building voltage drop

[VirgilJ]

As on a previous thread I'm building a new 400lb studio glass furnace to replace the current 100lb glass furnace.
Now going Moly elements
9 elements 9/18 312/500 60amp 12.5 volts each
wired in 3 series of 3each
40 kva transformer 4 to 1 step down
furnace will run on separate 200amp circuit

building has 400amps in single phase 240 to 248.8 volt It VARIES!!

I've been running checks on voltage to determine the build on the transformer...requested by the company I'm buying it from.
That's how I know that my voltage VARIES. The question I have is:
IS the following a problem:

This mornings Volt report:

Just a few lights on:
248.8 volts

100lb glass furnace temp. raised to engage elements:
244.3 volts

100lb furnace engaged and 15.5 amp compressor running:
242.3 volts

100lb furnace engaged and 15.5 amp compressor running and 30 amp kiln on: 240.6 volts

THEN add more lights and large fan: 240.2

 

[Skogsgurra]

I wouldn't say it is a problem.

Most utilities have voltage limits between +/-6 % up to +/-10 %. Your variation is at most +3.7 % down to +.1 %

Feel priviliged!

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[itsmoked]

That's why you'll have a temp controller. It will alter its control output to deal with that change in available power.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[7anoter4]

It seems to me the building feeder 2*1*350 MCM AL is too small.
The voltage drop -I appreciate the feeder length is about 1000 ft. long- for only 60 A is 248.8-240.6=8.2 volt.
For 400 A-if this will be the building rated load- will be 248.8-195.6=53.2 volt
In order to compensate the drop you have to substitute 2*2*1*500 MCM [2 parallel feeders of 500 MCM Al] for one feeder of 350 MCM Al. Then the drop will be 15 volt for 400 A.

 

[jghrist]

Is the 60A the existing 100lb furnace or the new 400lb furnace?
What is the transformer impedance?
Are the voltages measured at the transformer or the load?
What is the load current when the voltages were measured?
7anoter4 - Am I missing something? Where did the 350 MCM AL and 1000 ft come from?

 

[7anoter4]

I agree with you jghrist, there are a lot of presumptions.
First of all VirgilJ in the o.p. mentioned: "As on previous thread…".
The "previous thread" was thread 238-298488 of 10.05.11 "Contoller(!) question for glass furnace"
In this previous thread is mentioned:
" I have 240 V single phase 400 A into the building off a 350[mcm!] aluminum underground line"
In the present thread the voltage in absence of load is 248.8 V.
If 15.5 A (compressor running") will add 2443.3-242.3=2 V and the cable 1*350 MCM al. has x=0.028277 ohm/1000 ft r=0.06374 ohm/1000 ft.
you'll get the length of the 2*(1*350) al. feeder at 60 c/sec,90oC as 950 ft[approx].
So if the supply voltage [neglecting transformer voltage drop!] is still 248.8 V and the last measured voltage is 240.6 V the load will be 63 A[approx.].
Of course, for 400 A one has to add the transformer drop and some drop on the medium voltage but for general explanation I think this could be enough.

 

[VirgilJ]

thanks for the replies
7anoter4
the underground line from the splitter box is about 400ft...and 700feet before that and it looks like a bigger box at that location...MAYBE a transformer???...I don't know about the utility company transformers. They said I had more than enough for 400amps...did not say anything about voltage.

jghrist
the existing 100lb furnace is scr current limited to 66amps
I don't know the transformer impedance.
The voltages I gave are being measured in an unused 30amp outlet in the building.
the exact current load I'm not sure other than what I stated above.
The 350 aluminum wire is what the power company guy told me I had when he came out and looked in my box...then talked to his company engineers about the rest of the their line (circuit) coming to my property.

7anoter4
the 248.8 to 240.6 included everything that would be normally running in the building PLUS the old furnace.
With the change it will be everything that would be normally running in the building MINUS the old furnace PLUS the new furnace.
I'm a little unclear why the present amount would be only 63 amps with everything running.

What else can I find out to be more clear for your help??
I can contact the utility company/
the transformer for the new furnace is being specially built...it will be a 4 to 1 step down transformer...past that I don't know.
Thanks!!
Virgil

 

[VirgilJ]

a little more on what I currently (the 100lber)have:
controller has 4-20 milliamp output capability
current limiting scr
transformer plate reads:
marcus transformers
model 15G45-20T
240 primary
45 secondary
kva 15
hz 60
single phase
type wm
temp 150 C
class 220
spec. 9155
connect: cu/al
Polarity: additive
impedance 4.3
weight 200

taps 2 220 3 230 4 240
series 12330-208-ENCL1

I ASSUME that I'll be getting a similar type only bigger.

This is what my supplier's consultant suggested today:
Make it two banks of four elements each and switch the secondary to a 50V unit thereby running 12.5V each.
8 x3830= 30640 watts. That leaves the transformer alone at 40KVA (38KVA including safety factor) and your breaker panel would need 160 AMPS including the safety factor. This would in turn mean only two spare elements needed and less copper in the transformer. Life is good.

 

[jghrist]

The transformer at the furnace will not affect the voltage on the rest of the circuit.