Lights dimming.

Hi. I’ve been using some incandescent lamps recently & they dim considerably when the 13kw heat pump starts & a strip light in a shed starts to flash on/off. Not sure where to start looking - is it likely to be a supply voltage drop or an internal problem? Advice gratefully received

Hi @Sean_m, 13kW is only 57A so shouldn’t cause a voltage drop of enough to cause a strip light to flash unless the starter is really on the way out. Incandescent lamps should not dim considerably, although maybe marginally.
Even pulling the 100A that most residential houses can draw (before blowing the incoming fuse), you shouldn’t have issues like that.

What kind of location is your property in? You don’t have a 10 mile drive do you?

It’s most likely a supply issue as your heat pump will be connected on a different circuit to your other lights, which should go straight from the CU to the meter with cables rated to handle more than 100A.
People have had similar issues before:


You don’t say whether the heat pump has a ‘soft start’ facility? We have a CTC GSi12 which does have a soft start and is limited to 40A anyway as we don’t need to use the full capacity.

You definitely need to get your electricity supplier to check out the supply to your property.



Also, if you’re in the north, you can check the substation demand maps to see if maybe your local supply is very close to or at capacity:

I suspect other distribution companies have similar maps.

Hi. Thank you for these comments. As you say, heat pump has its own supply from the intake. We are rural, but the pole transformer is 150m away. Installed in 1970s.
Pumps start independently & are soft I believe so max start load shouldn’t be too excessive. If it just dimmed & recovered that would be understandable.
I can put a meter on the supply & see what the drop is- is there an acceptable level?

Many thanks

I can put a meter on the supply & see what the drop is- is there an acceptable level?
The UK supply spec is 230V(-6+10%), so 216V at the lowest.

It’s typically still closer to the 240V mark though as nothing much has really changed technically since the 1960s when the spec changed.