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Chris Lewis
 
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Default Electric baseboard heater question

According to KT :
(Chris Lewis) wrote in message
...
According to KT :
Hi. We've got a couple of electric baseboard heaters that appear to
be connected to a relay that apparently runs to a wall mounted
thermostat. We're trying to track down a power problem. Service
disconnect on the circuit box trips soon after baseboards start
heating.


_Service_ disconnect? Really? Takes out the whole house? Are you sure
you don't mean branch circuit breaker? (probably a pair of breakers tied
together).


It is actually the service disconnect, which is a 100 A breaker, and
unfortunately does take out the whole house.


Let's see if I can cut to the chase he

You have a 100A main panel, with a 100A subpanel. The heaters are
fed off the 100A subpanel (is the stove, dryer too?). The heaters
are on two 20A 240V circuits. When the heaters turn on, the main
(not subpanel) breaker (service disconnect) trips.

Points to consider:

1) "To code", the maximum current the heater circuits can draw in total
is 32A (80% of 2x20). If it's more than 20A apiece the branch breakers
should trip (but see below regarding massive shorts).

2) You have a low voltage thermostat circuit. This means: a small
transformer converts 220V to 24V. The thermostat (you have only
one thermostat, right?) acts as switch to control 24V to the relay.
The coil on the relay is 24V. The contacts that the relay opens/closes
are 240V driving the heaters. Most furnaces have low voltage or "millivolt"
thermostat circuits - the relay is inside the furnace, transformer often
mounted nearby. With electric heat, the relay has to switch a lot more
current.

3) The thing circuit corresponding to the 'Thermostat Circuit (24V 0.15
Amps/Line Voltage circuit 240 V AC Switched 25A).' powers the transformer
(at least). The small box you see is probably the transformer.

I'm not an electrician, and I've not worked with a low voltage thermostat
system for electric heating since the mid 70's, so, I'm not quite sure
of the current practise/hardware is. That "small device" could actually
be both the transformer and relay (one thermostat: if there's two 220V
circuits going into and out of it - total of four cables _plus_ smaller
wires going to the thermostat. If there are two thermostats involved,
there are likely two relays hidden somewhere).

Or, the relay is parked somewhere else. By code, the relay needs to be
"accessible". But that may mean that it's in the wall behind something.
[We installed ours in the wall cavity with the in-wall forced air
heater. Access by unscrewing the heater from the wall.]

There's just about nothing the heaters do on their own that would trip
a 100A breaker before their own 20A breakers. That goes for the relay[s],
transformer[s] and thermostat[s] too.

I can think of two possibilities:

1) "tired" main breaker. But the stove and dryer simultaneously should
be able to trip it. Probably not this.

2) One of the heaters has a temperature-induced short. Ie: when it heats
up, a hot-ground short occurs and the main breaker simply trips faster
than the 20A breaker on a massive short.

It's remotely possible this is the relay (or even the transformer connection),
but I'd suspect a heater first. You should be able to smell ozone, or hear
it go "bang", or find scorch marks in the device that's shorting out. Or all
three. 100A+ shorts aren't subtle things. They usually make their presence
known. The lights should dim momentarily when this happens for example.

If you find burnt contacts on the relay, be aware that they may be the _result_
of a heater problem. So you'll have to check the heaters out too.

You're really going to need an amprobe (and someone who knows how to use
one) to diagnose this if you can't sniff out the problem directly.

Ie: is the main breaker really seeing 100A when it trips? Do any of
the heater circuits spike 100A when the main goes?

For anything other than a solid diagnosis of a heater/relay/transformer
going bad and spiking 100A, you'll need an electrician.

[Other highly remote possibilities: temperature-induced short _inside_
panel. Etc. This requires experience to diagnose.]

Be _careful_. 100A breakers tripping implies that there's a lot of
energy going _somewhere_. Best not to be _you_. Don't be standing
around touching the panel or heaters when this thing is likely to trip.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.