SURVEY: Powering big iron in your home.

From: James Rice <jrice_at_texoma.net>
Date: Tue Jun 4 20:19:08 2002

Gunther, I don't collect "big iron" or even mini's but I still hold a
master electrician ticket.

1.Breaker - Breakers do and can safely run warm. I've been out of the
trsade for 6 years now, but if I remember, some household class breakers
were rated to run as much as 30-40 degrees C above ambient and still be
in spec. Temps measured with a IF scanner used to show that heavily
loaded breakers ran as hot as 150-175F in regular service. Some brands
;ike Zinsco-Sylvania, Federal Pacific and old GE panels were known to
break down at lower loads than others. We had the best luck with The
SquareD QO series or C-H. I never liked the Square D "Homeline"
series...too much cost savings at the expense of ease of service and
durability. I always thought they were "engineered to a price, not to a
standard". The QO breakers are even available in an electrically
operated remote version.


2.Split bolts are an approved splicing method, just not used as much on
small wire sizes. I rarely used them for small cables, just those #2
or larger. I routinely used split bolts on cables up to 1000MCM cross
section. Just tighten the shit out of them, pad them well with scotch
rubber splicing pads and tape the hell out of them. Never had a split
bolt splice of mine go bad.


Your #10 wire is under-sized. #8 is commonly used on 50 amp circuits in
some areas because the common household breaker will trip before a
continous load of 50 amps is reached. Most are designed to trip under
continous loads of 90% of the label rating. To relieve your concern,
you can apply some contact paste (Look for it under the Burndy brand at
a supply house) to you cables before torquing the screws. It seems to
keep screw connections cooler.

You might want to check your neutral current. Ideally it should be 0,
however that isn't common. It most of the load is single phase, then
doubling the neutral size might be a wise move.


> Hi,
>
> I wanted to conduct some survey of the common practices (which
> may or may not be good practices) to powering your very own mini
> and maxi big iron in your home. I think this topic may be
> embarrassing so PLEASE let's wait 10 days or so before flaming
> and blaming each other for our risky ways of squeezing the last
> amp out of our feeds. I would hope that in the end we could
> summarize in a calm and sensible way what the general suggestions
> might be to a safe but reasonable way of running equipment.
>
> Here I go then. As you know, I have two hot-spots:
>
>
>
> (1) 2 VAX 6000-460 and 8 to 10 RA90 and 2 RA8x and one HSC90
> that I run in my basement. I have a dedicated 60 A 220 V 4-wire
> line going right into that room. I have terminated that line in
> a circuit breaker box and feed the computers with a 25 A 220 V
> line. This is about 6 to 9 foot cable of AWG #10 that I run without
> a metal conduit. (The other breakers feed German household stuff
> that we still have in use here and there.)
>
> I noticed that when I have everything on, the 25 A breaker gets
> warm. Fairly warm. Not hot, but markedly warm. I made sure the
> wires are screwed very tight and the breaker is seated right, but
> still it gets warm. That worries me a bit.
>
> Also, I think that my #10 wire is under-dimensioned after all.
> Since all the consumers are essentially doing some (more or less)
> good job of load balancing, eventually the neutral wire gets
> quite a bit of load, possibly beyond 25 A. Theoretically up to
> 50 A. So, I should have used #8 or even #6 then.
>
> However, the practical problem is that you can't fit #6 wire into
> a breaker contact screw. And even #8 is going to be on the low
> end with 50 A. The best thing would be if neutral was twice as
> thick here. I am considering to put in two lines in parallel from
> the same breaker through the same conduit (if any) the outlets
> very close together.
>
>
>
> (2) The VAX 11/780, UNIBUS, TU78, and TE16 in the garage.
> They run off the same line (so that line is no longer dedicated.)
> I spliced a branch off with split bolts and lots of electric tape.
> for insulation. The branch terminates in a dedicated box with
> the original outlets. The initially #8 AWG is distributed through
> #10 wires to the various outlets. Even some #12 to the two
> standard 110V outlets in that box.
>
> My concern there is, of course, that (a) the split bolt construction
> may develop resistance and increased heat dissipation. Also (b) the
> way I downsize and distribute the #6 feet to smaller wires in the
> outlet box may lead to overcurrent if the devices aren't connected
> where they should. I did my best to have the big suckers on thick
> AWG and shorter length than the moderate consumers. On the other
> hane, turning the VAX 11/780 on is an extremely rare event right now
> and will probably be in the future. Some hours per week perhaps.
>
>
>
> I was concerned about what to do whe I have my VAX party (when
> everything works.) If we want to turn it all on at the same time,
> something might happen...
>
> I regularly check the warmth of that breaker downstairs and now that
> that spplit-bolt tap is insulated I can check that for heat. But it's
> nothing I want to run 24/7 without attention. So, I know I have to do
> something about it. I usually take the circuit off line when I am
> done with it.
>
> The problem of course is that to do it properly would require
> upgrading the main feed, replacing the main breaker panel running
> thousands of new wires, and sub-panels, where each machine essentially
> has its own breaker, and of course the outlets and plugs need to be
> all proper. Easy to spend $2000 or more in material and labor for that
> upgrade. For equipment we run a few hours per week.
>
>
>
>
> I'm really interested in hearing how others with minis do it. (If you
> don't have a mini yourself but want to teach the dangers of electricity
> and the demands of the NEC, please hold off for 10 days, O.K.?) Any
> good or bad example is welcome.
>
> regards,
> -Gunther
>
>
>
Received on Tue Jun 04 2002 - 20:19:08 BST