Job from Hell

[Roblawro65]

I have a switchgear with a duel Feeder Main-tie-main. the mains are 3000amp.the designer has us installing 24 sets each in 4" conduit with 4 each 500mcm XHHN 90c in each Conduit. 12 sets per 3000Amp Breaker, this monster is only 90 meters away from the main Switchgear,but he is using 75c 380amps for his calculation.the total system voltage drop is only 1.25%
I think he nuts because we don't have the switchgear yet. it seems to me it would be cheaper to up the wire size or get 90c connections.

Does this sound right to anyone?

 

[augie47]

disregard.. I was thinking "kirk-key" type feed not dual fed.

 

[dicklaxt]

12 per phase is 4560 amp capacity does the spec call for that much cushion or did I read something wrong?If you were to lose a transformer and shed load by about 33%(no spares) and operate tie closed,it would be close but still seems high as if there were spares they would normally not be running anyway but could with tie open,,

I guess the design engr has a reason or has a directive from the client.

dick

 

[Roblawro65]

I'm reading the specs. 10% Spare,20%,future,20%Spare Amps,100% continus load he says Voltage drop,but the total distance of anything feed from this switchgear is only a 100 meters away and the projected load calculation is 1650 amps draw.i don't get,
can anybody say that have heard of 90c termination for switchgear

 

[tkb]

If you are getting paid for it, then do what he designed.

 

[Roblawro65]

And it is Kirk-key type in a way, it's a split bus Switchgear Main-tie-main feed from City power with a Generator back up

 

[Roblawro65]

I'm a General Contractor and i'm Paying a Design Engineer,

 

[growler]

I'm a General Contractor and i'm Paying a Design Engineer,

There is a forum section on this site for electrical calculations and engineering. The engineers may be able to give more information about your questions.

Really the engineer that you are paying should be able to answer your questions or you should be looking for a different engineer. Opinions on a forum are just opinions and would have to be verified by the engineer that seals the drawings anyway.

Have you questioned this engineer and what answers did he give?

 

[Roblawro65]

the truth is i trust the code and the people with real experince in installing switchgear,i've hired a consultant to double check the engineer,but i don't trust thoses snakes and if i find out on my own i'll feel much better about it

 

[augie47]

If you are paying the engineer, as Growler states, ask him.
If it's an interlocked system so only one of the mains can be on at any one time, I will say I don't see the need for (12) sets,
IMHO, you can not use the 90? rating unless your breakers are so rated but 8 sets of 500 should be adequate for 3000 amp (10 if you derate for 4 ccc).
But, prior to deviating from engineered drawings, check with the engineer.

 

[growler]

i've hired a consultant to double check the engineer,but i don't trust thoses snakes and if i find out on my own i'll feel much better about it

If you were to list which are of the country you are in then some of the PE's here may be more likely to contact you or give advice.

Face it, everyone is looking for new customers. I would assume there are some darn good engineers that visit this site. You get what you pay for and you need to find an engineering firm that you can trust.


Remember that this is just an Opinion.

 

[ohmhead]

Well were doing a job right now and it has 6 x 3000 amp mains each gets 8 runs of 500 mcm cu just like the above post . I dont use meters when i measure ! Our transformers from main sections each is 55 feet thats one way wire length . Your distance is 295 feet 90 meters thats far for a service run and a generator feeding it we dont have tie breakers and we have a switchboard just for emergency 2500 amp from generator . But its not 295 feet away i think you engineer is looking good with 12 runs hes giving it and extra 1520 amps maybe its for the future ooooor he likes to spend money on wire but it is a long distance 295 feet ? AMP & VOLTS LONG DISTANCE AT LOTS OF LOAD = MORE WIRE