also in the picture's... are you guys allowed to bundle your wires out of the breaker box? Looks like it could create quite a heat load.

Erik

The NEC allows NM cable to be bundled with no restrictions for 24". After that, you can still bundle, but you need to apply a derating factor. Read all about it in 310.15(B).

- Jim Katen, Oregon

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NM cable must be derated whenever 2 or more NM cables containing 2 or more current carrying conductors are bundled together and pass through wood framing that is draft or fire stopped using any type of sealant, even foam. You must derate each conductor by using Table 310.15(B)(2)(a)

It does not matter if they are bundled for less than 24". The derating still applies as long as they are in firestop/draftstop material. I believe this started in the 05 code.

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Originally posted by Jeff Remas

NM cable must be derated whenever 2 or more NM cables containing 2 or more current carrying conductors are bundled together and pass through wood framing that is draft or fire stopped using any type of sealant, even foam. You must derate each conductor by using Table 310.15(B)(2)(a)

It does not matter if they are bundled for less than 24". The derating still applies as long as they are in firestop/draftstop material. I believe this started in the 05 code.

That's correct. Of course, the cables in the picture don't pass through wood framing that's fire or draft stopped. But I take your hint so I'll revise my statement:

The NEC allows NM cable to be bundled with no restrictions for 24" unless they're fire or draft stopped. After that, you can still bundle, but you need to apply a derating factor. Read all about it in 310.15(B) and 334.80.

- Jim Katen, Oregon

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Originally posted by Jim Katen

Originally posted by Jeff Remas

NM cable must be derated whenever 2 or more NM cables containing 2 or more current carrying conductors are bundled together and pass through wood framing that is draft or fire stopped using any type of sealant, even foam. You must derate each conductor by using Table 310.15(B)(2)(a)

It does not matter if they are bundled for less than 24". The derating still applies as long as they are in firestop/draftstop material. I believe this started in the 05 code.

That's correct. Of course, the cables in the picture don't pass through wood framing that's fire or draft stopped. But I take your hint so I'll revise my statement:

The NEC allows NM cable to be bundled with no restrictions for 24" unless they're fire or draft stopped. After that, you can still bundle, but you need to apply a derating factor. Read all about it in 310.15(B) and 334.80.

- Jim Katen, Oregon

Jim, I actually was not hinting anything, just following the severe thread drift in progress........[^]

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Originally posted by JEuriech

When we derate cables due to bundling, what would be an acceptable repair? Take out a 20 amp breaker and replace it with a 15 amp breaker? What would you do for #14 wire? Would the only repair be to separate the cables?

Thanks

Jeff Euriech

Peoria Arizona

You could unbundle the cables.

You could protect them at a lower level. (Probably not a realisitic alternative.)

You might not have to do anything. Sometimes you can use a higher ampacity for derating purposes than you can for overcurrent protection. In those cases, the existing overcurrent protection might be ok even after the conductors are derated. NM cable comes to mind here.

- Jim Katen, Oregon

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Hi,

Turns one of the contacts in the knife switch mechanism was a bit corroded and arching.

Since this is a Canadian installation, I’m not sure how relevant American codes are. In any case, the wires split off after about 30 cm.

(I always thought each 120v leg was one phase off from the other, which is why they could be combined to give 240v. But then that all comes from one 25kv line... anyway, sorry if I used the wrong terminology!)

Happy Christmas everyone!

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Originally posted by alabama1984

Hi,

Turns one of the contacts in the knife switch mechanism was a bit corroded and arching.

Ouch, that's not good. Did they replace the damaged parts?

Since this is a Canadian installation, I’m not sure how relevant American codes are. In any case, the wires split off after about 30 cm.

I wasn't aware that there was a province called Alabama. Must be one of those northern ones that no one ever talks about.

(I always thought each 120v leg was one phase off from the other, which is why they could be combined to give 240v. But then that all comes from one 25kv line... anyway, sorry if I used the wrong terminology!)

It's all one phase. The two legs carry power from two different poles of the same transformer winding. While one is "pushing" the other is "pulling" but they're pushing & pulling opposite ends of the same phase of power.

To get more than one phase, you'd need to have more than one winding at the transformer.

Don't feel bad. Pretty much everyone slips up and mixes up poles & phases now & then -- electricians do it all the time. It's just nice to know the difference so that when you're writing down things that matter, you can use the correct term.

- Jim Katen, Oregon

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