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350 mcm for 300 amps? #36289 04/03/04 09:04 AM
Joined: Jul 2002
Posts: 74
S
scjohn Offline OP
Member
I will be running my service (208v-3phase-300 amp) to an office building next week. The run is 100 ft. from poco transformer to meter base, and around 80 feet from meter to the 300 amp main. The engineer specs 350 mcm for the run. With copper going through the roof I cant see why I shouldnt be able to run 300 thhn. Total connected load is around 225, no electronics. I did a vd calc and 300 keeps me well within the 3%. Its a 4 wire system, but 310-15-4 states the neutral is not considered when it is used to carry the unbalance. Is there something obvious that I am missing? Thought I would ask, before I call the engineer.
Thanks, John

Work Gear for Electricians and the Trades
Re: 350 mcm for 300 amps? #36290 04/03/04 11:18 AM
Joined: Aug 2003
Posts: 1,374
R
Ryan_J Offline
Moderator
You should be fine. You could actually use 300KCMIL and use the round-up rule (240.4). You are right, the nuetral probably won't be a current carrying conductor, but even if it were you would be alright because for derating you can use the 90 degree column.

One thing, however: I notice that you talk about the connected load. 230.42 requires that you use the calculated load for the purpose of sizing service conductors. HAve you done a load calculation?


Ryan Jackson,
Salt Lake City
Re: 350 mcm for 300 amps? #36291 04/03/04 11:32 AM
Joined: Apr 2001
Posts: 507
G
golf junkie Offline
Member
"The engineer specs 350 mcm for the run. With copper going through the roof I cant see why I shouldnt be able to run 300 thhn."

Because the engineer spec'd 350 and you bid 350. In my experience if I was to ask to run the 300 two things might happen.

1) (mostly likely) the engineer would stick with his original decision for the 350.

2) (not as likely) They might accept the 300 and ask for a deduct.

On engineered jobs the service conductors are often oversized. The increase in coppper prices is not the engineers problem, it's your's.

GJ

Re: 350 mcm for 300 amps? #36292 04/03/04 01:34 PM
Joined: Jul 2002
Posts: 74
S
scjohn Offline OP
Member
Golf Junkie, when I bidded this job 6 months ago 350 was actually a little lower than 300 is now. Its not like I am asking to substitute 12 gauge with 14 gauge, or install residential grade devices instead of commercial grade. What potential benefit is there in running 350 mcm as opposed to 300?
John

Re: 350 mcm for 300 amps? #36293 04/03/04 02:24 PM
Joined: Dec 2000
Posts: 4,283
electure Offline
Member
I think GJ's right.
The benefit is to you. You won't have to buy 350 over and above the price of the 300, pull out the 300, and do the job over with the 350

I don't know what you'd want to do with the 720' of 300kcm that had been pulled out, but I wouldn't want it sitting around the shop here.

Re: 350 mcm for 300 amps? #36294 04/03/04 03:54 PM
Joined: Sep 2003
Posts: 650
W
winnie Offline
Member
scjohn,

Do your bids have an expiration on them? If you bid for a particular job, then IMHO providing less than what you bid to provide is essentially stealing.

But the other side of the coin is that copper prices have gone through the roof, and you bid this job _6 months_ ago, and your underlying costs have gone up quite a bit.

IMHO it is _not_ immoral to take a bid off the table after a period of time. Have you now signed a contract based upon the bid and are starting work, only to get sticker shock on the cost of the wire, or is the customer asking you to start the job now, and you think that things should be renegotiated because of the delay?

-Jon

Re: 350 mcm for 300 amps? #36295 04/03/04 06:10 PM
Joined: Jul 2002
Posts: 74
S
scjohn Offline OP
Member
We are in the final stage of the job. The service is one of the last things to do. I collected a rough draw 5 months ago. I dont know why it took so long for them to get us back in for trim. Someone stated in a previous post that eng. specs. usually go large on the service. Are they in bed with suppliers? Who is cheating who? This engineer also spec. bolt in panels, a $1200.00 surge suppressor, an aic rating of 24, which I talked him down to 22. A lighting package in excess $15,000 with low voltage lights galore all for a shell with cubicles. I spoke with the gc and the owner and we got the lighting down to $8000. I have basically helped the owner/gc every way possible to cut cost that was not within my bid, so I guess I neeed to go to them, instead of the engineer, and try to explain why they do not need a 350 mcm service so I do not lose profit on this job. I didnt mean for this thread to be morally oriented, but now that it has become such, I again ask is there a need for 350 mcm on a 300 amp service? Any engineers wish to reply??
Thanks, John

Re: 350 mcm for 300 amps? #36296 04/03/04 06:46 PM
Joined: Apr 2002
Posts: 558
C
caselec Offline
Member
John,

What happens at a later time if additional loads are added to this service and the calculated load is now at 286 amps? Who cheated who? If I was the owner and requested a 300 amp service I should get a 300 amp service not a 285 amp service. Most of projects I would on I am also the designer so I have the freedom to select the conductor sizes. The cost difference between the 300 and 350 can’t be a very high percentage of the total project. Since you bid the job with 350’s if you decide to use 300’s are you going to reduce your bill for this work accordingly?

Curt


Curt Swartz
Re: 350 mcm for 300 amps? #36297 04/03/04 06:54 PM
Joined: Sep 2003
Posts: 650
W
winnie Offline
Member
scjohn,

I have to apologize; it was inappropriate for me to moralize this discussion. We all take it as a given that doing less work, or installing smaller wire than is _agreed_ is not appropriate.

The real issue is: One of the promised materials is far in excess of what the customer needs. Materials prices have changed suddenly and unexpectedly, such that delivering the promised goods at the promised price means that you don't make your deserved profit.

There are two questions that this discussions can help answer: 1) Would installing smaller wire result in a safe and proper installation. 2) How can you pitch this change to the customer, so that they will agree to the change and both of you will come away from the deal reasonably satisfied.

The answer to 1) seems to be yes.

I wish I had better customer relations skills so that I could help you with 2)

I personally think that there are innumerable ways that one can 'oversize' in order to 'increase safety', which are not at all really necessary nor which actually increase safety. Engineers oversized things whenever they are not spending their own money! It has nothing to do with being in the pockets of suppliers, or trying to cheat anyone. It is simply 'bigger is better' combined with 'someone else's money'.

-Jon

Re: 350 mcm for 300 amps? #36298 04/03/04 06:56 PM
Joined: Jul 2002
Posts: 74
S
scjohn Offline OP
Member
Caselec, 300 mcm is rated at 320 amps -310-16.With what your saying the engineer should have spec a 400 amp main with 500 mcm feeders. The main is what it is no more, no less.
John

[This message has been edited by scjohn (edited 04-03-2004).]

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