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3-Phase heater on 2 cables

3-Phase heater on 2 cables

3-Phase heater on 2 cables

(OP)
Hello everyone.

Pretty green Electrical Engineer here, I have a slight complication, but I thoguht i'd weight in your thoguhts before submitting my project for review.

I have a project that requires a simple cable replacement for a 138kV CB. The problem arises when replacing the 3C#6 cable for the 3 phase heaters. The AC JB's CB is 40A. Our client's standards only approve cables 2C#6 and 4C#8.
According to CEC the #6AWG is rated for 65A. Which would be ideal but they only come in 2C cables.
According to CEC the #8AWG is rated for 45A. But after de-rating for temp (45 celsius) and number of conductors (3) the new rating would be 36.45A (45A*.9*.9 Correct me if i am wrong) Which would not be protected by the CB.

So I am curious as per your expertise if it's a big deal to run 2 runs of 50m cable of 2C#6. I imagine this would mess with one of the phases as per capacitive effect of cables.

your thoughts?


Thanks.

RE: 3-Phase heater on 2 cables

If you're pulling in the same conduit there is no issue, but I think you are too pessimistic regarding the #8. I'm not in Canada, but in the US, there would be no need to de-rate for having three conductors in one conduit. The derating starts with MORE than 3 current-carrying conductors. Also, if you have a calculated ampacity of 37 A, it is acceptable to protect with a 40 A breaker. So-called "next larger size" rule in the NEC. This applies up to and including 800 A.

RE: 3-Phase heater on 2 cables

(OP)
Thanks for your answer dpc, you are soemone whose knowledge I've come to respect on this forums.

You are right that I am being a tad pessimistic. De-rating it for 3 conductors (actually 2 current carrying) would be a factor of 1 (negligible) but I was actually using the factor for 40 C not 45 C. which is 0.85 which still brings me down to 38.25A

I looked into another rules that resembles what you are saying, where if i dont have a conductor with the exact ampacity i am allowed to pick based on overcurrent protection rating or setting. The one for 40A CB allows me to pick a conductor with ampacity between 36-40A. up to 600A

That being said, the reason why I don't do this is because my superior (senior designer) has told me I run the risk of damaging the conductors and was told that the client runs parallel runs for different phases all the time anyway.

Your vote is to go with the 4C#8, dcp?

RE: 3-Phase heater on 2 cables

I'd be OK with the #8, I think. The low voltage ampacity tables, at least in the NEC are very conservative. Also, presumably the heaters are controlled by thermostat, so they probably won't be on when it is 45 deg C?

I don't see a problem either way, as long as all three phases are in the same conduit - still not quite clear on that.

RE: 3-Phase heater on 2 cables

If in doubt about the 3c/#8 AWG, use three runs of #6 AWG two conductor cable, one cable for each phase.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: 3-Phase heater on 2 cables

(OP)
Thank you for your reply waross. that's definitely an option. From what i can gather, running a phase on a different wire is not that big of an issue (which was my main question) but I am being a bit OCD about that.

Dcp has a point about the thermostats, it didnt even cross my mind. This is why I like asking you veterans questions like this. I find i learn new angles I never even thought about.
I have decided i will submit my design with the 4C#8 cable.

Thank you both.

- Sn0

RE: 3-Phase heater on 2 cables

Beware of the anti-condensation heater.
In colder climates I see heaters on a thermostat to keep the switch warm in cold weather.
Condensation may occur at any temperature. In addition to the cold weather heaters, anti condensation heaters are often installed. These are typically two heaters in series across rated voltage so that each heater receives one-half rated voltage. These are left on 24/7/12. The circuit is often equipped with a current monitoring relay to alarm on loss of the anti-condensation heaters.
Don't confuse the two types of heaters and don't share cables, if anti-condensation heaters are fitted.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: 3-Phase heater on 2 cables

(OP)
Thanks for the heads up waross. On this particular circuit there are 2 space heaters only one wiht thermostat and 3 tank heaters all with thermostats and all of them paralelled. According to specs there's no condensation heaters for this particular CB. I will definitely keep that in mind in the future, though. Thank you.

RE: 3-Phase heater on 2 cables

Sn0 -
I'm definitely not green - but sometimes I'm a bit slow, and I'm under the NEC, not CEC. A few questions:
What does 138KV have to do with heaters?

The ampacity numbers for the #6 and #8 look like 75C low voltage, not 138KV. Leads me to believe the heaters are maybe 575V or 480V.

The client standards not approving 3/c (and 4/c) cables of any size (well, #14 and up) sounds really daft. I sure they have heard of three phase power. That doesn't make any sense at all. What gives with that? Did the standards change since the original installation? The original could use 3/c - #6 and now that is prohibided? That makes even less sense.

Paralleling cables less than 1/0 is specifically forbiden by the NEC (unless 400Hz) Doesn't the CEC have a similar prohibition?

Are the cables to be in an existing raceway as in conduit? Could you fit three 2/c-#8 cables in the existing conduit?

NEC rules for heater branch circuits requires 125% ampacity although feeders only require 100%. Is there a similar requirement for CEC? This only matters if the heater FLA X 1.25 is over the ampacity of the #8 - and still 40A or below.

Just curious. I thinking that I don't under the install at all, and as I said, I'm under the NEC and may not understand the CEC issues.

ice (seeking enlightemment)

Harmless flakes working together can unleash an avalanche of destruction

RE: 3-Phase heater on 2 cables

iceworm,

These are low voltage heaters used for heating high voltage SF6 circuit breakers in cold weather. In the US, the large majority of these are installed in utility substations that are exempt from the NEC anyway.

RE: 3-Phase heater on 2 cables

(OP)
Hello iceworm,

As dcp stated these are for 138kV circuit breakers for substation, which need keep the gas above a certain temp.

The existing cable was a 3C#6 which is no longer manufactured, so it is extremely hard and expensive to get one with the same specs. This is why the client has come up with this standard (well one of the reasons anyway). Which in a way, it makes our job easier because you only have to pick whatever has been tested and proven to be reliable and drives prices lower if they are readily available.

I remember a past project for a smaller client they had a weird cable too i cant remmeber what it was but in order to try and get one like-for-like i would have had to special order it from a neighbouring province's utility's which would have taken way longer to get and much more expensive. Instead I decided to run 2 cables with half the conductors. Much simpler in that case.

I have never seen a rule under the CEC like the one you describe for heaters. In fact I checked and I couldn't find anything describing conductors for heaters. Your rule sounds a lot like CEC's on motors though.

I could fit 2C#8 (to my knowledge there is no restriction in space in the cable trenches), my conundrum arises from the need to want to have one single cable carry all of the phases/conductors while maintaining code.

I have never heard that substations (in Canada of course) are except from Code. But i'd like to pride myself in keeping up with it as I was taught in school. Not to mention im right out of school and I would like to do everythign (literally) by the book. I figure once you stop doing so, complacency sets in.

Hope I explained your question.

- Sn0

RE: 3-Phase heater on 2 cables

Pretty sure the CEC has a similar size restriction on parallel conductors. Waross probably knows - Bill?

RE: 3-Phase heater on 2 cables

Ah --- heaters for sf6 cb. And the heaters are 3ph. Are they 480V, 575V, 208V? - I guess they could also be 240D? Doesn't matter much, just curious.

Interesting they are 3ph. Two of my clients have HV services, 2MVA primaries with sf6 CBs. The heaters are 1ph pads, about 1' square, SS tie-wrapped to the bottom of the sf6 reservoir. I recall they are about 1KW each. Leads come out of the CB insulation blankets to a connection box. Cable from there to the low voltage CB panel is MC-HL - I think it is similar to what you guys call "tek" cable.

If I had to use a 3c cable, I'd be looking real hard for a substitute that met or exceded spec. One would think there is something available out there

If the 2/c cable is all you have, and multiple cables are a must, hopefully the cable has a non-ferrous sheath. If not, that could be a problem. However, separate phase entry into a ferrous J-box should not be an issue. As I recall, the CEC doesn't mind up to 200A.

Only other thing I would suggest is to see if you can split up the heater into three separate 1ph loads. If available, that could drop your load to 23A each (or even a bit less)

ice

Harmless flakes working together can unleash an avalanche of destruction

RE: 3-Phase heater on 2 cables

Scope
This Code covers all electrical work and electrical equipment operating or intended to operate at all voltages
in electrical installations for buildings, structures, and premises, including factory-built relocatable and
non-relocatable structures, and self-propelled marine vessels stationary for periods exceeding five months and
connected to a shore supply of electricity continuously or from time to time, with the following exceptions:
(a) installations or equipment employed by an electric, communication, or community antenna distribution
system
utility in the exercise of its function as a utility, as recognized by the regulatory authority having
jurisdiction, and located outdoors or in buildings or sections of buildings used for that purpose;
For years the wording of this clause was "installations or equipment employed by an electric utility in the exercise of its function as a utility"
This has been added in later years. ( communication, or community antenna distribution
system)

Bill
--------------------
"Why not the best?"
Jimmy Carter

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