ECN Forum Forums General Topics of the Trade General Discussion Area 350 mcm for 300 amps?

John,

I have never seen 300 amp service equipment with 90C rated terminals. You need to use the 75C column for these conductors.

Curt

If part of the run is underground, wouldn't the 75°C {THWN} rating likely apply?

Aside from that, 350-kcmil seems like part of the bid-process "level playing field." As being on the side of the specifier and his client, presupposing different calculations and ratings not specified could be viewed as shortchanging the building owner.

Caslec, it is an underground service with thhn, not thwn.
John

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

John,

You can’t use the 90C rating if your terminals are only rated for 75C.

Curt

John,
THHN is not suitable for use in wet locations. Underground raceways are wet locations.
Don

I accidentally clicked post before I finished typing………

I will admit that all of the THHN I have seen is also rated as THWN but THWN is also only rated as a 75C conductor unless it’s THWN-2. You still can not use the 90C column if your terminals are only rated for 75C.

Curt

OK CURT, than the 300 mcm will burn clear through before the 300 main will trip???
This thread is turning way away from its start. Please show me where 350 mcm is needed in this application, Curt!
John

John

If the total load on this service is 225 amps you are correct 240.4(B) would permit the installation of 300 kcmil protected by a 300 amp breaker or fuse. If the calculated load on this service is >285 amps 350 kcmil cables would be required. If you signed a contract to install the electrical system according to the plans you need to follow them.

Curt

Lots of posts on all aspects of this. I will try to summarize;

Using table 310.16:

350Kcmil Cu is rated for 260A if 60C insulation and terminations, 310A is 75C insulation and terminations, and 350A if 90C insulation and terminations.

For 300Kcmil, the values are 240A, 285A, and 320A.

(Assuming the ambient temperatures, etc...)

Most type THHN wire is dual rated as THWN. You can use whichever rating applies to the circumstances; eg. if the wire is in a wet location, then you cannot use the wire as THHN, but can use it as THWN. THWN wire is only rated at 75C insulation. If the wire is dual rated to THWN-2 then you can use it at 90C.

If you want to be able to use the 90C ampacity, then all of the terminations will also need to be rated for 90C.

If your wire is rated for 75C in wet locations, or if your terminations are rated for 75C, then you will need to use the 75C ampacity for the wire. This means 350 Kcmil to supply 300A.

However if the calculated load is <285A, then you could use 300 Kcmil wire protected with a 300A breaker.

Ryan, did I get the above correct?

-Jon

This is not quite true. It is true within the limits of contract law, but presuming that the contract is legal, and that the customer held up their end of the contract, then I believe that scjohn can reasonably _request_ a change.

If scjohn has treated the GC and customer well, then it is not in their interest to leave him screwed over because copper prices went through the roof. It is also not in their interest to pay him more or get less copper, so they will have to balance those issues, and scjohn will have to be persuasive.

But this should be a lesson to the rest of us; include in the fine print in your contracts reasonable 'escape' clauses. Bids should be of limited duration, and work should proceed at a reasonable pace. If the customer or another contractor delays the work (delays getting you paid), then your bid should accommodate changes in your costs.

What would happen if you started a job, then something happened to put it on hold for a couple of years...you've given people raises, copper and steel prices have changed, etc. What do your contracts say? Would you be left holding the bag?

scjohn was not paid up front, and the job waited six months from the bid and start of work (more or less; I don't know the exact timing). Why should he front out of pocket to buy the copper up front, if he has to wait months before he could install it and get paid. What would happen to _you_ in the same situation. Would you end up eating the price increase, or do you bid in a way that lets you pass the price increase on, yet still make a _fair_ profit?

-Jon